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[vsnet-grb-info 39567] LIGO/Virgo/KAGRA S250818k: JinShan optical observations of AT2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41503 SUBJECT: LIGO/Virgo/KAGRA S250818k: JinShan optical observations of AT2025ulz DATE: 25/08/22 18:13:18 GMT FROM: J. An <jiean0813(a)foxmail.com> J. An, X. Liu, Z.P. Zhu, S.Q. Jiang, L.B. He, D. Xu (NAOC), S.Y. Fu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration: We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493; Bruni et al., GCN 41500; Santos et al., GCN 41501; Becerra et al., GCN 41502), possibly connected with the GW event S250818k (LVK Collaboration, GCNs 41437, 41440), using the 100C telescope of the JinShan project, located at Altay, Xinjiang, China. Our first epoch observation started on 2025 Aug 20.61 UT, i.e., 2.55 days after the GW trigger time, in the Sloan g-, r-, and i- filters with 1 hr exposure for each filter. By image subtraction with the Legacy Survey template, we obtained a provisional magnitude of r = 22.8 +/- 0.3 (AB) at 2025 August 20.63 UT for AT2025ulz. We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41503. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39566] LIGO/Virgo/KAGRA S250818k: GTC/OSIRIS Optical Detection of AT2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41502 SUBJECT: LIGO/Virgo/KAGRA S250818k: GTC/OSIRIS Optical Detection of AT2025ulz DATE: 25/08/22 17:59:55 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra(a)roma2.infn.it> Rosa L. Becerra (U Rome), Alan M. Watson (UNAM), and Eleonora Troja (U Rome) report on behalf of a larger team: We observed the field of AT2025ulz/ZTF25abjmnps, reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440), with the OSIRIS instrument mounted on the GTC telescope. Our observations began at 2025-08-21 21:40 UTC (3.9 days after the trigger) and were carried out in the r and z filters with an average airmass of ~1.3 and excellent seeing. After image subtraction against Legacy Survey templates (Dey et al. 2019), we detect the source with a preliminary magnitude of r~23.4 AB, consistent with the near-contemporaneous observations of Malesani et al. (GCN 41492). We also confirm a very red color, in agreement with earlier observations (Hall et al., GCN 41433; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Perley et al., GCN 41480). Further observations are planned. We thank the staff at the GTC, especially Antonio García Rodríguez, Antonio Marante Barreto, and Antonio Cabrera for the rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41502. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39565] LIGO/Virgo/Kagra S250818k: STEP/T80N upper limits on 2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41501 SUBJECT: LIGO/Virgo/Kagra S250818k: STEP/T80N upper limits on 2025ulz DATE: 25/08/22 17:12:58 GMT FROM: André Santos at Centro Brasileiro de Pesquisas Físicas (CBPF) <andsouzasanttos(a)gmail.com> A. Santos (CBPF), C. R. Bom (CBPF), C. D. Kilpatrick (Northwestern), L. Santana-Silva (CBPF), P. Darc (CBPF), Gabriel Teixeira (CBPF), C. Mendes de Oliveira (IAG-USP) report on behalf of the STEP collaboration We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; P. D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493), possibly associated with the GW superevent S250818k (LVK Collaboration, GCN 41437, GCN 41440), using the T80N 0.8-m robotic telescope using the 1.4 x 1.4 FoV T80N-Cam imager. The observations started on Aug 21, 21:46 UT (~3.85 days after trigger). We obtained 600s (2x300s)and 1200s (4x300s) stacked exposures in r and i bands respectively, with the T80-Cam centered at the position of the reported transient. Subtracting Pan-SSTARRS templates available at MAST archive using photpipe (Rest et al. 2005), we do not detect any sources in our difference images and derive 3-sigma limiting magnitudes of r > 22.9 and i > 22.8 mag for the transient. We thank the OAJ Data Processing and Archiving Department (DPAD) for reducing and calibrating the OAJ data used in this work, as well as the distribution of the data products through a dedicated web portal. View this GCN Circular online at https://gcn.nasa.gov/circulars/41501. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39564] LIGO/Virgo/KAGRA S250818k: 3 GHz MeerKAT observations of AT2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41500 SUBJECT: LIGO/Virgo/KAGRA S250818k: 3 GHz MeerKAT observations of AT2025ulz DATE: 25/08/22 16:13:06 GMT FROM: Aishwarya L Thakur at INAF-IAPS, Rome <aishth(a)outlook.com> G. Bruni [1], L. Piro [1], G. Gianfagna [1] and A. L. Thakur [1] report: We observed the field of AT2025ulz (announced by Stein et al., GCN 41414), which is proposed as a candidate counterpart for the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440) with the MeerKAT radio telescope under the program SCI-20241101-GB-01 (PI: Bruni) at 3 GHz (S4 band) starting on Aug 21, 16:30 UT (at 3.6 days post-trigger) for a total of 2 hours (1.5 hours on source). J1939-6342 was used for flux scale calibration, and J1609+2641 for phase referencing. Data was processed with the SARAO Science Data Processor (SDP) continuum pipeline. The image RMS was 6.4 uJy/beam. We detect radio emission at the position of the transient, and we measure an integrated flux density of 77 +/- 16 uJy (corresponding to a ~ 10-sigma detection). However, given the northern declination of the field, the angular resolution is insufficient to disentangle any potential contamination from the host with this epoch of observation alone. Further MeerKAT observations are planned. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. —------------------------ [1] INAF-IAPS View this GCN Circular online at https://gcn.nasa.gov/circulars/41500. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39563] GRB 250806A: Gemini observations of host galaxy candidate
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41499 SUBJECT: GRB 250806A: Gemini observations of host galaxy candidate DATE: 25/08/22 16:12:48 GMT FROM: gsriniv2(a)umd.edu C. Sevilla (Cornell), G. Srinivasaragavan (UMD), D. A. Perley (LJMU), A. Y. Q. Ho (Cornell), A. Bochenek (LJMU) report: We observed the candidate host galaxy of GRB 250806A (Xie et al., GCN 41243, Malesani et al. GCN 41279) beginning UTC 2025-08-13 13:14:40 with the Gemini Multi-Object Spectrograph on Gemini North under program GN-2025B-Q-125 (PI: Srinivasaragavan). We use a 1 arcsecond slit, the R400 grating, and the GG455 filter. We obtained four exposures of 450 seconds each, two at a central wavelength of 520 nm and two at 525 nm. We obtain a redshift of 0.367 from galactic Hβ, [O II], and [O III] emission lines. At this redshift, using the 4 - 120 keV flux from Bouchet et al. (GCN 41287) and observed burst time of 20 seconds, we estimate the burst has an isotropic equivalent energy of ~ 8e49 erg. We thank the Gemini Contact Scientists and the Gemini Helpdesk for their assistance in observing and data reduction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41499. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39562] IPN triangulation of GRB 250821B (long/bright)
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41498 SUBJECT: IPN triangulation of GRB 250821B (long/bright) DATE: 25/08/22 15:46:04 GMT FROM: Dmitry Svinkin at Ioffe Institute <svinkin(a)mail.ioffe.ru> A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin on behalf of the HEND/Mars Odyssey team, D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team, A. Goldstein, M. S. Briggs, C. Wilson-Hodge, and E. Burns on behalf of the Fermi GBM team, and W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr, and A.S. Gardner on on behalf of the GRS-Odyssey GRB team, report: The bright, long-duration GRB 250821B (Ferm-GBM detection: The Fermi GBM team, GCN 41473; Smith and Meegan, GCN 41477; BALROG localization: Preis and Greiner, GCN 41475; SVOM-GRM detection: Tan et al., GCN 41486) was detected by Fermi (GBM trigger 777485272), SVOM (GRM), Konus-Wind, and Mars-Odyssey (HEND), at about 58067 s UT (16:07:47). We have triangulated it to a preliminary, 3 sigma error box whose coordinates are: --------------------------------------------- RA(2000), deg Dec(2000), deg --------------------------------------------- Center: 29.373 (01h 57m 30s) -82.081 (-82d 04' 51") Corners: 26.171 (01h 44m 41s) -82.255 (-82d 15' 19") 32.420 (02h 09m 41s) -81.925 (-81d 55' 30") 32.427 (02h 09m 42s) -81.883 (-81d 52' 58") 26.212 (01h 44m 51s) -82.213 (-82d 12' 48") --------------------------------------------- The error box area is 127 sq. arcmin, and its maximum dimension is 56 arcmin (the minimum one is 2 arcmin). The Sun distance was 106 deg. This localization may be improved. The IPN localization is consistent with, but reduces the area of, the Fermi-GBM (GCN 41473) and BALROG (GCN 41475) localizations. A triangulation map and HEALPix FITS file are posted at http://www.ioffe.ru/LEA/GRBs/GRB250821_T58069/IPN The HEALPix triangulation map is the multi-order HEALPix in units of probability density. The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular. View this GCN Circular online at https://gcn.nasa.gov/circulars/41498. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39561] GRB 250821A: Kilonova-Catcher optical upper limits
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41497 SUBJECT: GRB 250821A: Kilonova-Catcher optical upper limits DATE: 25/08/22 15:19:10 GMT FROM: Damien Turpin at CEA-Saclay <dturpin-astro(a)hotmail.com> M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), M. Pillas (ULiege), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration: We observed the field of GRB 250821A detected by Swift/BAT (Dichiara et al., GCN 41462) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iTelescope T32 telescope located at Siding Spring Observatory and operated by M. Freeberg. Our observations started at T0+17 min. In our stacked frames, subtracted from the PanSTARRS DR2 template image, we do not detect any optical counterpart inside the Swift/XRT enhanced position (Beardmore et al., GCN 41472). We report our follow-up results in the table below: +---------------+-----------+-----------+----------------------+-------------+ | Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument | +===============+===========+===========+======================+=============+ | 0.86 | 20 x 180s | Ic (Vega) | 19.4 (U.L., 5 sigma) | T32 | +---------------+-----------+-----------+----------------------+-------------+ All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog. We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/) View this GCN Circular online at https://gcn.nasa.gov/circulars/41497. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39560] EP250821a: Swift/XRT counterpart fading
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41496 SUBJECT: EP250821a: Swift/XRT counterpart fading DATE: 25/08/22 14:41:37 GMT FROM: Simone Dichiara at Pennsylvania State University <sbd5667(a)psu.edu> S. Dichiara (PSU), S. R. Oates (Lancaster U.), J.A. Kennea (PSU), K.L. Page (U. Leicester) report on behalf of the Swift-XRT and Swift-UVOT team: Swift-XRT has performed further follow-up observations of the EP-WXT transient EP250821a (Hu et al., GCN 41459), collecting a total of 3.0 ks of Photon Counting (PC) mode data between T0+61.0 ks and T0+72.9 ks. The source detected in XRT in the first 1.9 ks of data previously reported as “Source 1” (Dichiara et al., GCN 41469), consistent with the optical counterpart (Li et al., GCN 41470, Yao et al., GCN 41485, An et al., GCN 41490), has faded more than 3 sigma in the latest X-ray observation. The count rate derived from the follow-up observation is 2.2 (+/-0.3) x 10^-2 cts/s. The light curve can be modelled with a power-law decay with an index of alpha=1.03 +/- 0.07. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 +/- 0.2. The best-fitting absorption column is 1.4 (+0.7, -0.6) x 10^21 cm^-2 , in excess of the Galactic value of 7.26 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.51 x 10^-11 (4.48 x 10^-11) erg cm^-2 count^-1. The observed (unabsorbed) flux obtained from the follow-up observations from T0+61.0 ks to T0+72.9 ks is thus 7.7 x 10^-13 (9.9 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/EP/EP_FIELD00059/ This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41496. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39559] GRB 250812A: SVOM/ECLAIRs refined analysis
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41495 SUBJECT: GRB 250812A: SVOM/ECLAIRs refined analysis DATE: 25/08/22 14:39:39 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> L. Bouchet, J.-L. Atteia , O. Godet, M. Brunet (IRAP), L. P. Xin(NAOC), Z.M. Wang(BNU) report on behalf of the SVOM/ECLAIRs team: Using the ECLAIRs event-by-event data downloaded through the X-band ground station, we report further analysis of ECLAIRs observations of GRB 250812A (SVOM burst-id sb25081201), triggered by ECLAIRs onboard SVOM (Xin et al., GCN 41322). The burst is faint and the 4-120 keV lightcurve is featureless. The time-averaged spectrum from T0-51 s to T0+3 s (T0 = 2025-08-12T02:46:03 UTC) in the energy range 4-120 keV is relatively well fitted by a powerlaw model with index -1.19+/-0.27. With this model, the fluence in 4-120 keV is about 7.5 (-1.9,+0.6) e-7 erg/cm^2. All quoted errors are at the 68% confidence level. The spectral index suggests that the Epeak value is above 120 keV. The source redshift is found to be z=2.571 (Van Dalen et al., GCN 41347; de Ugarte Postigo et al., GCN 41348). At this redshift, GRB 250812A lies in the region of type II GRBs in the Amati plot for values of Epeak ≤ 200 keV. We note that the calibration of SVOM/ECLAIRs is ongoing thus these results are preliminary. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. The SVOM/ECLAIRs point of contact for this GCN circular is: Laurent Bouchet (laurent.bouchet(a)irap.omp.eu) View this GCN Circular online at https://gcn.nasa.gov/circulars/41495. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39558] GRB 250821A: Swift-BAT refined analysis
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41494 SUBJECT: GRB 250821A: Swift-BAT refined analysis DATE: 25/08/22 14:31:30 GMT FROM: Rahul Gupta at NASA GSFC <rahul.gupta(a)nasa.gov> A. Y. Lien (U Tampa), S. D. Barthelmy (GSFC), S. Dichiara (PSU), R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), C. B. Markwardt (GSFC), M. J. Moss (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team): Using the data set from T-240 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250821A (trigger #1344131) (Dichiara, et al., GCN Circ. 41462). The BAT ground-calculated position is RA, Dec = 358.427, -28.631 deg which is RA(J2000) = 23h 53m 42.6s Dec(J2000) = -28d 37' 53.3" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 64%. The mask-weighted BAT light curve exhibits a short-duration main emission starting from ~T-0.5 to ~T+1.2 sec, followed by a weaker, extended emission which last up to ~ T+18 sec. T90 (15-350 keV) is 1.52 +- 0.37 sec (estimated error including systematics). The time-averaged spectrum from T-0.46 to T+1.20 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.83 +- 0.41. The fluence in the 15-150 keV band is 1.3 +- 0.3 x 10^-07 erg/cm2. The 1-sec peak photon flux measured from T+0.06 sec in the 15-150 keV band is 0.9 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1344131 View this GCN Circular online at https://gcn.nasa.gov/circulars/41494. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39557] LIGO/Virgo/KAGRA S250818k: Pan-STARRS coverage of the skymap and candidate counterparts
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41493 SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS coverage of the skymap and candidate counterparts DATE: 25/08/22 14:09:35 GMT FROM: James Gillanders at University of Oxford <jhgillanders.astro(a)gmail.com> S. J. Smartt, J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard). We surveyed the skymap of the gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560). Following on from the reported optical candidate AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Perley et al., GCN 41480, D’Avanzo et al. GCN 41489), we have been taking targeted grizy imaging of this transient (Gillanders et al., GCN 41454). At this specific region we have proprietary data to make significantly deeper reference stacks than those that are available through the STScI public website, and monitoring with Pan-STARRS is ongoing. For the rest of the northern portion of the GW skymap, we surveyed the region in the i-band over three nights: 2025 Aug 20, 21, 22. We covered ~288 square degrees and ~33% of the bilby.fits skymap 90% area each night, with multiple 45s (Aug 20) and 120s (Aug 21, 22) exposures. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). Transient candidates were identified and run through our standard filtering procedures, including rejection of artefacts with machine learning tools and cross-matching with galaxy, stellar and solar-system catalogs (e.g., Smith et al., 2020, PASP, 132, 1; Smartt et al., 2024, MNRAS 528, 2299). The depth of our images are typically i ~ 21 - 21.5 per exposure, with the first night having better seeing and transparency conditions. We detected 68 candidate extragalactic transients, of which 30 were already registered on the IAU Transient Name Server (TNS) as having been discovered and detected before S250818k merger time. We have registered the other 38 sources on the TNS. To down-select for optical counterpart candidates of S250818k, we applied the following filters: 1. Spatially associated with a host galaxy that has a photometric or spectroscopic redshift within the 2-sigma limit from LVK of 0.025 < z < 0.08, 2. Has some indication of lightcurve evolution over the three nights of observation, 3. We rejected all faint transients spatially coincident with the nucleus of galaxies as being due to either AGN variability or possible subtraction artefacts. We found no plausible candidate for the optical counterpart of S250818k other than AT2025ulz. Six sources with plausible kilonova-like absolute magnitudes and in hosts with consistent redshifts (within errors) with S250818k were found (AT2025uuw, AT2025uso, AT2025uuf, AT2025utr, AT205usy, AT2025uxs), but their lightcurves are either rising or are flat and show no evolution over 48hrs, and they do not appear to be viable candidates at this point. Discovery details and magnitudes of the ATs can be found on the TNS. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and NNX14AM74G, issued through the SSO Near-Earth Object Observations Program. Data processing is enabled by Queen's University Belfast and the University of Oxford, enabled through STFC grants ST/Y001605/1, ST/T000198/1 and ST/X001253/1, the Royal Society, and the Hintze Centre for Astrophysical Surveys. View this GCN Circular online at https://gcn.nasa.gov/circulars/41493. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39556] LIGO/Virgo/KAGRA S250818k: NOT optical observations of AT2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41492 SUBJECT: LIGO/Virgo/KAGRA S250818k: NOT optical observations of AT2025ulz DATE: 25/08/22 11:30:51 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> D. B. Malesani (DAWN/NBI and Radboud), A. Boye (DTU Space), L. Izzo (INAF/OAC and DARK/NBI), G. Leloudas (DTU Space), J. An (NAOC), X. Liu (NAOC), D. Xu (NAOC), M. Fraser (UCD), S. J. Brennan (MPE), J. Broe Bendsten (SDU), I. A. Koch (SDU), S. Lund Wagner (SDU), J. Magaard Knudsen (SDU), J. Hein Pedersen (SDU), J. P. U. Fynbo (DAWN/NBI), R. Holmberg Rasmussen (NOT and Aarhus), K. Valeckas (NOT and NBI), M. De Pasquale (U. Messina) report on behalf of a larger collaboration: We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480), possibly connected with the GW event S250818k (LVK Collaboration, GCNs 41437, 41440), using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were started on 2025 Aug 20.875 UT (2.82 days after the GW trigger time). We adopted the filters r, i, and z (exposure times of 1200, 1200, and 1500 s, respectively). Image subtraction using the Legacy sky survey as reference reveals a transient at coordinates (J2000): RA = 15:51:54.19 Dec = +30:54:08.3 with an estimated error of < 0.5". These are consistent with the ZTF position (Stein et al., GCN 41414). The transient is located 1" southwest of the nucleus of its host galaxy, at a projected distance of 1.6 kpc (z = 0.0848; Karambelkar et al., GCN 41436; Banerjee et al., GCN 41476). Using nearby calibrators from the Legacy Survey, we obtained a provisional magnitude measurement of r = 23.2 ± 0.2 (AB), at a mean time of 2025 August 20.883 UT. We caution that we expect to refine our results after securing a deep template with the same instrument and filter. View this GCN Circular online at https://gcn.nasa.gov/circulars/41492. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39555] EP250821a: Swift/UVOT further observations.
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41491 SUBJECT: EP250821a: Swift/UVOT further observations. DATE: 25/08/22 09:44:34 GMT FROM: Samantha Oates at University of Birmingham <samantha.oates(a)alumni.ucl.ac.uk> S. R. Oates (Lancaster U.) and S. Dichiara (PSU) report on behalf of the Swift-XRT and Swift-UVOT team: Swift/UVOT has performed further follow-up observations of the Einstein Probe/WXT-detected source EP250821a (Hu et al., GCN 41459). The UVOT optical counterpart reported in (Dichiara et al., GCN 41469) has faded, consistent with the behaviour reported by Li et al. (GCN 41470) and Yao et al. (GCN 41485). The refined UVOT position is RA (J2000) 19:19:32.56 Dec (J2000) -43:22:52.3 with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence). We note that this position supersedes that reported in Dichiara et al. (GCN 41469), which is incorrect. We apologise for any confusion. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag/3sigUL v 61584 72904 799 >19.9 u 5186 11029 1897 18.88+\-0.08 (reported in GCN 41469) u 68377 68575 195 >19.8 uvw1 67966 68372 399 >20.0 uvm2 67555 67961 399 >19.8 uvw2 61180 72857 1168 >20.2 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.07 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41491. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39554] EP250821a: VLT X-shooter spectroscopic redshift z = 0.577
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41490 SUBJECT: EP250821a: VLT X-shooter spectroscopic redshift z = 0.577 DATE: 25/08/22 09:01:58 GMT FROM: nrt3(a)le.ac.uk J. An (NAOC), V. Abril-Melgarejo (LUX-Paris Obs.), R. A. J. Eyles-Ferris (U. Leicester), D. Xu (NAOC), A. L. Thakur (INAF-IAPS), A. J. Levan (Radboud U.), P. G. Jonker (Radboud U.), D. B. Malesani (DAWN/NBI and Radboud), S. R. Oates (U. Lancaster), G. Corcoran (UCD), A. Martin-Carrillo (UCD), V. D’Elia (ASI/SSDC), N. R. Tanvir (U. Leicester) report on behalf of the Stargate collaboration: We observed the location of the optical counterpart and coincident likely host galaxy (Dichiara et al., GCN 41469; Li et al. GCN 41470; Yao et al. GCN 41485) of the X-ray transient EP250821a detected by Einstein Probe (Hu et al. GCN 41459; Liang et al., GCN 41467), using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 600 s each. Observations started on 2025-08-22 at approximately 04:10 UT (20.9 hr after the EP trigger). In a preliminary reduction of the spectra, we detect absorption lines of Fe II (fine structure), Mg II, Ca II, Na I, and emission lines of [O II], [O III], H alpha, H beta, H gamma, [N II], [S II], from which we infer a redshift of z = 0.577. We note that our spectroscopic redshift measurement is roughly consistent with the photometric redshift value of 0.64 +/- 0.06 provided for the catalogued object in Legacy Survey that was noted by Li et al. (GCN 41470). From the X-shooter acquisition imaging we estimate a magnitude of r = 20.9 at 2025-08-22 03:41 UT, approximately 20.4 hr post-trigger. Given the Legacy Survey host magnitude of r = 21.79 +/- 0.02, this suggests a significant transient contribution at this epoch. We acknowledge excellent support from the ESO observing staff in Paranal, in particular Celia Desgrange, Lorena Faundez, Camila de Sa Freitas. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495) View this GCN Circular online at https://gcn.nasa.gov/circulars/41490. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39553] LIGO/Virgo/KAGRA S250818k: GRAWITA TNG NIR observations of AT2025ulz
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41489 SUBJECT: LIGO/Virgo/KAGRA S250818k: GRAWITA TNG NIR observations of AT2025ulz DATE: 25/08/22 08:32:41 GMT FROM: Paolo D'Avanzo at INAF - OAB <paolo.davanzo(a)inaf.it> P. D'Avanzo, (INAF-OAB), M.T. Botticella (INAF - OACn), E. Cappellaro (INAF-OAPd), L. Izzo (INAF - OACn), A. Rossi (INAF-OAS), V. D’Elia (ASI-SSDC), S. Piranomonte (INAF-OAR), G. Greco (INFN), M.G. Bernardini (INAF-OAB), S. Ronchini (GSSI), B. Patricelli (Univ. Pisa), O. S. Salafia, G. Ghirlanda (INAF-OAB), N. Elias-Rosa (INAF-OAPd), Fabio Ragosta (Università degli studi di Napoli “Federico II”), R. Salvaterra (INAF-IASF Milano), E. Brocato (INAF-OAR), V. Lorenzi, A. Garcia De Gurtubai Escudero (INAF-TNG), on behalf of GRAWITA report: We observed the optical transient AT2025ulz (Stein et al., GCN Circ. 41414; Busmann et al., GCN Circ. 41421; Hall et al., GCN Circ. 41433; Karambelkar et al., GCN Circ. 41436; O’Connor et al., GCN Circ. 41452; Gillanders et al., GCN Circ. 41454; Liu et al., GCN Circ. 41461; Perley et al., GCN Circ. 41480) possibly connected to the GW candidate event S280818k (LVK collaboration, GCN Circ. 41437 & GCN Circ. 41440) with the Italian 3.6m TNG telescope, located in Canary Islands (Spain), equipped with the near-infrared camera NICS in imaging mode. A series of images were obtained with the J filter starting on 2025-08-20T20:27:00 UT (i.e. 2.80 days post the GW T0), with the H filter starting on 2025-08-20T21:42:37 UT (i.e. 2.85 days post the GW T0) and with the K filter starting on 2025-08-20T23:00:15 UT (i.e. 2.90 days post the GW T0). The exposure time for each filter was 50 minutes. The transient host galaxy is clearly detected in all our images, while no source is detected at the position of AT2025ulz down to the following 3sigma upper limits (AB magnitudes, calibrated against the 2MASS catalogue and obtained after subtracting a 2D galaxy model obtained with galfit, Peng et al., 2010, AJ, 139, 2097): J > 21.7 mag H > 21.3 mag K > 20.5 mag View this GCN Circular online at https://gcn.nasa.gov/circulars/41489. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39552] GRB 250821A: VLT/FORS2 Optical Observation
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41488 SUBJECT: GRB 250821A: VLT/FORS2 Optical Observation DATE: 25/08/22 08:32:30 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra(a)roma2.infn.it> Rosa L. Becerra (U Rome), Niccolò Passaleva (U Rome), Yu-Han Yang (U Rome) and Eleonora Troja (U Rome) report on behalf of the ERC BHianca team: We observed the field of GRB 250821A (Dichiara et al., GCN 41462) with the FORS2 imager on the ESO VLT UT1 (Antu). Observations began 18.4 hours after the trigger and were carried out in the R filter with an average airmass of ~1.0. We do not detect any new source at the enhanced Swift-XRT position (Beardmore et al. GCN 41472) down to the following 3-sigma limit R>26.1 AB mag calibrated with using nearby stars in the PanSTARRS PS1 DR2 catalogue (Flewelling et al. 2020) and without Galactic extinction correction. We thank the staff at the VLT for the rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41488. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39551] GRB 250821A: GECAM-B detection
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41487 SUBJECT: GRB 250821A: GECAM-B detection DATE: 25/08/22 07:46:56 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> Chen-Wei Wang, Shao-Lin Xiong and Yue Huang (IHEP) report on behalf of GECAM team: GECAM-B was triggered on-ground by GRB 250821A at 2025-08-21T10:46:18.000 UTC (denoted as T0), which is also detected by Swift/BAT (S. Dichiara et al., GCN#41462). According to the GECAM-B light curves in about 40-6000 keV, this burst mainly consists of a single pulse with a duration (T90) of 1.5 +1.5/-0.5 s. The GECAM-B light curve can be found here: https://www.bursthub.cn//admin/static/gecamgrb250821A.png Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS). View this GCN Circular online at https://gcn.nasa.gov/circulars/41487. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39550] GRB 250821B: SVOM/GRM observation
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41486 SUBJECT: GRB 250821B: SVOM/GRM observation DATE: 25/08/22 06:53:40 GMT FROM: tanwj(a)ihep.ac.cn SVOM/GRM team: Wen-Jun Tan, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Frédéric Daigne (IAP), Stéphane Schanne (CEA) and Jean-Luc Atteia (IRAP) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by GRB 250821B (SVOM burst-id sb25082109) at 2025-08-21T16:07:47.500 UTC (T0), which is also detected by FERMI/GBM (GBM team, GCN #41473 and GCN #41477). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single pulse with a T90 of 2.5 +0.3/-0.2 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250821B.png In addition, the position of this burst, as determined by Fermi/GBM (RA= 39.95, Dec = -77.13, GCN #41473), is located at about 65 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. However, ECLAIRs detected this burst above 50 keV, through the shield. With this localization given by FERMI/GBM, the time-averaged spectrum from T0-0.5 to T0+3.5 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.89 +/-0.05 and the cutoff energy, parameterized as Epeak, is 289 +37/-31 keV. The event fluence (10-1000 keV) in this time interval is (1.1 +/-0.1)E-05 erg/cm^2. The localization of GRB 250821B in the 'Amati' relation diagram is shown at: https://www.bursthub.cn//admin/static/grb250821B_amati.png The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM point of contact for this burst is: Wen-Jun Tan (IHEP)(tanwj(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/41486. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39549] EP250821a: SVOM/VT optical observation
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41485 SUBJECT: EP250821a: SVOM/VT optical observation DATE: 25/08/22 03:16:13 GMT FROM: zhyao(a)bao.ac.cn Z. H. Yao, Y. N. Ma, L. P. Xin, H. L. Li, Y. L. Qiu, C. Wu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM performed a Target of Opportunity observation of EP250821a detected by EP/WXT (Hu et al., GCN 41459). SVOM/VT began observing the field at 2025-08-21T11:23:41.5 UTC, 4.087 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. An uncatalogued source is found using VT X-band data, within the error box of EP/FXT (Hu et al., GCN 41459; Liang et al., GCN 41467), Swift/XRT and UVOT (Dichiara et al., GCN 41469) and Las Cumbres (Li et al., GCN 41470) at R.A., Dec 289.88564, -43.38115 degrees: RA (J2000) = 19:19:32.55 Dec (J2000) = -43:22:52.14 with an uncertainty of 0.5 arcsec. The source is detected in both VT_R and VT_B and was fading by ~0.5 mag between the first 2 VT observing sequences, the magnitudes are: T-T0 (h) | Exposure Time (s) | Band | Mag (AB) | Mag err --------------------|-------------------|------|----------|-------- 4.087 | 70 | VT_B | 19.44 | 0.12 4.087 | 70 | VT_R | 18.49 | 0.05 6.363 | 70 | VT_B | 19.93 | 0.05 6.363 | 70 | VT_R | 19.02 | 0.03 Our photometry was not corrected for Galactic extinction. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Centre for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/41485. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39548] Fermi GRB 250821B: Global MASTER-Net observations report
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41484 SUBJECT: Fermi GRB 250821B: Global MASTER-Net observations report DATE: 25/08/22 02:45:40 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 250821B ( Fermi GBM team, GCN 41473) errorbox 26834 sec after notice time and 35938 sec after trigger time at 2025-08-22 02:06:45 UT, with upper limit up to 18.8 mag. The observations began at zenith distance = 63 deg. The sun altitude is -50.3 deg. The galactic latitude b = -39 deg., longitude l = 295 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2974796 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 35953 | 2025-08-22 02:06:45 | MASTER-OAFA | (02h 39m 51.65s , -77d 15m 11.6s) | C | 30 | 18.2 | 36003 | 2025-08-22 02:07:35 | MASTER-OAFA | (02h 39m 51.48s , -77d 14m 09.1s) | C | 30 | 18.2 | 36081 | 2025-08-22 02:08:38 | MASTER-OAFA | (02h 39m 55.46s , -77d 15m 05.3s) | C | 60 | 18.5 | 36162 | 2025-08-22 02:09:59 | MASTER-OAFA | (02h 39m 48.90s , -77d 16m 01.7s) | C | 60 | 18.4 | 36246 | 2025-08-22 02:11:23 | MASTER-OAFA | (02h 39m 55.59s , -77d 15m 36.4s) | C | 60 | 18.4 | 36370 | 2025-08-22 02:13:27 | MASTER-OAFA | (02h 39m 50.79s , -77d 14m 15.1s) | C | 60 | 18.6 | 36494 | 2025-08-22 02:15:31 | MASTER-OAFA | (02h 39m 49.56s , -77d 15m 45.5s) | C | 60 | 18.2 | 36592 | 2025-08-22 02:17:09 | MASTER-OAFA | (02h 39m 51.22s , -77d 13m 46.7s) | C | 60 | 18.7 | 36673 | 2025-08-22 02:18:30 | MASTER-OAFA | (02h 39m 44.40s , -77d 14m 42.5s) | C | 60 | 18.8 | 36749 | 2025-08-22 02:19:46 | MASTER-OAFA | (02h 39m 43.93s , -77d 13m 40.2s) | C | 60 | 18.5 | 36829 | 2025-08-22 02:21:06 | MASTER-OAFA | (02h 39m 50.13s , -77d 14m 36.9s) | C | 60 | 18.8 | 36918 | 2025-08-22 02:22:34 | MASTER-OAFA | (02h 39m 43.66s , -77d 15m 33.3s) | C | 60 | 17.7 | 37024 | 2025-08-22 02:24:21 | MASTER-OAFA | (02h 39m 46.86s , -77d 15m 02.4s) | C | 60 | 17.9 | 37132 | 2025-08-22 02:26:09 | MASTER-OAFA | (02h 39m 43.59s , -77d 13m 42.4s) | C | 60 | 17.9 | 37229 | 2025-08-22 02:27:46 | MASTER-OAFA | (02h 39m 42.71s , -77d 15m 06.5s) | C | 60 | 17.9 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41484. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39547] GRB 250821A: NOT z-band upper limit
by GCN Circulars 22 Aug '25

22 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41483 SUBJECT: GRB 250821A: NOT z-band upper limit DATE: 25/08/22 02:33:46 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> M. Beyer Stjerne (KU), K. A. Grimstrup (KU), W. K. Krarup Mirsal (KU), C. Lind-Thomsen (KU), F. Hjorth Gammelgaard (KU), I. L. Róbertsdóttir (KU), J. P. U. Fynbo (DAWN/NBI), K. Valeckas (NOT and KU), R. Holmberg Rasmussen (NOT and Aarhus), D. B. Malesani (DAWN/NBI) and Radboud), report on behalf of a larger collaboration: We observed the field of the Swift GRB 250821A (Dichiara et al., GCN 41462) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. A total of 45 min of imaging was secured using the SDSS z filter, with a mean epoch 2025 Aug 22.075 UT (15.0 hr after the Swift/BAT trigger). No new sources are detected consistent with the Swift/XRT position (Beardmore et al., GCN 41472), down to a 3-sigma limit of z > 22.8 (AB), calibrated against nearby stars from the Pan-STARRS catalog. Our result is consistent with, and extends to redder wavelengths, the non-detections reported by Strausbaugh & Cucchiara (GCN 41466) and Oates & Dichiara (GCN 41471). View this GCN Circular online at https://gcn.nasa.gov/circulars/41483. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39546] GRB 250821C: Fermi GBM Final Real-time Localization
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41482 SUBJECT: GRB 250821C: Fermi GBM Final Real-time Localization DATE: 25/08/21 23:52:36 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 23:42:00 UT on 21 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250821C (trigger 777512525.658232 / 250821988). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 104.3, Dec = -1.1 (J2000 degrees, equivalent to J2000 06h 57m, -1d 06'), with a statistical uncertainty of 15.0 degrees. The angle from the Fermi LAT boresight is 90.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250821988/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250821988/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250821988/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41482. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39545] GRB 250821A: KAIT optical upper limit
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41481 SUBJECT: GRB 250821A: KAIT optical upper limit DATE: 25/08/21 23:35:44 GMT FROM: Weikang Zheng at UC Berkeley <weikang(a)berkeley.edu> WeiKang Zheng (UCB) and Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team: The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at Lick Observatory, observed the field of GRB 250821A (Dichiara et al., GCN 41462) starting at 12:02:57, Aug. 21 UT, ~1.28 hours after the burst. A set of 9x60s clear (roughly R) filter images were obtained under twilight conditions. We do not detect any new optical counterpart candidate within the Enhanced XRT error circles (Beardmore et al., GCN 41472) in the co-add image. Our upper limit is about 20.0 (Vega) mag at a mid time of 1.30 hours after the burst. Our result is consistent with the optical upper limit reported by other groups (Strausbaugh et al., GCN 41466; Oates et al., GCN 41471; Lipunov et al., GCN 41478). View this GCN Circular online at https://gcn.nasa.gov/circulars/41481. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39544] LIGO/Virgo/KAGRA S250818k: Continued fading and reddening from Keck/LRIS imaging observations
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41480 SUBJECT: LIGO/Virgo/KAGRA S250818k: Continued fading and reddening from Keck/LRIS imaging observations DATE: 25/08/21 23:06:59 GMT FROM: Daniel Perley at Liverpool JMU <d.a.perley(a)ljmu.ac.uk> Daniel A. Perley (LJMU), Mansi M. Kasliwal (Caltech), Viraj Karambelkar (Caltech), Michael Lundquist (WMKO), Xander Hall (CMU), Tomas Ahumada (Caltech), Kritti Sharma (Caltech), and Sam Rose (Caltech) report on behalf of the larger ZTF and GROWTH collaboration: The location of AT2025ulz (ZTF25abjmnps; Stein et al., GCN 41414), a candidate counterpart to S250818k (LVK Collaboration, GCNs 41437, 41440), was imaged using the Low-Resolution Imaging Spectrometer (LRIS) on the Keck I 10m Telescope on the consecutive nights of 2025 August 20 and 21 (UT), simultaneously in g-band and i-band on each night. On the first night weather conditions were good, and two exposures of 30s each were acquired in each band starting at 2025-08-20 07:04 UT. The weather was cloudy on the second night. Three 300s exposures were taken, but due to highly variable atmospheric extinction only the highest-transmission image (starting at 2025-08-21 07:45 UT) was used for this analysis. The 5-sigma limiting magnitudes of the images are g>24.8, i>23.9 (08-20) and g>23.9, i>23.2 (08-21). While determining the magnitude of the transient is limited by the depth of available reference images, we performed difference imaging between the two nights in each band. Subtracting the 08-20 image from the 08-21 image in each filter, there is a clear detection of a negative point source at the reported location of the transient in the g-band difference image, but no significant residual flux in the i-band difference image. The difference flux at the transient position from forced photometry is -1.60 +/- 0.20 microJy (g-band) and +0.31 +/- 0.38 (i-band). This suggests the transient is still rapidly fading in g-band, and likely reddening in g-i color. This observation is consistent with the kilonova hypothesis and inconsistent with the re-brightening proposed by Liu et al. (GCN 41461). View this GCN Circular online at https://gcn.nasa.gov/circulars/41480. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39543] GRB 250821A: Swift-XRT refined Analysis
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41479 SUBJECT: GRB 250821A: Swift-XRT refined Analysis DATE: 25/08/21 22:32:18 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> M.A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), A. Melandri (INAF-OAR), J.A. Kennea (PSU) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 7.4 ks of XRT data for GRB 250821A, from 150 s to 29.3 ks after the trigger. The data are entirely in Photon Counting (PC) mode. The light curve can be modelled with a power-law decay with a decay index of alpha=1.32 (+/-0.20). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.59 (+0.42, -0.28). The best-fitting absorption column is 3 (+13, -2) x 10^20 cm^-2, consistent with the Galactic value of 1.6 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.2 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3 (+13, -2) x 10^20 cm^-2 Galactic foreground: 1.6 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.59 (+0.42, -0.28) If the light curve continues to decay with a power-law decay index of 1.32, the count rate at T+24 hours will be 1.0 x 10^-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.3 x 10^-15 (4.6 x 10^-15) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01344131. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41479. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39542] Swift GRB 250821A: Global MASTER-Net observations report
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41478 SUBJECT: Swift GRB 250821A: Global MASTER-Net observations report DATE: 25/08/21 22:28:53 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the Swift GRB 250821A ( S. Dichiara et al., GCN 41462) errorbox 41464 sec after notice time and 41495 sec after trigger time at 2025-08-21 22:17:52 UT, with upper limit up to 18.0 mag. The observations began at zenith distance = 72 deg. The sun altitude is -33.2 deg. The galactic latitude b = -78 deg., longitude l = 24 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2974552 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 41585 | MASTER-Kislovodsk | C | 180 | 17.4 | 41585 | MASTER-Kislovodsk | C | 180 | 16.6 | 41787 | MASTER-Kislovodsk | C | 180 | 18.0 | 41787 | MASTER-Kislovodsk | C | 180 | 17.2 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41478. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39541] GRB 250821B: Fermi GBM Detection
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41477 SUBJECT: GRB 250821B: Fermi GBM Detection DATE: 25/08/21 21:02:40 GMT FROM: Jacob Smith at Fermi-GBM Team <jrs0118(a)uah.edu> Jacob Smith (UAH) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team: At 16:07:47.46 UT on 21 August 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250821B (trigger 777485272/250821672). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 39.95, Dec = -77.13 (J2000 degrees, equivalent to J2000 2h 39m, -77d 7'), with a statistical uncertainty of 1.00 degrees. (radius, 1-sigma containment, statistical only; there is additionally a systematic error which we have characterized as a core-plus-tail model, with 90% of GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg systematic error [Connaughton et al. 2015, ApJS, 216, 32]). The angle from the Fermi LAT boresight is 67 degrees. The GBM light curve consists of a single emission episode with a duration (T90) of about 2.3 s (50-300 keV). The time-averaged spectrum from T0-0.06 to T0+2.62 s is best fit by a Band function with Epeak = 250 +/- 10 keV, alpha = -0.67 +/- 0.03, and beta = -2.24 +/- 0.06. The event fluence (10-1000 keV) in this time interval is (1.19 +/- 0.01)E-05 erg/cm^2. The 0.064-sec peak photon flux measured starting from T0+0.576 s in the 10-1000 keV band is 58 +/- 1.9 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/ View this GCN Circular online at https://gcn.nasa.gov/circulars/41477. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39540] LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of AT 2025ulz
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41476 SUBJECT: LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of AT 2025ulz DATE: 25/08/21 20:58:24 GMT FROM: Andrew Levan at Radboud University <a.levan(a)astro.ru.nl> Smaranika Banerjee (Stockholm University), Maria-Teresa Botticella (INAF - Capodimonte Obs.), Seán J. Brennan (MPE), Enrico Cappellaro (Padua Obs.), Ting-Wan Chen (NCU Taiwan), Paolo D'Avanzo (INAF - Brera Obs.), Max De Pasquale (Univ. Messina), Rob A. J. Eyles-Ferris (Leicester), Morgan Fraser (UCD), James H. Gillanders (Oxford), Ben Gompertz (Birmingham), Nusrin Habeeb (Leicester), Luca Izzo (INAF/OAC and DARK/NBI), Peter G. Jonker (Radboud), Andrew J. Levan (Radboud and Warwick), Daniele Bjørn Malesani (DAWN/NBI and Radboud), Antonio Martin-Carrillo (UCD), Sam Oates (Lancaster U.), Matt Nicholl (QUB), Andrea Rossi (INAF - Bologna), Om Sharan Salafia (INAF - Brera Obs.), Nikhil Sarin (Cambridge), Steve Schulze (Northwestern), Stephen J. Smartt (Oxford), Danny Steeghs (Warwick), Nial R. Tanvir (Leicester), Aishwarya L. Thakur (INAF-IAPS) report for the ENGRAVE collaboration: “We report preliminary analysis of ESO VLT observations obtained by the ENGRAVE collaboration of AT 2025ulz (Stein et al., GCN 41414), which was discovered within the 3D localization of the GW alert S250818k (LVK Collaboration, GCNs 41437, 41440). Following the announcement of the discovery of AT 2025ulz, we triggered optical imaging and spectroscopy, as well as NIR imaging using FORS2, X-shooter and HAWK-I. Using X-shooter, we obtained total exposure times of 3480, 3600, 3600 s in the UVB, VIS and NIR arms respectively, between 2025 Aug 20.99 and Aug 21.04 UT. As AT 2025ulz is faint, we acquired the target using a blind offset. We see no obvious broad features in our extracted spectrum that would be suggestive of either a KN or a SN. We do, however, see galaxy features, with prominent lines from the Balmer series (alpha, beta, gamma and delta), Paschen-alpha, [OII], [OIII], [OI], [NII], [SII] in emission, and Ca II in absorption, which securely place the redshift at z = 0.0848 (consistent with Karambelkar et al., GCN 41436). With HAWK-I we obtained 690 s of Ks-band imaging between Aug 20.96 and Aug 20.98 UT. While the host galaxy of AT 2025ulz is well detected, we see no obvious point source corresponding to the transient itself. We note that, in the optical, template subtraction is required to detect AT 2025ulz, and, as we have no deep templates, we are currently unable to do this in the NIR. We estimate a limiting magnitude of K > 23 (AB mag) for the image, and K > 22 at the location of AT 2025ulz. We obtained 3000 s of FORS2 long-slit spectroscopy using the 300I grism at the position of AT 2025ulz between Aug 21.01 and Aug 21.05 UT, but again see no obvious signal from the transient. We also examined the 3x60 s I-band FORS2 acquisition images, but see no clear point source. With the data in hand, we cannot confirm a signal that is kilonova-like. Neither of the two VLT spectra show signatures of a transient source and there is no detectable K-band emission to around -16 (AB mag) absolute magnitude. The host galaxy contamination makes faint object observations challenging without deep templates. If AT 2025ulz is associated with a KN, then preliminary modeling of the optical fluxes reported (Busman et al. GCN 41421, Hall et al. GCN 41433, O'Connor et al. GCN 41452, Hall et al. GCN 41453, Gillanders et al. GCN 41454, Mo et al. GCN 41456, Lui et al. GCN 45461) indicate that it must have a fairly large ejecta mass (of perhaps ~0.05 Msun) and the lack of K-band detection indicates it may be lanthanide-poor. If AT 2025ulz is a cooling tail following shock breakout in a core-collapse supernova (as seen for example in SN 1993J), then we should expect the lightcurve to begin to rise again approximately four days from now. Further ENGRAVE observations of AT 2025ulz will be carried out at the VLT. We acknowledge Célia Desgrange, Lorena Faundez, Camila de Sa Freitas, Elisa Garro, Cecilia Bustos, Claudia Cid, Miguel Lopez, Francesca Lucertini and the observing staff in Paranal for their excellent support.“ View this GCN Circular online at https://gcn.nasa.gov/circulars/41476. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39539] GRB 250821B: BALROG localization (Fermi Trigger / GRB 250821672)
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41475 SUBJECT: GRB 250821B: BALROG localization (Fermi Trigger / GRB 250821672) DATE: 25/08/21 19:57:25 GMT FROM: Jochen Greiner at MPE <jcgrog(a)mpe.mpg.de> T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report: The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger at 16:07:47 on 21 Aug. 2025 were manually fitted for spectrum and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427; Berlato et al. 2019, ApJ 873, 60). The best-fit position is: RA(2000.0) = 60.0 deg Decl.(2000.0) = -84.7 deg The 1 sigma statistical error radius is 2.0 deg. We estimate an additional systematic error of 2 deg. Further details are available at: https://grb.mpe.mpg.de/grb/GRB250821672/ The Healpix map can be downloaded from: https://grb.mpe.mpg.de/grb/GRB250821672/healpix The location parameters are available as JSON at: https://grb.mpe.mpg.de/grb/GRB250821672/json View this GCN Circular online at https://gcn.nasa.gov/circulars/41475. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39538] GRB 250821B: BALROG localization (Fermi Trigger / GRB 250821672)
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41475 SUBJECT: GRB 250821B: BALROG localization (Fermi Trigger / GRB 250821672) DATE: 25/08/21 19:57:25 GMT FROM: Jochen Greiner at MPE <jcgrog(a)mpe.mpg.de> T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report: The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger at 16:07:47 on 21 Aug. 2025 were manually fitted for spectrum and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427; Berlato et al. 2019, ApJ 873, 60). The best-fit position is: RA(2000.0) = 60.0 deg Decl.(2000.0) = -84.7 deg The 1 sigma statistical error radius is 2.0 deg. We estimate an additional systematic error of 2 deg. Further details are available at: https://grb.mpe.mpg.de/grb/GRB250821672/ The Healpix map can be downloaded from: https://grb.mpe.mpg.de/grb/GRB250821672/healpix The location parameters are available as JSON at: https://grb.mpe.mpg.de/grb/GRB250821672/json View this GCN Circular online at https://gcn.nasa.gov/circulars/41475. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39537] LIGO/Virgo/KAGRA S250818k : Tautenburg observations, upper limits
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41474 SUBJECT: LIGO/Virgo/KAGRA S250818k : Tautenburg observations, upper limits DATE: 25/08/21 19:06:59 GMT FROM: Sylvio Klose at TLS Tautenburg <klose(a)tls-tautenburg.de> S. Klose, B. Stecklum, A. Nicuesa Guelbenzu, and F. Ludwig (all Tautenburg) report: We observed the optical transient ZTF25abjmnps/AT2025ulz (Stein et al., GCN 41414), possibly associated with the gravitational wave event S250818k (GCN 41437), with the Tautenburg 1.34-m Schmidt telescope using the Sloan filter set. Observations were performed on 20-08-2025 between 20:06 and 20:39 UT as well as between 21:57 and 22:30 UT. They reached a 3-sigma upper limit in r and i of 22.3 and 22.4 mag (AB), respectively. Photometric calibration was performed using Pan-STARRS DR2. At the position of the optical transient (OT) discovered by Stein et al. (GCN 41414; see also Busmann et al., GCN 41421; Hall et al., GCN 41433), no point source superimposed the underlying host galaxy was detected / could be identified. Given the closeness of the OT to the host galaxy's nucleus (Liu et al., GCN 41461) we caution that the photometric limits are preliminary. View this GCN Circular online at https://gcn.nasa.gov/circulars/41474. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39536] GRB 250821B: Fermi GBM Final Localization of a Bright Burst
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41473 SUBJECT: GRB 250821B: Fermi GBM Final Localization of a Bright Burst DATE: 25/08/21 19:00:29 GMT FROM: Jacob Smith at Fermi-GBM Team <jrs0118(a)uah.edu> The Fermi GBM team reports the detection of a likely LONG GRB "At 16:07:47.46 UT on 21 August 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250821B (trigger 777485272/250821672). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 39.95, Dec = -77.13 (J2000 degrees, equivalent to J2000 2h 39m, -77d 7'), with a statistical uncertainty of 1.00 degrees. The angle from the Fermi LAT boresight is 67 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250821672/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250821672/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250821672/…" View this GCN Circular online at https://gcn.nasa.gov/circulars/41473. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39535] GRB 250821A: Enhanced Swift-XRT position
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41472 SUBJECT: GRB 250821A: Enhanced Swift-XRT position DATE: 25/08/21 18:23:49 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) report on behalf of the Swift-XRT team. Using 1629 s of XRT Photon Counting mode data and 2 UVOT images for GRB 250821A, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 358.43974, -28.65327 which is equivalent to: RA (J2000): 23h 53m 45.54s Dec (J2000): -28d 39' 11.8" with an uncertainty of 2.6 arcsec (radius, 90% confidence). This position may be improved as more data are received. The latest position can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). This circular was automatically generated, and is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41472. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39534] GRB 250821A: Swift/UVOT Upper Limits
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41471 SUBJECT: GRB 250821A: Swift/UVOT Upper Limits DATE: 25/08/21 17:55:04 GMT FROM: Samantha Oates at University of Birmingham <samantha.oates(a)alumni.ucl.ac.uk> S. R. Oates (Lancaster U.) and S. Dichiara (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250821A 147 s after the BAT trigger (Dichiara et al., GCN Circ. 41462). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 41463) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag/3sigUL white (FC) 147 297 148 >20.8 u (FC) 455 475 19 >18.3 white 147 1715 281 >21.1 v 381 1593 136 >19.1 b 480 1692 127 >19.9 u 455 1667 136 >19.6 uvw1 431 1643 58 >18.7 uvm2 406 1618 136 >19.1 uvw2 357 1569 136 >19.4 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.015 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41471. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39533] EP250821a: Rapidly fading optical source detected in Las Cumbres followup
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41470 SUBJECT: EP250821a: Rapidly fading optical source detected in Las Cumbres followup DATE: 25/08/21 16:53:21 GMT FROM: Wenxiong Li at NAOC <liwenxiong1992(a)gmail.com> Wenxiong Li (NAOC), Iair Arcavi (TAU), Ido Keinan (TAU), David Sand (U of Arizona) We observed the position of EP250821a (Hu, Liang and Jin, GCN 41459; Liang, Hu and Jin, GCN 41467) with a Las Cumbres 1m telescope at Siding Spring Observatory 1.25 hours after the Einstein Probe WXT trigger and again 6.5 hours after the trigger. We took 2x300s exposures in each visit in the broad optical w band. We find a rapidly fading source at RA=289.8853 Dec=-43.3813 (slightly offset from the position of the Swift counterpart; Dichara et al. GCN 41469) within the EP/FXT error circle and measure the following preliminary photometry calibrated to the r band: Epoch 1: MJD 60908.362 Mag 18.11 +- 0.03 Epoch 2: MJD 60908.535 Mag 19.32 +- 0.02 We find a faint red likely extended source at this position in Legacy Survey data (https://www.legacysurvey.org/viewer) Addition followup is encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/41470. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39532] EP250821a: Swift XRT and UVOT counterpart detection
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41469 SUBJECT: EP250821a: Swift XRT and UVOT counterpart detection DATE: 25/08/21 16:30:52 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> S. Dichiara (PSU), S. R. Oates (Lancaster U.), J.A. Kennea (PSU), K.L. Page (U. Leicester) report on behalf of the Swift-XRT and Swift-UVOT team: Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected source EP250821a (Hu et al., GCN 41459), collecting 1.6 ks of Photon Counting (PC) mode data between T0+4.9 ks and T0+10 ks after the trigger. A likely counterpart has been found. The details of this source are: Source 1 (SWIFT J191932.6-432250): ================================== RA (J2000.0): 289.8861 = 19 19 32.66 Dec (J2000.0): -43.3807 = -43 22 50.5 Error: 3.6 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 2.2 arcmin from the Einstein Probe/WXT position. Mean rate: 0.263 +/- 0.015 ct s^-1 Mean flux: (9.44 +/- 0.53)e-12 erg cm^-2 s^-1 Peak rate: 0.440 +/- 0.099 ct s^-1 Peak flux: (1.58 +/- 0.35)e-11 erg cm^-2 s^-1 ECF: 3.58e-11 erg cm^-2 ct^-1 assuming NH=1.38e+21 cm^-2, gamma=1.93 determined from a spectral fit. XMM UL: 5.5e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is 2.9-sigma above this 3-sigma upper limit. The source may be fading, at the 1.1-sigma level. All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated. The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/EP/EP_FIELD00059. Source 1 was also detected in the UVOT u-band image. The preliminary UVOT position is: RA (J2000.0): 289.883 = 19 19 31.86 Dec (J2000.0): -43.3816 = -43 22 53.8 Preliminary detection u-band magnitude in 1897 s exposure is 18.88 +/- 0.08, using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) We note that the source is also listed in the Guide Star Catalog (Lasker et al, 2008) and it has clearly brightened from its catalogued brightness (Bmag = 22.3). This circular is an official product of the Swift XRT and UVOT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41469. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39531] LIGO/Virgo/KAGRA S250818k: GOTO counterpart search and upper limits
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41468 SUBJECT: LIGO/Virgo/KAGRA S250818k: GOTO counterpart search and upper limits DATE: 25/08/21 16:29:36 GMT FROM: kendall.ackley(a)warwick.ac.uk K. Ackley, A. Kumar, D. O'Neill, M. J. Dyer, J. Lyman, K. Ulaczyk, S. Belkin, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, D. Pollacco, J. Casares Velazquez, T. Killestein report on behalf of the GOTO collaboration: We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022; Dyer et al. 2024) in response to the LIGO/Virgo/KAGRA low-significance event S250818k (The LIGO-Virgo-KAGRA Collaboration, GCN 41437). As part of our strategy for LVK O4 follow-up, targeted observations of the event between 2025-08-18 01:22:29 UT (2.4 minutes post-trigger) and 2025-08-21 05:29:32 UT (76.2 hours post trigger) with GOTO-North and between 2025-08-18 08:34:23 UT (7.2 hours post-trigger) and 2025-08-19 09:29:09 UT (32.2 hours post-trigger) with GOTO-South. We covered a total of 521.4 deg2 (46.0%) of the localisation probability of the Bilby skymap (The LIGO-Virgo-KAGRA Collaboration, GCN 41440). Exposures were taken in L-band (400-700 nm passband) and the average 5 sigma depth was L = 19.95 AB magnitudes with GOTO-1 (North), L = 19.74 AB with GOTO-2 (North), L = 19.05 AB with GOTO-3 (South), and L = 19.70 AB with GOTO-4 (South). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using recent survey observations of the same pointings. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks. No significant counterpart sources that can be associated with S250818k have been identified. We do not detect emission of the candidate counterpart AT 2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O’Connor et al. GCN 41452) down to a 3-sigma limiting magnitude of L = 20.3 AB at 2025-08-18 20:53:41 UTC (~19.6 hours post-trigger). Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction. GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/41468. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39530] EP250821a: Preliminary analysis of the EP-WXT and EP-FXT observations
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41467 SUBJECT: EP250821a: Preliminary analysis of the EP-WXT and EP-FXT observations DATE: 25/08/21 16:27:06 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y. F. Liang, D. F. Hu (PMO, CAS), C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team: The X-ray transient EP250821a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 41459). We report on the further results of WXT and FXT observations. The analysis of the WXT data shows that the event started at T0=2025-08-21T07:17:04 (UTC) and lasted for at least 80 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.33 x 10^20 cm^-2 and a photon index of 1.2 (-/+0.5). The derived average unabsorbed 0.5-4 keV flux is 1.3(-/+0.4) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 320 s after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 289.8862, DEC = -43.3808 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.25 x 10^21 cm^-2 and a photon index around 3.5. The derived average unabsorbed 0.5-10 keV flux is about 2.6 x 10^(-10) erg/s/cm^2 during the time interval from 0 to 1000 seconds after the start of the observation.The results from FXT may have been affected by pile-up effect, particularly in the brightest time interval. Therefore, the values reported here should be considered preliminary and are intended to provide a rough estimate of the source intensity. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41467. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39529] GRB 250821A: LCOGT Optical Upper Limits
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41466 SUBJECT: GRB 250821A: LCOGT Optical Upper Limits DATE: 25/08/21 13:14:26 GMT FROM: Robert Strausbaugh at Eastern Illinois University <rstrausbaugh(a)eiu.edu> R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on behalf of a larger collaboration: We observed the Swift GRB 250821A field (Dichiara et al., GCN 41462) with the LCOGT 1-meter Sinistro instrument at the Siding Spring Observatory, Australia site, on August 21, from 12:00 to 12:32 UT (corresponding to 1.23 to 1.76 hours after the GRB trigger time) with the SDSS r and i filters. We performed a series of 3x300s exposures in i-band and r-band. We do not detect an uncatalogued source within the Prompt Swift XRT error region (Evans et al., GCN 41463) in either band. The following 5-sigma upper limits are calculated using the PanSTARRS catalog as reference: r > 23.0 i > 22.3 These magnitudes are not corrected for galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41466. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39528] GRB 250814A: CMO SAI and SAO RAS izJ observations.
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41465 SUBJECT: GRB 250814A: CMO SAI and SAO RAS izJ observations. DATE: 25/08/21 13:09:03 GMT FROM: Alina Volnova at IKI RAS <alinusss(a)gmail.com> A. Volnova (IKI), A. Moskvitin (SAO RAS), A. Pozanenko (IKI), N. Pankov (HSE, IKI), P. Minaev (IKI), S. Kotov (SAO RAS), A.M. Tatartnikov (SAI MSU), V.K. Postnikova (SAI MSU) report on behalf of IKI-GRB-FuN: We observed the field of GRB 250814A detected by Swift/BAT and Fermi/GBM (Caputo et al. GCN Circ. 41358; Fermi team, GCN Circ. 41357) and possibly associated with the sub-threshold GW trigger S250814bg (LVK, GCN Circ. 41364) with 2.5-meter SAI-25 telescope of the Caucasian Mountain Observatory of Sternberg Astronomical Institute (SAI CMO), equipped with the IR camera ASTRONIRCAM. The observations in the J-filter were carried out on 2025-08-16 starting UT 23:20:51. We also observed this field with the BTA 6-m telescope of SAO RAS, equipped with the Scorpio-2 multi-mode focal reducer (Afanasiev & Moiseev, 2011, BaltA, 20, 363), and obtained series of images in SDSS i,z bands on 2025-08-17 starting 20:47:31 UT. The two NIR sources reported previously (Karambelkar et al. GCN Circ. 41377; Castro-Tirado et al., GCN Circ. 41381; Ahumada et al. GCN Circ. 41382; Pankov et al. GRB Circ. 41389; D'Avanzo et al. GCB Circ. 41391; Kumar et al. GCN Circ. 41392; D'Avanzo et al. GCN Circ. 41408) are clearly detected. The preliminary photometry of the sources Source 1 and Source 2 according to D'Avanzo et al. (GCB Circ. 41391) is: date |UT start|t-T0,d |Filter|Exp,s|Source 1 |Source 2 |UL3sigma|FWHM"|Telescope/Site ========================================================================================= 25-08-16|23:20:51|2.68958|J |4224 |21.72(0.07)|21.96(0.08)|22.9 |1.2 |SAI-25/CMO 25-08-17|20:47:31|3.56255|z |1200 |22.39(0.04)|22.46(0.04)|24.2 |1.4 |BTA/SAO 25-08-17|21:25:21|3.58647|i |1200 |23.39(0.05)|23.31(0.05)|24.6 |1.4 |BTA/SAO All magnitudes are in AB system and not corrected for the Galactic extinction. The photometry is calibrated using Pan-STARRS i,z magnitudes and 2MASS J magnitudes for the following stars: RA Dec i_PS1 z_PS1 J_2MASS* 20:25:12.01 +48:24:08.4 16.924(0.004) 16.703(0.008) 15.750 20:25:17.82 +48:24:04.6 17.234(0.003) 16.952(0.005) 15.627 * - J magnitudes of the reference stars are in Vega system. (We are grateful to the management of the observatories for observational time and the staff for excellent observations.) View this GCN Circular online at https://gcn.nasa.gov/circulars/41465. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39527] LIGO/Virgo/KAGRA S250818k: 10 GHz VLA observations of AT2025ulz
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41464 SUBJECT: LIGO/Virgo/KAGRA S250818k: 10 GHz VLA observations of AT2025ulz DATE: 25/08/21 12:54:35 GMT FROM: Roberto Ricci at INAF-IRA <ricci(a)ira.inaf.it> Authors: R. Ricci, M. Yadav, E. Troja (U Rome) report: We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O’Connor et al. GCN 41452) possibly associated to the GW candidate S250818k (LVKC GCN 41437) with the Very Large Array in X-band at the centre frequency of 10 GHz with a bandwidth of 4 GHz on Aug 21st 2025, 3.1 days after the GW trigger. After reducing the data using the latest version of the CASA VLA pipeline (v6.6.1) and standard imaging procedures, we found no detection at the optical transient position down to a 3-sigma flux density upper limit of 30 microJy/beam. We thank the VLA staff for promptly executing the observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41464. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39526] GRB 250821A: Prompt enhanced Swift-XRT position
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41463 SUBJECT: GRB 250821A: Prompt enhanced Swift-XRT position DATE: 25/08/21 11:57:10 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Using 1.6 ks of promptly downlinked XRT event data for GRB 250821A, we find an enhanced XRT position of the afterglow: RA, Dec: 358.44056, -28.65369 which is equivalent to: RA (J2000) = 23 53 45.73 Dec (J2000) = -28 39 13.3 with an uncertainty of 2.9 arcseconds (radius, 90% confidence). Analysis of the promptly available data is online at http://www.swift.ac.uk/sper/1344131. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41463. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39525] GRB 250821A: Swift detection of a burst
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41462 SUBJECT: GRB 250821A: Swift detection of a burst DATE: 25/08/21 11:00:36 GMT FROM: K.L. Page at U Leicester <klp5(a)leicester.ac.uk> S. Dichiara (PSU), R. A. J. Eyles-Ferris (U Leicester), R. Gupta (NASA GSFC), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 10:46:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250821A (trigger=1344131). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 358.412, -28.618 which is RA(J2000) = 23h 53m 39s Dec(J2000) = -28d 37' 05" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 20 sec. The peak count rate was ~2476 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 10:48:40.1 UT, 142.8 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 358.43969, -28.65380 which is equivalent to: RA(J2000) = 23h 53m 45.53s Dec(J2000) = -28d 39' 13.7" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 155 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.65 x 10^20 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 147 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers none of the XRT error circle. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.015. Burst Advocate for this burst is S. Dichiara (sbd5667 AT psu.edu) Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/) View this GCN Circular online at https://gcn.nasa.gov/circulars/41462. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39524] LIGO/Virgo/KAGRA S250818k: WFST pre-discovery limits and follow-up observations of AT 2025ulz
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41461 SUBJECT: LIGO/Virgo/KAGRA S250818k: WFST pre-discovery limits and follow-up observations of AT 2025ulz DATE: 25/08/21 09:00:21 GMT FROM: Zhengyan Liu at USTC <ustclzy(a)mail.ustc.edu.cn> Z. Y. Liu, Z. L. Xu, D. Z. Meng, J.-A. Jiang, W. Zhao, M. X. Cai, Z. G. Dai, L. L. Fan, N. Jiang, X. Kong, T. G. Wang (USTC), Y. Z. Fan, J. J. Geng, Z. P. Jin, X. F. Wu (PMO) report on behalf of the WFST Collaboration: We report the pre-discovery upper limits and follow-up photometry results of AT2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414), possibly associated with the sub-threshold gravitational wave (GW) event S250818k (GCN 41437), using the Wide Field Survey Telescope (WFST Collaboration; Wang et al., 2023). After checking WFST archive data, we found u- and g-band 30s exposures taken at MJD 60901.66, about 3.4 days prior to the GW event trigger. The images were processed with the WFST pipeline, using stacked WFST multi-exposures taken in February 2025 as templates. We do not found any residuals in difference images in the two bands. We report the following pre-discovery 3-sigma upper limits and time since the S20250818k trigger (t-t0): MJD t-t0 (days) AB mag Band 60901.64 -3.42 > 22.1 u 60901.67 -3.38 > 22.4 g We conducted WFST follow-up observations of AT2025ulz with 120s exposures in g and r bands and a 180s exposure in u band beginning at 2025-08-20T14:24:03 UTC. After image subtraction, a clear source was detected in g band, and faint sources detected in u and r bands, which are close to the detection limit. The preliminary AB magnitudes of AT2025ulz are (all photometries are not corrected for Galactic extinction): MJD t-t0 (days) AB mag Band 60907.60 2.54 22.71+/-0.10 g 60907.61 2.55 22.78+/-0.16 r 60907.65 2.59 23.24+/-0.24 u The g-r and u-g color from WFST observations are approximately -0.1 mag and 0.5 mag, respectively, which became bluer compared with the previous reports (Hall et al., GCN 41433; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454). Additionally, our results show a re-brightening trend for AT2025ulz, which may be related to the shock-cooling process mentioned in Gillanders et al. (GCN 41454). We also note that AT2025ulz is significantly more luminous than AT 2017gfo at the same phase, especially in g and u bands assuming a host redshift 0.0848 reported by Karambelkar et al. (GCN 41436). The angular separations between the detected source in g band and the host center are approximately 0.85 and 0.83 arcseconds using host coordinates in PS1 DR2 (Chambers et al., 2016) and SDSS DR16 (Ahumada et al., 2020), respectively. Given the close distance between the source and the bright host nucleus, and the peculiar photometric evolution in the past 24 hours, we suggest that the variability might be influenced by the host-nuclear activity. Further deep multiband imaging follow-ups are highly encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/41461. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39523] LIGO/Virgo/KAGRA S250818k: EP/FXT observation of AT 2025ulz
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41460 SUBJECT: LIGO/Virgo/KAGRA S250818k: EP/FXT observation of AT 2025ulz DATE: 25/08/21 08:38:43 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> R.-Z. Li (YNAO, CAS), X.-P. Xu, H. Sun, D.-Y. Li, Y. Liu, W. Yuan (NAO ,CAS) and B. Zhang (HKU) on behalf of the Einstein Probe (EP) team: We performed a ToO observation with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) for AT2025ulz, reported by Stein et al. (GCN 41414) as a possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440). The observation was carried out from 2025-08-20 08:31:13 (UTC) to 2025-08-20 13:19:05 (UTC), with a total exposure time of about 9 ks. The midpoint time of this observation is 2025-08-20T10:55:09 (UTC), approximately 57.6 hours after the trigger (LVK Collaboration, GCN 41437). The source was not detected with EP/FXT with an upper limit of 1.1e-3 cts/s at 0.5-10 keV. The estimated flux upper limit is 1.68e-14 erg/cm^2/s (90% C.L., at 0.5-10 keV). Given the redshift of z = 0.0848 (GCN 41436), this gives a rough luminosity upper limit of 3.2e41 erg/s. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41460. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39522] EP250821a: Einstein Probe detection of an X-ray transient
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41459 SUBJECT: EP250821a: Einstein Probe detection of an X-ray transient DATE: 25/08/21 08:25:08 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> D. F. Hu, Y. F. Liang (PMO, CAS), C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250821a. The transient triggered EP-WXT (ID: 01709200464) at 2025-08-21T07:18:29 (UTC). The WXT position of the source is R.A. = 289.856 deg, DEC = -43.410 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 289.8828 deg, DEC = -43.3816 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the telemetry data is received. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41459. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39521] GRB 250819A: SVOM/GRM observation of a possible “long” duration type I burst
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41458 SUBJECT: GRB 250819A: SVOM/GRM observation of a possible “long” duration type I burst DATE: 25/08/21 06:02:19 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> SVOM/GRM team: Chen-Wei Wang, Wen-Jun Tan, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Tais Maiolino (LUPM), Stéphane Schanne (CEA) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by a burst GRB 250819A (SVOM trigger reference: sb25081903) at 2025-08-19T17:49:46.000 UTC (T0), which is also detected by Insight-HXMT/HE (Wang et al., GCN#41457). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multi-pulses with a T90 of 7.4 +3.2/-2.0 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb250819A.png The SVOM/GRM on-ground localization of this burst is (J2000): RA: 354.6 deg DEC: -18.8 deg Error: 12.6 deg (1sigma, statistical only) We caution that the calibration of SVOM/GRM is undergoing and this localization is subject to systematic errors. In addition, the position of this burst, as determined by GRM, is located at about 32 degrees from the SVOM optical axis, which is inside the ECLAIRs field of view and consistent with a sub-threshold detection of ECLAIRs. The preliminary ECLAIRs localization is RA: 338.3 deg, DEC: -19.8 deg, with an error estimated as 30 arcmin, close to the edge of ECLAIRs field of view. With the localization from ECLAIRs, the time-averaged spectrum from T0-1.5 to T0+8.0 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.98 +0.13/-0.10 and the cutoff energy, parameterized as Epeak, is 1740 +1250/-750 keV. The event fluence (10-1000 keV) in this time interval is (4.11 +0.35/-0.34)E-06 erg/cm^2. Thus GRB 250819A is well consistent with Type I GRBs in the 'Amati' relation diagram, as shown at: https://www.bursthub.cn//admin/static/grb250819A_amati.png Based on the location of this GRB in 'Amati' relation diagram, a possible merger origin is suggested. A SVOM ToO has been programmed. Follow-up observations are strongly encouraged. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/41458. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39520] GRB 250819A: Insight-HXMT detection
by GCN Circulars 21 Aug '25

21 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41457 SUBJECT: GRB 250819A: Insight-HXMT detection DATE: 25/08/21 05:50:42 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> Chen-Wei Wang, Wen-Jun Tan, Yue Huang, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team: At 2025-08-19T17:49:46.000 (T0), Insight-HXMT/HE detected an intermedium duration burst GRB 250819A. The Insight-HXMT/HE light curve mainly consists of multi-pulses with a T90 of 7.5 +3.2/-2.9 s. The 1s peak rate, measured from T0-0.25 s, is 1153 cnts/sec. The total counts from this burst is 4769 counts. The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb250819A.png All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope. Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.or View this GCN Circular online at https://gcn.nasa.gov/circulars/41457. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39519] LIGO/Virgo/KAGRA S250818k: WINTER J-band observations of AT 2025ulz
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41456 SUBJECT: LIGO/Virgo/KAGRA S250818k: WINTER J-band observations of AT 2025ulz DATE: 25/08/20 18:41:15 GMT FROM: Geoffrey Mo at MIT <gmo(a)mit.edu> Geoffrey Mo (MIT), Robert Stein (JSI), Mansi Kasliwal (Caltech), Viraj Karambelkar (Caltech), Danielle Frostig (CfA), Nathan Lourie (MIT), and Robert Simcoe (MIT) report: We observed AT2025ulz (Stein et al., GCN 41414), a possible counterpart to the LVK sub-threshold event S250818k (LVK, GCN 41437, 41440), with the 1.2 sq. degree near-IR WINTER camera on the Palomar 1-m telescope (Lourie et al. 2021, Frostig et al. 2024). Our observations were conducted in J-band beginning at 2025-08-19 03:09:09 UTC, approximately 26 hours after the merger. Our observations consisted of 30 individual 120s dithers. The images were processed using the WINTER data reduction pipeline (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565) using images from the UKIRT Hemisphere survey (Dye et al. 2018) as references for image subtraction. We do not detect any source at the position of AT2025ulz in our difference image. We place a J-band upper limit of m > 19.3 AB mag. WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research. View this GCN Circular online at https://gcn.nasa.gov/circulars/41456. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39518] GRB 250818B: 10 GHz VLA detection
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41455 SUBJECT: GRB 250818B: 10 GHz VLA detection DATE: 25/08/20 18:16:07 GMT FROM: Roberto Ricci at INAF-IRA <ricci(a)ira.inaf.it> R. Ricci, M. Yadav, E. Troja (U Rome) on behalf of a larger collaboration: We observed the short-duration GRB 250818B discovered by SVOM (GCN 41405) with Very Large Array in X-band at the centre frequency of 10 GHz with a bandwidth of 4 GHz on Aug 20th 2025, 2.3 days after the burst. After reducing the data in latest CASA version (v6.6.1) with standard procedures a detection was found at a position compatible with GOTO (GCN 41406) and NOT (GCN 41426) optical transient. We estimate a flux density of approximately 120 microJy. We thank the VLA staff for promptly executing the observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41455. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39517] LIGO/Virgo/KAGRA S250818k: Pan-STARRS grizy-band imaging and photometry of AT2025ulz
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41454 SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS grizy-band imaging and photometry of AT2025ulz DATE: 25/08/20 18:11:46 GMT FROM: James Gillanders at University of Oxford <jhgillanders.astro(a)gmail.com> J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard). We observed the optical transient AT2025ulz (Stein et al., GCN 41414), the transient plausibly associated with the (sub-threshold) gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560). Our observation consisted of 6x150s exposures in each of the grizy filters in both telescopes. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). From these difference images, we measure the following preliminary AB magnitudes, from averages of all forced photometry from the two GPC1 and GPC2 cameras: MJD t-t0 (days) Filter AB mag 60907.30 2.25 g 22.9 +/- 0.4 60907.31 2.26 r 22.2 +/- 0.2 60907.33 2.27 i 21.8 +/- 0.2 60907.34 2.28 z 21.4 +/- 0.2 Here, t0 corresponds to the GW trigger time (LVK Collaboration, GCN 41437). Our difference imaging and photometric measurements indicate a consistent flux excess at the position of AT2025ulz (Stein et al., GCN 41414), although we note that these current magnitude estimates are derived from difference images generated with reference images shallower than the target images (900s vs. ~300s, respectively). We caution these are preliminary, given the differencing issues and the closeness of the source to the bright nucleus and body of the host. Comparing our current Pan-STARRS gr-band data to that previously reported by Busmann et al. (GCN 41421) and Hall et al. (GCN 41433), we find that AT2025ulz appears to have faded further, if both sets of photometry are accurate. We can certainly conclude that no re-brightening (which may be expected in SNe IIb or shock cooling) is yet taking place. Work is ongoing to generate a deeper reference image, and thus improved magnitude estimates. Further multi-band observations are scheduled. View this GCN Circular online at https://gcn.nasa.gov/circulars/41454. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39516] LIGO/Virgo/KAGRA S250818k: Swift observations of AT 2025ulz
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41453 SUBJECT: LIGO/Virgo/KAGRA S250818k: Swift observations of AT 2025ulz DATE: 25/08/20 17:43:21 GMT FROM: xjh(a)andrew.cmu.edu Xander J. Hall (Carnegie Mellon U.), Robert Stein (UMD), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report: We requested ToO observations of AT 2025ulz, reported by Stein et al. (GCN 41414) as a possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440). The observation was taken with a midpoint time of 2025-08-19T11:48:55.755, approximately 30 hours after the trigger, with a duration of 3.5ks. The source was not detected with XRT with an upper limit of 3.8 * 10^-3 cts/s at 0.3 - 10 keV. Given the redshift of z = 0.0848 (GCN 41436), this gives a rough luminosity upper limit of 3.3 * 10^42 erg/s. We clearly detect a source at the position of AT2025ulz in UVOT observations, conducted in the UVM2 filter, with a magnitude of m=22 AB. However, this flux is consistent with archival detections of the host galaxy in GALEX. We therefore find no evidence of transient UV emission in UVOT data. We thank the Swift team, Observatory Duty Scientists and the PI for promptly scheduling and making these observations possible. View this GCN Circular online at https://gcn.nasa.gov/circulars/41453. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39515] LIGO/Virgo/KAGRA S250818k: Multi-band Gemini GMOS Detections
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41452 SUBJECT: LIGO/Virgo/KAGRA S250818k: Multi-band Gemini GMOS Detections DATE: 25/08/20 17:28:14 GMT FROM: Brendan O'Connor at Carnegie Mellon University <boconno2(a)andrew.cmu.edu> B. O’Connor (CMU), J. Freeburn (UNC), X. J. Hall (CMU), Malte Busmann (LMU), I. Andreoni (UNC), A. Palmese (CMU), D. Gruen (LMU), L. Hu (CMU), T. Cabrera (CMU), K. Kunnumkai (CMU), A. Amsellem (CMU) report on behalf of a larger collaboration: Using Gemini GMOS-N, we observed the source AT 2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414) discovered during ZTF follow-up of the low-significance gravitational wave event S250818k (GCN 41437, 41440; https://gracedb.ligo.org/superevents/S250818k) which is a candidate binary neutron star merger. Following the discovery of rapid fading and reddening of the source with the Fraunhofer Telescope at Wendelstein Observatory (FTW; see Busmann et al. GCN 41421, Hall et al. GCN 41433), we carried out observations in the grizY filters with GMOS-N under photometric observing conditions starting at 2025-08-20 05:29:56 UT. This corresponds to 2.17 days after the gravitational wave detection (GCN 41437). A visual inspection reveals that the transient is significantly detected on top of the host galaxy. The source is clearly off-nuclear, but lies on top of the galaxy’s disk. We performed difference imaging with respect to Legacy Survey templates that further reveal the source. The brightness of the transient is g~23.0 AB mag and r~22.6 AB mag. The photometry is not correct for Galactic extinction. Our data reveals continued rapid fading and reddening of the transient in the 9.4 hours following the FTW report (Hall et al. GCN 41433). This would imply a steepening of the fade rate in g-band. We caution that the galaxy appears over subtracted in our difference imaging, which may affect the preliminary photometry. Additional analysis is underway. Further observations are planned and additional follow-up is encouraged. We thank the staff of the Gemini Observatory, in particular Jen Andrews, Brian Lemaux, and Jen Miller, for their excellent support in rapidly scheduling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41452. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39514] LIGO/Virgo/KAGRA S250818k: ATLAS pre-discovery limits for AT2025ulz
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41451 SUBJECT: LIGO/Virgo/KAGRA S250818k: ATLAS pre-discovery limits for AT2025ulz DATE: 25/08/20 15:30:45 GMT FROM: S. Srivastav at Oxford <shubhamsrivastav(a)gmail.com> S. Srivastav (Oxford), K. W. Smith, S. J. Smartt (Oxford/QUB), D. R. Young, M. Nicholl, M. D. Fulton, T. Moore, A. Aamer, C. R. Angus, M. McCollum, S. Sim, J. Weston, X. Sheng (QUB), L. Shingles (GSI/QUB), J. Sommer (LMU), J. Gillanders, H. Stevance, L. Rhodes, A. Andersson (Oxford), L. Denneau, J. Tonry, H. Weiland, A. Lawrence, R. Siverd (IfA, University of Hawaii), N. Erasmus, W. Koorts (South African Astronomical Observatory), J. Anderson (ESO), A. Jordan, V. Suc (UAI, Obstech) A. Rest (STScI), T.-W. Chen (NCU), C. Stubbs (Harvard): We report pre-discovery upper limits from ATLAS observations of AT2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414), possibly associated with the sub-threshold gravitational wave event S250818k (GCN 41437). ATLAS observed the field most recently before the GW event on MJD 60903.32, 1.74 days prior to the trigger. The observations were made as part of regular survey operations in the o-band. During the August 2025 lunation, we have been experimenting with a different cadence and longer exposures. The data on MJD 60903.32 were taken as 4 x 110 sec exposures (compared to 30 sec exposures in normal operations). These longer exposures provide a 3-sigma upper limit of o > 21.6 mag (AB) after stacking the 4 individual measurements. The discovery r-band mag of r = 21.29 +/- 0.13 (Stein et al., GCN 41414) implies a significant 3-sigma rise in 1.74 days, or a new transient appearing on that timescale. The Asteroid Terrestrial-impact Last Alert System [ATLAS] project is primarily funded to search for Near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, STFC grants ST/Y001605/1, ST/X001253/1, the Royal Society and Schmidt Sciences. ATLAS-Teide is an IAC instrument included in the present “Strategic plan of the Canarian Observatories”, funded by the European Union NextGenerationEU EQC2021-007122-P and ICT2022-007828 projects. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, Queen's University Belfast, University of Oxford, the Space Telescope Science Institute, the South African Astronomical Observatory, The Millennium Institute of Astrophysics (MAS), Chile, and the Instituto de Astrofisica De Canarias. View this GCN Circular online at https://gcn.nasa.gov/circulars/41451. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39513] GRB 250814A: Swift/UVOT Upper Limits
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41450 SUBJECT: GRB 250814A: Swift/UVOT Upper Limits DATE: 25/08/20 15:20:29 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18(a)psu.edu> M. H. Siegel (PSU) and R. Caputo (GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250814A 179 s after the BAT trigger (Caputo et al., GCN Circ. 41358). No optical afterglow consistent with the XRT position (Salvaggio et al., GCN Circ. 41379) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u_FC 179 429 246 >20.3 white 460 1537 264 >21.8 v 510 1412 97 >20.2 b 436 1511 117 >20.2 u 179 1486 343 >20.5 w1 560 1462 97 >19.5 m2 708 1263 58 >18.6 w2 486 1556 130 >20.0 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.066 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41450. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39512] GRB 250813B: Swift/UVOT Upper Limit
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41449 SUBJECT: GRB 250813B: Swift/UVOT Upper Limit DATE: 25/08/20 15:17:24 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18(a)psu.edu> M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250813B 9.6 ks after the SVOM/ECLAIRs trigger (Xie et al., GCN Circ. 41352). We do not detect any afterglow consistent with the XRT position (Page et al., GCN Circ. 41359) or optical position (Jelinek et al., GCN Circ. 41355). Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u 9599 16347 1576 >21.07 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.072 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41449. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39511] IPN triangulation of GRB 250816A (short)
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41448 SUBJECT: IPN triangulation of GRB 250816A (short) DATE: 25/08/20 14:52:01 GMT FROM: Dmitry Svinkin at Ioffe Institute <svinkin(a)mail.ioffe.ru> D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team, A. Goldstein, M. S. Briggs, C. Wilson-Hodge, and E. Burns on behalf of the Fermi GBM team, report: The short-duration GRB 250816A (Fermi-GBM detection: The Fermi GBM team, GCN 41397; SVOM-GRM detection: Wang et al., GCN 41400) was detected by Fermi (GBM trigger 777000615), SVOM (GRM), and Konus-Wind at about 5410 s UT (01:30:10). The burst was outside the coded field of view of the BAT. We have triangulated it to a preliminary, 3 sigma error box whose area is 7.8 sq. deg, and its maximum dimension is 32.2 deg (the minimum one is 18.7 arcmin). The Sun distance was ~46 deg. This localization may be improved. A triangulation map and HEALPix FITS file are posted at http://www.ioffe.ru/LEA/GRBs/GRB250816_T05407/IPN The HEALPix triangulation map is the multi-order HEALPix in units of probability density. The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular. View this GCN Circular online at https://gcn.nasa.gov/circulars/41448. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39510] IPN triangulation of GRB 250814C (short)
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41447 SUBJECT: IPN triangulation of GRB 250814C (short) DATE: 25/08/20 14:48:51 GMT FROM: Dmitry Svinkin at Ioffe Institute <svinkin(a)mail.ioffe.ru> A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin on behalf of the HEND/Mars Odyssey team, D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team, E. Burns on behalf of the IPN, Y. Zhang, C. Wang, S. Xiong, J. Wei, and B. Cordier on behalf of the SVOM-GRM team, S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, A. Tohuvavohu, and J. DeLaunay on behalf of the Swift-BAT team, and W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr, and A.S. Gardner on on behalf of the GRS-Odyssey GRB team, report: The short-duration GRB 250814C (SVOM-GRM detection: Wang et al., GCN 41385) was detected by Konus-Wind, SVOM (GRM), Swift (BAT), and Mars-Odyssey (HEND) at about 12263 s UT (03:24:23). The burst was outside the coded field of view of the BAT. We have triangulated it to a preliminary, 3 sigma error box whose coordinates are: --------------------------------------------- RA(2000), deg Dec(2000), deg --------------------------------------------- Center: 164.655 (10h 58m 37s) +47.526 (+47d 31' 35") Corners: 164.814 (10h 59m 15s) +47.549 (+47d 32' 56") 164.789 (10h 59m 09s) +47.597 (+47d 35' 48") 164.497 (10h 57m 59s) +47.504 (+47d 30' 13") 164.521 (10h 58m 05s) +47.456 (+47d 27' 21") --------------------------------------------- The error box area is 39 sq. arcmin, and its maximum dimension is 14 arcmin (the minimum one is 3 arcmin). The Sun distance was 37 deg. A triangulation map and HEALPix FITS file are posted at http://www.ioffe.ru/LEA/GRBs/GRB250814_T12259/IPN/ The HEALPix triangulation map is the multi-order HEALPix in units of probability density. The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular. View this GCN Circular online at https://gcn.nasa.gov/circulars/41447. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39509] IPN triangulation of GRB 250804B (short)
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41446 SUBJECT: IPN triangulation of GRB 250804B (short) DATE: 25/08/20 14:44:09 GMT FROM: Dmitry Svinkin at Ioffe Institute <svinkin(a)mail.ioffe.ru> A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin on behalf of the HEND/Mars Odyssey team, D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team, E. Burns on behalf of the IPN, S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, A. Tohuvavohu, and J. DeLaunay on behalf of the Swift-BAT team, and W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr, and A.S. Gardner on on behalf of the GRS-Odyssey GRB team, report: The short-duration GRB 250804B (SVOM-GRM detection: Wang et al., GCN 41229; Insight-HXMT detection: Wang et al., GCN 41284) was detected by Konus-Wind, Swift (BAT), SVOM (GRM), Insight (HXMT), and Mars-Odyssey (HEND) at about 17092 s UT (04:44:52). The burst was outside the coded field of view of the BAT. We have triangulated it to a preliminary, 3 sigma error box whose coordinates are: --------------------------------------------- RA(2000), deg Dec(2000), deg --------------------------------------------- Center: 40.607 (02h 42m 26s) -46.768 (-46d 46' 04") Corners: 40.397 (02h 41m 35s) -46.987 (-46d 59' 12") 40.852 (02h 43m 24s) -46.590 (-46d 35' 25") 40.815 (02h 43m 16s) -46.548 (-46d 32' 52") 40.360 (02h 41m 26s) -46.944 (-46d 56' 39") --------------------------------------------- The error box area is 84 sq. arcmin, and its maximum dimension is 31 arcmin (the minimum one is 3 arcmin). The Sun distance was 105 deg. This localization may be improved. A Swift ToO has been requested for this burst (GCN 41231, 41239). Only source #8 detected by Swift-XRT (https://www.swift.ac.uk/xrt_products/TILED_GRB00138/) is inside the IPN box. A triangulation map and HEALPix FITS file are posted at http://www.ioffe.ru/LEA/GRBs/GRB250804_T17091/IPN The HEALPix triangulation map is the multi-order HEALPix in units of probability density. The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular. View this GCN Circular online at https://gcn.nasa.gov/circulars/41446. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39508] GRB 250818B: FTW optical and NIR observations show reddening of the counterpart
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41445 SUBJECT: GRB 250818B: FTW optical and NIR observations show reddening of the counterpart DATE: 25/08/20 14:06:15 GMT FROM: Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann(a)physik.lmu.de> Malte Busmann (LMU), Jennifer Fabà (LMU), Vani Jain (LMU), Xiaoxiong Zhou (LMU), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report: We observed the counterpart of the short GRB 250818B (Wang et al., GCN 41405; Kumar et al., GCN 41406; Ferro et al., GCN 41407; Yao et al., GCN 41409; Yheng et al., GCN 41417; Fong et al., GCN 41419; Li et al., GCN 41424; Bendtsen et al., GCN 41426; Moskvitin et al., GCN 41428; An et al., GCN 41430; Kumar et al., GCN 41431; Siegel et al., GCN 41435; Dimple et al., GCN 41442) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J-band simultaneously for 35x 180 s starting at 2025-08-19T01:13:45 UT (0.91 days after the trigger) and again for 30x 180 s starting at 2025-08-20T01:27:02 (1.9 days after the trigger). In the two epochs, we detect the counterpart at r = (21.12 +/- 0.02) mag (1st epoch), r = (22.26 +/- 0.08) mag (2nd epoch). We note a significant reddening between the two epochs, where the r-J color increases by nearly 0.6 mag. The cause of the color evolution is unclear, but it is potentially consistent with either a red host galaxy or an intrinsic color evolution of the counterpart. Further observations are encouraged. The r-band magnitudes are calibrated against the PS1 catalog, and the J-band is calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction. We thank Michael Schmidt from the Wendelstein Observatory for obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41445. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39507] GRB 250818B: Kilonova-Catcher optical afterglow detection
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41444 SUBJECT: GRB 250818B: Kilonova-Catcher optical afterglow detection DATE: 25/08/20 13:19:56 GMT FROM: Damien Turpin at CEA-Saclay <dturpin-astro(a)hotmail.com> R. Hellot, M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), M. Pillas (ULiege), D. Akl (NYUAD), M. Molham (NRIAG), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration: We observed the field of GRB 250818B detected by SVOM/ECLAIRs and SVOM/GRM (Wang et al., GCN 41405) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a TEC160FL telescope operated by M. Freeberg and the CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot. Our observations started at T0+5hr. In our stacked frames, we detect an uncatalogued optical source consistent with the afterglow candidate reported by several teams: GOTO (Kumar et al., GCN 41406), Swift/XRT (Ferro et al., GCN 41407), SVOM/VT (Yao et al., GCN 41409), KAIT (Zheng et al., GCN 41417), Keck (Fong et al., GCN 41419), EP/FXT (Li et al.; GCN 41424), NOT (Bentdsen et al., GCN 41426), SAO/RAS (Moskvitin et al., GCN 41428), TRT (An et al., GCN 414430), the 1m Ledesi telescope (Kumar et al., GCN 41431), Swift/UVOT (Siegel et al., GCN 41435) and the Liverpool telescope (Dimple et al., GCN 41442). We report some of our follow-up results in the table below: +---------------+-----------+------------+----------------+--------------+ | Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument | +===============+===========+============+================+==============+ | 4.97 | 15 x 300s | sdssr (AB) | 19.47 +/- 0.07 | CDK17-AITP | | 7.53 | 11 x 300s | sdssr (AB) | 20.17 +/- 0.16 | TEC160FL | +---------------+-----------+------------+----------------+--------------+ All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog. We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/) View this GCN Circular online at https://gcn.nasa.gov/circulars/41444. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39506] LIGO/Virgo/KAGRA S250818k: AstroSat CZTI non-detection and upper limits
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41443 SUBJECT: LIGO/Virgo/KAGRA S250818k: AstroSat CZTI non-detection and upper limits DATE: 25/08/20 13:06:27 GMT FROM: Gaurav Waratkar at IIT Bombay <gauravwaratkar(a)iitb.ac.in> G. Waratkar (IITB), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: We have carried a search [1] for X-ray candidates in Astrosat CZTI data in a 100 sec window around the trigger time of the event S250818k (UTC 2025-08-18 01:20:06; LVK Collaboration, GCN Circ. 41437). We use the Bilby.offline0.multiorder.fits,0 map (https://gracedb.ligo.org/api/superevents/S250818k/files/Bilby.offline0.mult…) for our analysis. CZTI is a coded aperture mask instrument that has considerable effective area for about 29% of the entire sky, but is also sensitive to brighter transients from the entire sky. At the time of the merger, Astrosat's nominal pointing is RA,DEC = 00:46:53.8, 42:11:36.4 (11.7242,42.1934), which is ~88 deg away from the maximum probability location, which severely reduces the effective area of CZTI. At the time of the merger event, the Earth-satellite-transient angle corresponding to maximum probability location is ~33 deg and hence is occulted by Earth in satellite's frame. In a time interval of 100 sec around the event, the region of the localisation map which is not occulted by Earth in the satellite's frame has a total probability of 0.59 (59%). We note that the source AT 2025ulz (Stein et al., GCN Circ. 41414) was occulted behind Earth for AstroSat at the time of the merger event. CZTI data were de-trended to remove orbit-wise background variation. We then searched data from three of the four independent, identical quadrants to look for coincident spikes in the count rates. Quadrant C being noisy was excluded from further analysis. Searches were undertaken by binning the data in 0.1s, 1s, and 10s respectively. Statistical fluctuations in background count rates were estimated by using data from 5 preceding orbits. We selected confidence levels such that the probability of a false trigger in a 100 sec window is 10^-3, after excluding one noisy quadrant. We do not find any evidence for any hard X-ray transient in this window, in the CZTI energy range of 20-200 keV. We use a detailed mass model of the satellite to calculate the direction-dependent instrument response for points in the visible sky. We then assume the source is modelled as a power law with photon index alpha = -1, and convert our count rate upper limits to direction-dependent flux limits. We obtain the following upper limits for source flux in the 20-200 keV band by taking a probability weighted mean over the visible sky: | Bin (s) | Flux Limit (ergs/cm^2/s) | Fluence Limit (ergs/cm^2) | |---------|--------------------------|---------------------------| | 0.1 | 1.47e-06 | 1.47e-07 | | 1.0 | 3.78e-07 | 3.78e-07 | | 10.0 | 8.00e-08 | 8.00e-07 | CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. CZTI EMGW detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=emgw [1] Waratkar et al. ApJ 976, 123 (2024) doi:10.3847/1538-4357/ad84e6 View this GCN Circular online at https://gcn.nasa.gov/circulars/41443. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39505] GRB 250818B: Liverpool Telescope optical detections
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41442 SUBJECT: GRB 250818B: Liverpool Telescope optical detections DATE: 25/08/20 11:30:44 GMT FROM: Dimple at University of Birmingham <dimplepanchal96(a)gmail.com> Dimple (U. Birmingham), B. P. Gompertz (U. Birmingham), and A. Kumar (RHUL) report on behalf of a larger collaboration: We conducted follow-up observations of GRB 250818B (Wang et al., GCN Circ. 41405) with the IO:O camera on the 2m robotic Liverpool Telescope (LT). Observations began at 03:46:39 UT on 2025-08-20, ~2 days after the SVOM/ECLAIRs trigger, and consisted of 5 x 240 s exposures in each of the SDSS g, r, i, and z filters. We detect the optical afterglow (Kumar et al., GCN Circ. 41406; Yao et al., GCN Circ. 41409; Zheng et al., GCN Circ. 41417; Bendtsen et al., GCN Circ. 41426; Moskvitin et al., GCN Circ. 41428; An et al., GCN Circ. 41430; Kumar et al., GCN Circ. 41431) in all filters, and measure an AB magnitude of r = 22.14 ± 0.10 (mid-time t0+2.03 days). Magnitudes are calibrated against nearby Pan-STARRS stars and are not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41442. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39504] LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41441 SUBJECT: LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations DATE: 25/08/20 11:04:15 GMT FROM: Cuán de Barra at UCD <cuan.debarra(a)ucdconnect.ie> For S250818k (GCN 41437, LVK Collaboration) and using the updated bayestar skymap, Fermi-GBM was observing 51.0% of the localization probability at event time. There was no Fermi-GBM onboard trigger around the event time of the LIGO/Virgo/KAGRA (LVK) detection of GW trigger S250818k . An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM also identified no counterpart candidates. The GBM targeted search, the most sensitive, coherent search for GRB-like signals, was run from +/-30 s around merger time, and also identified no counterpart candidates. Part of the LVK localization region is behind the Earth for Fermi, located at an RA=66.9, Dec=-25.2 with a radius of 68.0 degrees. We therefore set upper limits on impulsive gamma-ray emission for the GW localization region visible to Fermi at merger time. Using the representative soft, normal, and hard GRB-like templates described in arXiv:1612.02395, we set the following 3 sigma flux upper limits over 10-1000 keV, weighted by GW localization probability (in units of 10^-7 erg/s/cm^2): Timescale Soft Normal Hard ------------------------------------ 0.128 s: 1.5 2.5 4.3 1.024 s: 0.66 0.87 1.5 8.192 s: 0.22 0.29 0.45 Assuming the median luminosity distance of 259.4 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^50 erg/s): Timescale Soft Normal Hard ------------------------------------ 0.128s: 0.018 0.029 0.081 1.024s: 0.008 0.010 0.028 8.192s: 0.003 0.003 0.008 View this GCN Circular online at https://gcn.nasa.gov/circulars/41441. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39503] LIGO/Virgo/KAGRA S250818k: Updated Sky localization and EM Bright Classification
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41440 SUBJECT: LIGO/Virgo/KAGRA S250818k: Updated Sky localization and EM Bright Classification DATE: 25/08/20 10:12:16 GMT FROM: Soichiro Morisaki at U. of Tokyo <soichiro.morisaki(a)ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250818k (GCN Circular 41437). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S250818k Based on posterior support from parameter estimation [1], under the assumption that the candidate S250818k is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass above one solar mass (HasNS) is 80%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is 80%. [2] HasRemnant is assumed to be zero when the heavier component mass is below 1 solar mass. Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 7%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is >99%. The source chirp mass falls with highest probability in the bin (0.1, 0.87) solar masses after parameter estimation [1], assuming the candidate is astrophysical in origin. For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 949 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 237 +/- 62 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe View this GCN Circular online at https://gcn.nasa.gov/circulars/41440. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39502] LIGO/Virgo/KAGRA S250818k: Pan-STARRS pre-detection limits for AT2025ulz
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41439 SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS pre-detection limits for AT2025ulz DATE: 25/08/20 09:52:22 GMT FROM: Matt Nicholl at Queens University Belfast <matt.nicholl(a)qub.ac.uk> M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), S. J. Smartt, K.W. Smith, J. Gillanders, S. Srivastav, H. Stevance, F. Stoppa, J. Tweddle (Oxford), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), K. C. Chambers, M. E. Huber, A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G, Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), T.-W. Chen (NCU), A. Rest (STScI), C. Stubbs (Harvard): We report pre-discovery limits on the source AT2025ulz, reported by Stein et al. (GCN 41414) and possibly associated with the sub-threshold gravitational wave candidate S20250818k (The LIGO-Virgo-Kagra Collaboration, GCN 41437), obtained with the Pan-STARRS telescope system (Chambers et al., 2016, ArXiv e-prints, 1612.05560). The Pan-STARRS system comprises of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters. In our primary NASA mission for Near-Earth Object discovery, we scan the visible night sky North of -50 degrees declination to magnitude depths m~22, weather and Moon permitting. Examining recent data obtained during normal Pan-STARRS survey coverage (see Fulton et al. 2025, MNRAS, 542, 541) at the location of AT2025ulz, we report the following pre-discovery upper limits and time since the S20250818k trigger (t-t0) Date t-t0 (days) Magnitude Band 60904.233 -0.82 <19.7 y 60895.314 -9.7 <21.0 i We note that the y-band non-detection was obtained only ~0.8 days before the GW signal. This indicates that there was no bright, red transient at this position less than a day prior. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). Further Pan-STARRS grizy imaging of this source, and coverage of the S20250818k skymap (northern banana) is underway in the i-band with PS1 and PS2 jointly. The discoveries from this program are a byproduct of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) NEO survey observations. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and Grant No. NNX14AM74G issued through the SSO Near-Earth Object Observations Program. Data are processed at Queen's University Belfast enabled through the STFC grants ST/P000312/1 and ST/T000198/1. View this GCN Circular online at https://gcn.nasa.gov/circulars/41439. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39501] GRB 250818A: AstroSat CZTI detection
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41438 SUBJECT: GRB 250818A: AstroSat CZTI detection DATE: 25/08/20 09:19:25 GMT FROM: Gaurav Waratkar at IIT Bombay <gauravwaratkar(a)iitb.ac.in> Harsha K. H. (IUCAA), Anuraag Arya (IITB), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 250818A which was also detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 41402), and Swift/BAT (Cenko et. al., GCN Circ. 41403). The source was clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-08-18 01:32:39.98 UTC. The measured peak count rate associated with the burst is 214 (+77, -0) counts/s above the background in the combined data of all quadrants, with a total of 6518 (+769, -832) counts. The local mean background count rate was 1187 (+3, -3) counts/s. We measure a T90 of 89 (+7, -5) s from the cumulative Veto light curve. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb View this GCN Circular online at https://gcn.nasa.gov/circulars/41438. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39500] LIGO/Virgo/KAGRA S250818k: Properties of the low-significance GW compact binary merger candidate potentially associated with AT 2025ulz
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41437 SUBJECT: LIGO/Virgo/KAGRA S250818k: Properties of the low-significance GW compact binary merger candidate potentially associated with AT 2025ulz DATE: 25/08/20 06:04:02 GMT FROM: gwangeon.seong(a)ligo.org The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration (LVK) report: We identified the compact binary merger candidate S250818k during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-18 01:20:06.030 UTC (GPS time: 1439515224.030). The candidate was found by the GstLAL SSM [1] and PyCBC Live [2] analysis pipelines. Two LVK Preliminary GCN Notices were issued within 5 minutes after the candidate was identified. Based on the analysis of gravitational-wave (GW) data alone, this candidate does not meet our criteria for a high-significance public alert as its false alarm rate is estimated by the online analysis to be 6.8e-08 Hz or about one in 5 months. However, we are issuing this Circular to confirm the properties of the GW candidate because of its potential association with the optical transient ZTF25abjmnps / AT 2025ulz (Stein et al, GCN 41414) which has been confirmed and reported to be fading and reddening rapidly (Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436). The GW candidate’s properties can be found at this URL: https://gracedb.ligo.org/superevents/S250818k The classification of the GW candidate, in order of descending probability, is Terrestrial (71%), BNS (29%), NSBH (<1%), or BBH (<1%). Initial data quality checks found that there is no evidence for presence of a glitch or other data quality issues. Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is >99%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is >99%. [3] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The source chirp mass falls with highest probability in the bin (0.1, 0.87) solar masses, assuming the candidate is astrophysical in origin. Three sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 28 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time. * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 2 days after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 786 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 259 +/- 74 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Hanna et al. (2024) arXiv 2412.10951 [2] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [4] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41437. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39499] LIGO/Virgo/KAGRA S250818k: Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz)
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41436 SUBJECT: LIGO/Virgo/KAGRA S250818k: Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz) DATE: 25/08/20 05:23:52 GMT FROM: Mansi Kasliwal at Caltech <mansikasliwal(a)gmail.com> Viraj Karambelkar (Caltech), Mansi M. Kasliwal (Caltech) and Xander J. Hall (CMU) report on behalf of the larger ZTF and GROWTH collaborations We observed ZTF25abjmnps (AT2025ulz; Stein et al. GCN 41414) starting UTC 2025-08-19 06:42 with the LRIS spectrograph on Keck I. We derive a redshift of 0.0848 based on host galaxy emission lines - this is consistent with the 3D GW localization for S250818k. Subtracting host galaxy light, we see a blue continuum excess from the transient, and no tell-tale supernova-like features. The data in-hand thus far appears consistent with a kilonova. However, continued monitoring (such as Busmann et al. GCN 41421, Hall et al. GCN 41433) is encouraged to rule out any future re-brightening from a faint and fast supernova Type IIb (e.g., Barna et al. 2025, arXiv:2506.15900). Panchromatic follow-up is ongoing and encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/41436. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39498] GRB 250818B: Swift/UVOT Detection
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41435 SUBJECT: GRB 250818B: Swift/UVOT Detection DATE: 25/08/20 04:23:17 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18(a)psu.edu> M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250818B 1.7 ks after the SVOM/ECLAIRs trigger (Wang et al., GCN Circ. 41405). An uncatalogued source consistent with the XRT position (Farro et al., GCN Circ. 41407) and the optical transient (Kumar et al., GCN Circ. 41406; Yao et al., GCN Circ. 41409; Zheng et al., GCN Circ. 41417; Bendtsen et al., GCN Circ. 41426; Moskvitin et al., GCN Circ. 41428; An et al., GCN Circ. 41430; Kumar et al., GCN Circ. 41431) is detected in the UVOT exposures. The preliminary UVOT position is: RA (J2000) = 03:04:13.45 = 46.05602 (deg.) Dec (J2000) = -03:07:31.2 = -3.12532 (deg.) with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence). Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u 1723 6610 1544 18.25+/-0.06 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.063 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41435. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39497] Fermi Gamma-ray Burst Monitor trigger 777342746/250820022 is not a GRB
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41434 SUBJECT: Fermi Gamma-ray Burst Monitor trigger 777342746/250820022 is not a GRB DATE: 25/08/20 01:39:55 GMT FROM: oindabimukherjee(a)gmail.com O. Mukherjee (USRA) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "The Fermi Gamma-ray Burst Monitor (GBM) trigger 777342746/250820022 at 00:32:21.59 UT on 20 August 2025, tentatively classified as a GRB, is in fact not due to a GRB. This trigger is likely due to local particles." View this GCN Circular online at https://gcn.nasa.gov/circulars/41434. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39496] LIGO/Virgo/KAGRA S250818k: FTW Fast reddening of AT 2025ulz
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41433 SUBJECT: LIGO/Virgo/KAGRA S250818k: FTW Fast reddening of AT 2025ulz DATE: 25/08/20 01:24:57 GMT FROM: xjh(a)andrew.cmu.edu Xander J. Hall (Carnegie Mellon U.), Malte Busmann (LMU), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report: We observed the source AT 2025ulz reported by Stein et al. (GCN 41414) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, r, i, z, and J bands starting at 2025-08-19T20:09:46 for 22 x 180 s. We took 11 observations in the giJ configuration and 11 in the rzJ configuration. In the difference imaging with templates from the Legacy Survey, we detect AT 2025ulz at g = (22.08 +/- 0.09) AB mag, r = (21.83 +/- 0.06) AB mag. We note significant reddening and fading from our previous report (GCN 41421), with the g band dropping 0.83 AB mag in 24.6 hours and r band now being brighter than g. The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction. Further analysis is underway. We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41433. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39495] GRB 250820A: Fermi GBM Final Real-time Localization
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41432 SUBJECT: GRB 250820A: Fermi GBM Final Real-time Localization DATE: 25/08/20 00:42:49 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 00:32:21 UT on 20 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250820A (trigger 777342746.588176 / 250820022). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 28.0, Dec = -70.8 (J2000 degrees, equivalent to J2000 01h 52m, -70d 47'), with a statistical uncertainty of 1.1 degrees. The angle from the Fermi LAT boresight is 169.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250820022/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250820022/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250820022/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41432. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39494] GCN Announcement: Client Credential Expiration Policy
by GCN Announcements 19 Aug '25

19 Aug '25

The GCN Team is announcing a new policy on https://gcn.nasa.gov. For security purposes, we will disable client credentials that you have not used to connect to a Kafka broker for the past 30 days. We will email you reminders 9 days before client credentials will expire. Once disabled, a client credential cannot be used again. To prevent a credential from expiring and being disabled, simply use it to connect to a Kafka broker. You may create new client credentials at any time. To review, create, delete, and manage your client credentials, see your account client credential settings (https://gcn.nasa.gov/user/credentials) For more details on this new feature and an archive of GCN news and announcements, see https://gcn.nasa.gov/news. For questions, issues, or bug reports, please contact us via: - Contact form: https://gcn.nasa.gov/contact - GitHub issue tracker: https://github.com/nasa-gcn/gcn.nasa.gov/issues --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39493] GRB 250818B: optical afterglow detection with Lesedi (GOTO25fzq)
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41431 SUBJECT: GRB 250818B: optical afterglow detection with Lesedi (GOTO25fzq) DATE: 25/08/19 14:54:41 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515(a)gmail.com> A. Kumar (RHUL), D. O’Neill (Birmingham), N. Rawat (SAAO), R. A. J. Eyles-Ferris (U. Leicester), R. Dastidar (UNAB), D. Buckley (SAAO), D. Steeghs (Warwick), J. R. Maund (RHUL), R. L. C. Starling (U. Leicester) and G. Ramsay (Armagh) report: We observed the optical afterglow GOTO25fzq (Kumar et al. GCN 41406) associated with GRB 250818B (Wang et al., GCN 41405), also reported by Yao et al. (GCN 41409), Zheng et al. (GCN 41417), Bendtsen et al. (GCN 41426), Moskvitin et al. (GCN 41428), and An et al. (GCN 41430). The observations are conducted using the 1-m Lesedi telescope at the South African Astronomical Observatory (SAAO), Sutherland, South Africa, between 2025-08-19 00:51:24 UT and 2025-08-19 04:05:00 UT (between 21.37 and 24.60 hours post-trigger) in g’, r’, i’, and z’ bands (6×450s in g and z, 6×400s in r and i bands). OT was clearly detected in the stacked g’- and r’-band images, whereas i’ and z’ images are severely affected by trailing. Preliminary aperture photometry yields the following magnitudes in the g’ and r’ bands: DATE-OBS (Start_time) T-T0 (hrs) Filter Exp (s) Magnitude (AB) -------------------------------------------------------------------------------------- 2025-08-19 00:51:24 UT 21.37 g’ 6x450s 21.39 ± 0.09 2025-08-19 01:36:43 UT 22.13 r’ 6x400s 21.58 ± 0.47 Although our r'-band image is also significantly affected by trailing, aperture photometry confirms a low-significance detection, yielding the reported magnitude. Photometric calibration was performed using reference stars from the SDSS catalogue. Magnitudes are not corrected for Galactic extinction. Follow-up observations are ongoing, and further analysis is in progress. Data were obtained using the 1-m Lesedi robotic telescope at the SAAO, equipped with the Mookodi low-resolution spectrograph and imager operating in fully robotic imaging mode. We thank the SAAO Instrumentation and Operations (IO) team for their continued support. View this GCN Circular online at https://gcn.nasa.gov/circulars/41431. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39492] GRB 250818B: TRT Optical Observation
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41430 SUBJECT: GRB 250818B: TRT Optical Observation DATE: 25/08/19 13:29:47 GMT FROM: Zipei Zhu at NAOC <zpzhu(a)nao.cas.cn> J. An (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Y. Fu (HUST), X. Liu, Z.P. Zhu, S.Q. Jiang, L.B. He, D. Xu (NAOC) report on behalf of a large collaboration: We carried out two-epoch observations of GRB 250818B (Wang et al., GCN 41405), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Fresno, California, U.S.A (SRO) and New South Wales, Australia (SBO), respectively. And we obtained several frames in the B, V, R & I band. The optical afterglow (Kumar et al., GCN 41406; Yao et al., GCN 41409; Zheng et al., GCN 41417; Bendtsen et al., GCN 41426; Moskvitin et al., GCN 41428) was clearly detected in our stack frames. The photometric results are presented in the following table: |mid time(h)|filter|mag(Vega)|site| |--|--|--|--| |7.75|R|19.98+/-0.06|SRO| |12.64|R|20.23+/-0.06|SBO| The presented magnitudes are calibrated with Pan-STARRS DR2 catalog and not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41430. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39491] Fermi GRB 250816A: Global MASTER-Net observations report
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41429 SUBJECT: Fermi GRB 250816A: Global MASTER-Net observations report DATE: 25/08/19 09:47:47 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) started inspect of the Fermi GRB 250816A ( Fermi GBM team, GCN 41397) errorbox 16 sec after notice time and 43 sec after trigger time at 2025-08-16 01:30:54 UT, with upper limit up to 12.5 mag. Observations started at twilight. The observations began at zenith distance = 68 deg. The sun altitude is -8.5 deg. The galactic latitude b = 50 deg., longitude l = 151 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2969505 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 49 | 2025-08-16 01:30:54 | MASTER-Kislovodsk | (10h 24m 32.65s , +60d 44m 23.8s) | C | 10 | 12.5 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41429. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39490] GRB 250818B: SAO RAS optical observations
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41428 SUBJECT: GRB 250818B: SAO RAS optical observations DATE: 25/08/19 08:13:33 GMT FROM: Alexander Moskvitin at SAO RAS <mosk(a)sao.ru> A. Moskvitin, O. Spiridonova, V. Vlasyuk, Yu. Sotnikova (SAO RAS), Tao An and Yuanqi Liu (Shanghai Astronomical Observatory) report on behalf of a larger collaboration. We observed the field of the SVOM short GRB 250818B (Wang et al., GCN 41405; also detected by Swift/XRT, Ferro et al., GCN 41407 and EP-FXT, Li et al., GCN 41424) with the 1-m SAO RAS telescope Zeiss-1000 equipped with CCD-photometer. We obtained 13 x 300 sec. images in Rc band on August 18, 23:15:02 -- August 19, 00:33:37 UT (t_mid - T0 = 20.4196 hours = 0.85082 days). The OT (Kumar et al., GCN 41406; Yao et al., GCN 41409; Zheng et al., GCN 41417; Fong et al., GCN 41419; Bendtsen et al., GCN 41426) is clearly detected in the stacked image with the brightness of R = 20.88 +/- 0.04 (R_lim = 23.0). This preliminary photometry is based on nearby Pan-STARRS objects (magnitudes were converted with the Lupton 2005 equations) and not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41428. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39489] Swift GRB 250818A: Global MASTER-Net observations report
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41427 SUBJECT: Swift GRB 250818A: Global MASTER-Net observations report DATE: 25/08/19 05:30:35 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 250818A ( S. B. Cenko et al., GCN 41403) errorbox 30 sec after notice time and 64 sec after trigger time at 2025-08-18 01:32:26 UT, with upper limit up to 18.4 mag. The observations began at zenith distance = 52 deg. The sun altitude is -43.8 deg. The galactic latitude b = 3 deg., longitude l = 314 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2971328 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 69 | MASTER-OAFA | C | 10 | 17.6 | 90 | MASTER-OAFA | C | 20 | 17.4 | 116 | MASTER-OAFA | C | 20 | 17.6 | 148 | MASTER-OAFA | C | 30 | 18.0 | 184 | MASTER-OAFA | C | 30 | 18.1 | 225 | MASTER-OAFA | C | 40 | 18.3 | 276 | MASTER-OAFA | C | 50 | 18.1 | 337 | MASTER-OAFA | C | 60 | 18.4 | 408 | MASTER-OAFA | C | 70 | 18.1 | 494 | MASTER-OAFA | C | 90 | 18.1 | 601 | MASTER-OAFA | C | 110 | 18.2 | 722 | MASTER-OAFA | C | 120 | 18.1 | 848 | MASTER-OAFA | C | 120 | 18.0 | 974 | MASTER-OAFA | C | 120 | 18.1 | 1100 | MASTER-OAFA | C | 120 | 18.1 | 1226 | MASTER-OAFA | C | 120 | 18.1 | 1352 | MASTER-OAFA | C | 120 | 18.1 | 1478 | MASTER-OAFA | C | 120 | 18.1 | 1604 | MASTER-OAFA | C | 120 | 18.0 | 1730 | MASTER-OAFA | C | 120 | 18.1 | 1856 | MASTER-OAFA | C | 120 | 18.3 | 1982 | MASTER-OAFA | C | 120 | 18.1 | 2108 | MASTER-OAFA | C | 120 | 18.1 | 2234 | MASTER-OAFA | C | 120 | 18.1 | 2361 | MASTER-OAFA | C | 120 | 18.0 | 2487 | MASTER-OAFA | C | 120 | 18.0 | 2613 | MASTER-OAFA | C | 120 | 17.9 | 2739 | MASTER-OAFA | C | 120 | 17.8 | 2865 | MASTER-OAFA | C | 120 | 17.9 | 2992 | MASTER-OAFA | C | 120 | 17.8 | 3118 | MASTER-OAFA | C | 120 | 17.9 | 3244 | MASTER-OAFA | C | 120 | 18.1 | 3370 | MASTER-OAFA | C | 120 | 18.1 | 3496 | MASTER-OAFA | C | 120 | 18.0 | 3622 | MASTER-OAFA | C | 120 | 18.1 | 3748 | MASTER-OAFA | C | 120 | 18.0 | 3874 | MASTER-OAFA | C | 120 | 18.1 | 4000 | MASTER-OAFA | C | 120 | 18.0 | 4126 | MASTER-OAFA | C | 120 | 18.0 | 4253 | MASTER-OAFA | C | 120 | 18.0 | 4379 | MASTER-OAFA | C | 120 | 18.1 | 4505 | MASTER-OAFA | C | 120 | 18.1 | 4631 | MASTER-OAFA | C | 120 | 17.9 | 4757 | MASTER-OAFA | C | 120 | 18.0 | 4884 | MASTER-OAFA | C | 120 | 18.0 | 5010 | MASTER-OAFA | C | 120 | 18.1 | 5135 | MASTER-OAFA | C | 120 | 18.0 | 5261 | MASTER-OAFA | C | 120 | 18.1 | 5388 | MASTER-OAFA | C | 120 | 18.1 | 5515 | MASTER-OAFA | C | 120 | 18.0 | 5640 | MASTER-OAFA | C | 120 | 18.0 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41427. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39488] GRB 250818B: NOT optical observations
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41426 SUBJECT: GRB 250818B: NOT optical observations DATE: 25/08/19 04:07:05 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> Jonas Broe Bendtsen (SDU), Johanne Hein Pedersen (SDU), Isabella Alexandra Koch (SDU), Sophie Lund Wagner (SDU), Julie Magaard Knudsen (SDU), Roar Holmberg Rasmussen (NOT and Aarhus), Kostas Valeckas (NOT and NBI), Johan P. U. Fynbo (DAWN/NBI), Daniele B. Malesani (DAWN/NBI and Radboud Univ.), Antonio de Ugarte Postigo (LAM), report on behalf of a larger collaboration: We observed the optical counterpart (Kumar et al., GCN 41406; Yao et al., GCN 41409; Zheng et al., GCN 41417; Fong et al., GCN 41419) of the short SVOM GRB 250818B (Wang et al., GCN 41405) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried in the SDSS r band out using the ALFOSC camera, for a total exposure time of 3x300 s. The optical afterglow is clearly detected in our images. Its coordinates, calibrated against the Gaia catalog, are: RA(J2000) = 03:04:13.483 Dec(J2000) = -03:07:30.74 with an error of 0.1". At a mean epoch 2025 Aug 19.128 UT (23.60 hr after the trigger), we measure r = 21.15 +- 0.04 (AB), calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41426. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39487] GRB 250818A: Swift-XRT refined Analysis
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41425 SUBJECT: GRB 250818A: Swift-XRT refined Analysis DATE: 25/08/19 04:02:04 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> D.N. Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 5.5 ks of XRT data for GRB 250818A, from 86 s to 83.2 ks after the trigger. The data comprise 247 s in Windowed Timing (WT) mode (the first 10 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The late-time light curve (from T0+4.4 ks) can be modelled with a power-law decay with a decay index of alpha=1.15 (+/-0.04). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.18 (+/-0.06). The best-fitting absorption column is 1.63 (+/-0.08) x 10^22 cm^-2, in excess of the Galactic value of 6.8 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.76 (+0.17, -0.16) and a best-fitting absorption column of 1.69 (+0.25, -0.23) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.6 x 10^-11 (1.2 x 10^-10) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.69 (+0.25, -0.23) x 10^22 cm^-2 Galactic foreground: 6.8 x 10^21 cm^-2 Excess significance: 7.2 sigma Photon index: 1.76 (+0.17, -0.16) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01343270. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41425. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39486] GRB 250818B: EP-FXT counterpart detection
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41424 SUBJECT: GRB 250818B: EP-FXT counterpart detection DATE: 25/08/19 02:25:18 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> R.-Z. Li (YNAO, CAS), Y.-C. Fu (BNU), W.-D. Zhang and Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250818B (SVOM/sb25081801, Wang et al. GCN 41405) at 2025-08-18 06:00:27 (UTC), about 2.52 hours after the SVOM/ECLAIRs trigger, with an exposure time of ~2 ks. One uncatalogued source is detected within the ECLAIRs error circle, and the source is spatially consistent with the counterpart reported in optical and X-ray bands (Kumar et al. GCN 41406, Ferro et al. GCN 41407, Yao et al. GCN 41409, Zheng et al. GCN 41417, Fong et al. GCN 41419). Preliminary analysis on this source are conducted, with the details listed as follows. Source 1: EPF_J030413.4-030730 RA (J2000): 46.056 Dec (J2000): -3.1247 Flux: 7.5432e-12 erg/s/cm2 (observed, 0.5-10 kev) Flux_err: 6.1924e-13 erg/s/cm2 (90% C. L.) The position uncertainty of the source is about 10 arcsec in radius (90% C.L. statistical and systematic). Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41424. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39485] GRB 250818A: Swift-BAT refined analysis
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41423 SUBJECT: GRB 250818A: Swift-BAT refined analysis DATE: 25/08/19 01:42:00 GMT FROM: Amy <yarleen(a)gmail.com> S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), M. J. Moss (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team): Using the data set from T-721 to T+482 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250818A (trigger #1343270) (Cenko et al., GCN Circ. 41403). The BAT ground-calculated position is RA, Dec = 213.816, -58.010 deg which is RA(J2000) = 14h 15m 15.8s Dec(J2000) = -58d 00' 36.5" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 44%. The mask-weighted light curve shows a multi-peaked structure that starts at ~T-15 s and ends at ~T+140 s. The main peak occurs at ~T+78 s. T90 (15-350 keV) is 106.86 +- 3.79 sec (estimated error including systematics). The time-averaged spectrum from T-15.92 to T+141.43 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.85 +- 0.04. The fluence in the 15-150 keV band is 1.2 +- 0.03 x 10^-05 erg/cm2. The 1-sec peak photon flux measured from T+77.63 sec in the 15-150 keV band is 5.5 +- 0.3 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1343270/ View this GCN Circular online at https://gcn.nasa.gov/circulars/41423. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39484] Fermi Gamma-ray Burst Monitor trigger 777251788/250818970 is not a GRB
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41422 SUBJECT: Fermi Gamma-ray Burst Monitor trigger 777251788/250818970 is not a GRB DATE: 25/08/19 01:41:04 GMT FROM: Jacob Smith at Fermi-GBM Team <jrs0118(a)uah.edu> Jacob Smith (UAH) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "The Fermi Gamma-ray Burst Monitor (GBM) trigger 777251788/250818970 at 23:16:23.51 UT on 18 August 2025, tentatively classified as a GRB, is in fact not due to a GRB. This trigger is likely due to local particles." View this GCN Circular online at https://gcn.nasa.gov/circulars/41422. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39483] LIGO/Virgo/KAGRA S250818k: FTW optical and NIR observations of AT 2025ulz
by GCN Circulars 19 Aug '25

19 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41421 SUBJECT: LIGO/Virgo/KAGRA S250818k: FTW optical and NIR observations of AT 2025ulz DATE: 25/08/19 01:00:05 GMT FROM: Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann(a)physik.lmu.de> Malte Busmann (LMU), Xander J. Hall (Carnegie Mellon U.), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report: We observed the source AT 2025ulz reported by Stein et al. (GCN 41414) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, r, i, z, and J bands starting at 2025-08-18T19:35:11 for 40 x 180 s. We took 30 observations in the giJ configuration and 10 in the rzJ configuration. In the difference imaging with templates from the Legacy Survey, we detect AT 2025ulz at g = (21.25 +/- 0.03) AB mag, r = (21.43 +/- 0.06) AB mag. The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction. We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41421. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39482] GRB 250818C: Fermi GBM Final Real-time Localization
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41420 SUBJECT: GRB 250818C: Fermi GBM Final Real-time Localization DATE: 25/08/18 23:26:54 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 23:16:23 UT on 18 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250818C (trigger 777251788.512804 / 250818970). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 203.8, Dec = 74.7 (J2000 degrees, equivalent to J2000 13h 35m, 74d 42'), with a statistical uncertainty of 1.0 degrees. The angle from the Fermi LAT boresight is 0.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818970/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818970/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818970/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41420. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39481] GRB 250818B: Keck redshift of the optical afterglow
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41419 SUBJECT: GRB 250818B: Keck redshift of the optical afterglow DATE: 25/08/18 22:13:52 GMT FROM: Wen-fai Fong at Northwestern University <wfong(a)northwestern.edu> W. Fong, A. C. Gordon (Northwestern), A. J. Levan (Radboud), N. R. Tanvir (Leicester), Y. Dong, A. Suresh, C. Liu (Northwestern) report: We observed the position of the SVOM short-duration GRB 250818B (Wang et al., GCN 41405) with a Swift/XRT counterpart (Ferro et al., GCN 41407). We obtained spectroscopy of the optical afterglow candidate detected by GOTO (Kumar et al., GCN 41406) and SVOM/VT (Yao et al., GCN 41409) using the Low-Resolution Imaging Spectrograph (LRIS) mounted on the Keck I telescope atop Mauna Kea in Hawaii (PI: Chang Liu; Program O397). We obtained 3x300-sec exposures using the 400/3400 grism on the blue side and the 400/8500 grating on the red side at an airmass of 1.2 in 0.9” seeing and clear conditions, starting on 2025 Aug 18 at 14:10 UT (approximately 10.68 hr post-burst). The spectrum spans approximately 3500 to 9500 Ang. The continuum is well-detected along with several prominent absorption features. In particular, we identify absorption features of FeII (2344, 2374, 2382), MgII (2796, 2803), MgI (2852), CaII H&K, and possibly AlII (1670), at a common redshift of z=1.216 which we consider to be the redshift of the GRB. This implies an optical luminosity of ~5e45 erg/s at 0.67 hr after the burst (using Kumar et al., GCN 41406), which is on the bright side compared to the handful of short GRBs observed at these epochs. We thank Keck Observatory staff Rita Morris and Josh Walawender for their assistance with our observing night. View this GCN Circular online at https://gcn.nasa.gov/circulars/41419. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39480] Fermi trigger No 776849714: Global MASTER-Net observations report
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41418 SUBJECT: Fermi trigger No 776849714: Global MASTER-Net observations report DATE: 25/08/18 22:09:57 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-Tunka robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Applied Physics Institute, Irkutsk State University) started inspect of the Fermi GRB250814.32 (trigger No 776849714,19h 52m 09.60s , +47d 27m 36.0s, R=8.61) errorbox 28727 sec after notice time and 28760 sec after trigger time at 2025-08-14 15:34:30 UT, with upper limit up to 19.5 mag. The observations began at zenith distance = 6 deg. The sun altitude is -21.4 deg. The galactic latitude b = 10 deg., longitude l = 82 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2968191 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 28851 | 2025-08-14 15:34:30 | MASTER-Tunka | (20h 33m 22.90s , +48d 36m 16.9s) | C | 180 | 16.4 | 28851 | 2025-08-14 15:34:30 | MASTER- | (20h 17m 19.59s , +48d 42m 56.2s) | C | 180 | 16.3 | 30056 | 2025-08-14 15:54:35 | MASTER-Tunka | (20h 33m 27.75s , +48d 35m 30.4s) | C | 180 | 16.3 | 30056 | 2025-08-14 15:54:35 | MASTER- | (20h 17m 24.16s , +48d 42m 05.5s) | C | 180 | 16.1 | 31332 | 2025-08-14 16:15:51 | MASTER-Tunka | (20h 33m 21.59s , +48d 34m 42.2s) | C | 180 | 16.3 | 31332 | 2025-08-14 16:15:51 | MASTER- | (20h 17m 18.27s , +48d 41m 13.3s) | C | 180 | 16.1 | 32228 | 2025-08-14 16:30:47 | MASTER-Tunka | (20h 33m 26.53s , +48d 35m 48.2s) | C | 180 | 16.4 | 32228 | 2025-08-14 16:30:47 | MASTER- | (20h 17m 22.76s , +48d 42m 16.4s) | C | 180 | 16.1 | 32361 | 2025-08-14 16:33:59 | MASTER-Tunka | (20h 33m 26.48s , +48d 34m 27.3s) | C | 60 | 19.5 | 32361 | 2025-08-14 16:33:59 | MASTER- | (20h 17m 23.23s , +48d 40m 55.6s) | C | 60 | 19.3 | 32433 | 2025-08-14 16:35:11 | MASTER-Tunka | (20h 33m 30.41s , +48d 35m 46.3s) | C | 60 | 19.5 | 32433 | 2025-08-14 16:35:11 | MASTER- | (20h 17m 26.76s , +48d 42m 14.4s) | C | 60 | 19.3 | 32505 | 2025-08-14 16:36:23 | MASTER-Tunka | (20h 33m 24.26s , +48d 35m 02.8s) | C | 60 | 19.4 | 32505 | 2025-08-14 16:36:23 | MASTER- | (20h 17m 20.90s , +48d 41m 30.9s) | C | 60 | 19.3 | 32577 | 2025-08-14 16:37:36 | MASTER-Tunka | (20h 33m 24.14s , +48d 36m 00.8s) | C | 60 | 19.4 | 32577 | 2025-08-14 16:37:36 | MASTER- | (20h 17m 20.53s , +48d 42m 28.1s) | C | 60 | 19.3 | 32650 | 2025-08-14 16:38:49 | MASTER-Tunka | (20h 33m 30.50s , +48d 34m 58.3s) | C | 60 | 19.4 | 32650 | 2025-08-14 16:38:49 | MASTER- | (20h 17m 27.29s , +48d 41m 25.8s) | C | 60 | 19.3 | 32723 | 2025-08-14 16:40:02 | MASTER-Tunka | (20h 33m 22.90s , +48d 33m 55.9s) | C | 60 | 19.4 | 32723 | 2025-08-14 16:40:02 | MASTER- | (20h 17m 20.07s , +48d 40m 23.1s) | C | 60 | 19.3 | 32797 | 2025-08-14 16:41:16 | MASTER-Tunka | (20h 33m 30.19s , +48d 34m 09.2s) | C | 60 | 19.4 | 32797 | 2025-08-14 16:41:16 | MASTER- | (20h 17m 27.34s , +48d 40m 36.2s) | C | 60 | 19.3 | 32870 | 2025-08-14 16:42:29 | MASTER-Tunka | (20h 33m 27.31s , +48d 35m 42.0s) | C | 60 | 19.4 | 32870 | 2025-08-14 16:42:29 | MASTER- | (20h 17m 24.00s , +48d 42m 08.7s) | C | 60 | 19.3 | 32942 | 2025-08-14 16:43:41 | MASTER-Tunka | (20h 33m 27.15s , +48d 34m 10.2s) | C | 60 | 19.4 | 32942 | 2025-08-14 16:43:41 | MASTER- | (20h 17m 24.35s , +48d 40m 36.5s) | C | 60 | 19.3 | 33018 | 2025-08-14 16:44:57 | MASTER-Tunka | (20h 33m 31.39s , +48d 35m 22.4s) | C | 60 | 19.4 | 33018 | 2025-08-14 16:44:57 | MASTER- | (20h 17m 28.24s , +48d 41m 48.4s) | C | 60 | 19.3 | 33094 | 2025-08-14 16:46:12 | MASTER-Tunka | (20h 33m 24.22s , +48d 34m 44.4s) | C | 60 | 19.4 | 33094 | 2025-08-14 16:46:12 | MASTER- | (20h 17m 21.30s , +48d 41m 10.3s) | C | 60 | 19.3 | 33169 | 2025-08-14 16:47:28 | MASTER-Tunka | (20h 33m 25.21s , +48d 35m 42.1s) | C | 60 | 19.4 | 33169 | 2025-08-14 16:47:28 | MASTER- | (20h 17m 21.99s , +48d 42m 07.7s) | C | 60 | 19.3 | 33241 | 2025-08-14 16:48:40 | MASTER-Tunka | (20h 33m 30.18s , +48d 34m 39.6s) | C | 60 | 19.5 | 33241 | 2025-08-14 16:48:40 | MASTER- | (20h 17m 27.28s , +48d 41m 05.0s) | C | 60 | 19.3 | 33315 | 2025-08-14 16:49:54 | MASTER-Tunka | (20h 33m 24.74s , +48d 33m 37.1s) | C | 60 | 19.4 | 33315 | 2025-08-14 16:49:54 | MASTER- | (20h 17m 22.16s , +48d 40m 02.3s) | C | 60 | 19.3 | 33389 | 2025-08-14 16:51:07 | MASTER-Tunka | (20h 33m 30.29s , +48d 33m 58.2s) | C | 60 | 19.4 | 33389 | 2025-08-14 16:51:08 | MASTER- | (20h 17m 27.60s , +48d 40m 23.0s) | C | 60 | 19.2 | 33463 | 2025-08-14 16:52:22 | MASTER-Tunka | (20h 33m 28.09s , +48d 35m 05.7s) | C | 60 | 19.4 | 33463 | 2025-08-14 16:52:22 | MASTER- | (20h 17m 25.02s , +48d 41m 30.4s) | C | 60 | 19.2 | 33536 | 2025-08-14 16:53:35 | MASTER-Tunka | (20h 33m 28.01s , +48d 33m 59.1s) | C | 60 | 19.3 | 33536 | 2025-08-14 16:53:35 | MASTER- | (20h 17m 25.29s , +48d 40m 23.5s) | C | 60 | 19.1 | 33608 | 2025-08-14 16:54:46 | MASTER-Tunka | (20h 33m 31.60s , +48d 35m 24.4s) | C | 60 | 19.3 | 33608 | 2025-08-14 16:54:46 | MASTER- | (20h 17m 28.43s , +48d 41m 48.6s) | C | 60 | 19.1 | 33680 | 2025-08-14 16:55:59 | MASTER-Tunka | (20h 33m 24.74s , +48d 34m 26.1s) | C | 60 | 19.4 | 33680 | 2025-08-14 16:55:59 | MASTER- | (20h 17m 21.87s , +48d 40m 50.0s) | C | 60 | 19.2 | 33753 | 2025-08-14 16:57:12 | MASTER-Tunka | (20h 33m 25.74s , +48d 35m 24.3s) | C | 60 | 19.4 | 33753 | 2025-08-14 16:57:12 | MASTER- | (20h 17m 22.56s , +48d 41m 47.6s) | C | 60 | 19.2 | 33828 | 2025-08-14 16:58:27 | MASTER-Tunka | (20h 33m 31.07s , +48d 34m 21.8s) | C | 60 | 19.4 | 33828 | 2025-08-14 16:58:27 | MASTER- | (20h 17m 28.21s , +48d 40m 45.1s) | C | 60 | 19.3 | 33902 | 2025-08-14 16:59:41 | MASTER-Tunka | (20h 33m 26.34s , +48d 33m 19.5s) | C | 60 | 19.4 | 33902 | 2025-08-14 16:59:41 | MASTER- | (20h 17m 23.82s , +48d 39m 42.3s) | C | 60 | 19.3 | 33974 | 2025-08-14 17:00:53 | MASTER-Tunka | (20h 33m 30.83s , +48d 33m 42.9s) | C | 60 | 19.4 | 33974 | 2025-08-14 17:00:53 | MASTER- | (20h 17m 28.18s , +48d 40m 05.7s) | C | 60 | 19.3 | 34052 | 2025-08-14 17:02:11 | MASTER-Tunka | (20h 33m 28.99s , +48d 34m 50.2s) | C | 60 | 19.5 | 34052 | 2025-08-14 17:02:11 | MASTER- | (20h 17m 25.96s , +48d 41m 12.6s) | C | 60 | 19.3 | 34125 | 2025-08-14 17:03:24 | MASTER-Tunka | (20h 33m 28.87s , +48d 33m 29.3s) | C | 60 | 19.4 | 34125 | 2025-08-14 17:03:24 | MASTER- | (20h 17m 26.28s , +48d 39m 51.3s) | C | 60 | 19.3 | 34199 | 2025-08-14 17:04:37 | MASTER-Tunka | (20h 33m 32.08s , +48d 34m 44.8s) | C | 60 | 19.4 | 34199 | 2025-08-14 17:04:37 | MASTER- | (20h 17m 29.07s , +48d 41m 06.9s) | C | 60 | 19.3 | 34275 | 2025-08-14 17:05:54 | MASTER-Tunka | (20h 33m 26.69s , +48d 34m 08.9s) | C | 60 | 19.4 | 34275 | 2025-08-14 17:05:54 | MASTER- | (20h 17m 23.88s , +48d 40m 30.2s) | C | 60 | 19.3 | 34349 | 2025-08-14 17:07:08 | MASTER-Tunka | (20h 33m 26.26s , +48d 35m 06.8s) | C | 60 | 19.4 | 34349 | 2025-08-14 17:07:08 | MASTER- | (20h 17m 23.14s , +48d 41m 27.8s) | C | 60 | 19.2 | 34422 | 2025-08-14 17:08:21 | MASTER-Tunka | (20h 33m 31.92s , +48d 34m 04.4s) | C | 60 | 19.4 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41418. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39479] GRB 250818B: KAIT optical observations
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41417 SUBJECT: GRB 250818B: KAIT optical observations DATE: 25/08/18 21:38:07 GMT FROM: Weikang Zheng at UC Berkeley <weikang(a)berkeley.edu> WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team: The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at Lick Observatory, observed the field of GRB 250818B (Wang et al., GCN 41405) with a set of 90x60s images in the clear (roughly R) filters, at a mid time of 8.0 hours after the trigger. We clearly detected the optical afterglow (Kumar et al., GCN 41406; Yao et al, GCN 41409) in the coadd image with a brightness of 19.9 +/- 0.2 mag (Vega). View this GCN Circular online at https://gcn.nasa.gov/circulars/41417. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39478] GRB 250818A: Swift/UVOT Upper Limits
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41416 SUBJECT: GRB 250818A: Swift/UVOT Upper Limits DATE: 25/08/18 17:50:17 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18(a)psu.edu> M. H. Siegel (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250818A 162 s after the BAT trigger (Cenko et al., GCN Circ. 41403). No optical afterglow consistent with the XRT position (Osborn et al., GCN Circ. 41403) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u (fc) 162 412 246 >20.3 b 418 438 19 >18.6 v 32212 32641 429. >20.2 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.602 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41416. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39477] LIGO/Virgo/KAGRA S250818t: Updated Sky localization
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41415 SUBJECT: LIGO/Virgo/KAGRA S250818t: Updated Sky localization DATE: 25/08/18 17:16:28 GMT FROM: lucy.thomas(a)ligo.org The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250818t (GCN Circular 41404). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S250818t For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 319 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1209 +/- 290 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040 View this GCN Circular online at https://gcn.nasa.gov/circulars/41415. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39476] LIGO/Virgo/KAGRA S250818k: Candidates from the Zwicky Transient Facility
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41414 SUBJECT: LIGO/Virgo/KAGRA S250818k: Candidates from the Zwicky Transient Facility DATE: 25/08/18 17:01:30 GMT FROM: Robert David Stein at JSI <rdstein(a)umd.edu> Robert Stein (JSI/UMD), Tomás Ahumada (Caltech) Mansi Kasliwal (Caltech), Theophile du Laz (Caltech), Utkarsh Pathak (IITB), Vishwajeet Swain (IITB), Anirudh Salgundi (UNC), Varun Bhalerao (IITB), Xander J. Hall (CMU) report, On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: We observed the localization region of the LVK trigger S250818k with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started observations in the g- and r-band beginning at 2025-08-18 04:02 UTC, approximately 2.7 hours after merger. We covered 25.2% (168.3 sq deg) of the reported localization region. This estimate accounts for chip gaps. Each exposure was 300s with a typical depth of 22 mag. The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019, Stein et al. 2021) was used to search the alerts database for candidates. We reject stellar sources (Tachibana and Miller 2018) and moving objects, and apply machine learning algorithms (Mahabal et al. 2019) , and removing candidates with history of variability prior to the merger time. We are left with 58 transient candidates, all lying within the 95.0% localization of the skymap. We perform additional vetting of these candidates, and identify those which appear to be hosted in galaxies at plausible redshifts for S250818k. We highlight one of these candidates, ZTF25abjmnps/AT2025ulz. +--------------------------------------------------------------------------------+ | ZTF Name | IAU Name | RA (deg) | DEC (deg) | Filter | Mag | MagErr | +--------------------------------------------------------------------------------+ | ZTF25abjmnps | AT2025ulz | 237.9757129 | +30.9023146 | r | 21.29 | 0.13 | +--------------------------------------------------------------------------------+ ZTF25abjmnps is in an elliptical galaxy with a Legacy Survey photometric redshift of z = 0.091 +/- 0.016, is therefore consistent with the estimated distance of S250818k. The lower 95th percentile limit of the photoz is 0.057, and at this distance S250818k would have an absolute g-band peak magnitude of M=-16.1. However, the higher redshifts would be more consistent with a supernova luminosity of M=-17. Forced photometry of this source reveals several detections in our data. The source appears to possibly be fading in g-band, but given the low SNR and short baseline it is difficult to constrain this. There are also several recent non-detections, but all upper limits are shallower than the magnitude of the transient in our images. We therefore cannot confirm whether the source is young, or if it predates the merger. We encourage observations of ZTF25abjmnps, to determine the nature of this source. Analysis of the remaining ZTF candidates is ongoing. Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC. GROWTH acknowledges the generous support of the NSF under PIRE Grant No 1545949. Alert database searches are done by AMPEL (Nordin et al. 2019) and Kowalski (Duev et al. 2019). The GROWTH India Telescope (GIT, Kumar et al., 2022) is set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. Its operations are partially supported by funding from the IIT Bombay alumni batch of 1994. The Fritz and SkyPortal projects acknowledge the generous support of The Gordon and Betty Moore Foundation. View this GCN Circular online at https://gcn.nasa.gov/circulars/41414. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39475] GRB 250818A: Enhanced Swift-XRT position
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41413 SUBJECT: GRB 250818A: Enhanced Swift-XRT position DATE: 25/08/18 16:24:03 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift-XRT team. Using 431 s of XRT Photon Counting mode data and 2 UVOT images for GRB 250818A, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 213.80012, -58.01403 which is equivalent to: RA (J2000): 14h 15m 12.03s Dec (J2000): -58d 00' 50.5" with an uncertainty of 2.4 arcsec (radius, 90% confidence). This position may be improved as more data are received. The latest position can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). This circular was automatically generated, and is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41413. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39474] GRB 250813B: SVOM/ECLAIRs refined analysis
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41412 SUBJECT: GRB 250813B: SVOM/ECLAIRs refined analysis DATE: 25/08/18 15:48:24 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> M. Brunet, O. Godet, H. Yang (IRAP), W.J. Xie (NAOC), B.T. Wang (YNAO, CAS) report on behalf of the SVOM/ECLAIRs team: Using the event-by-event data downloaded through the X-band ground station, we report further analysis of ECLAIRs observations of GRB 250813B (SVOM burst-id sb25081303). The burst that triggered ECLAIRs onboard (Xie et al. GCN 41352/[41354](https://gcn.nasa.gov/circulars/41354?view=index&query=250813B…) consists of multiple peaks. By considering only the data before the slew of the platform, the T90-value is 26.8 +0.4/-0.5 s in the 4-120 keV energy band. We note that ECLAIRs detected some emission up to at least T0+90s (T0 = 2025-08-13T22:51:12 UTC). The time-averaged spectrum from T0-5.5 s to T0+36 s in the energy range 4-120 keV is best fitted by a cutoff power law. The powerlaw index is -0.936 +0.031/-0.032 and the cutoff energy, parameterized as Epeak, is 104 +12/-10 keV. With this model, the total flux in 4-120 keV is (1.10 +0.01/-0.03)e-5 erg/cm^2/s. With a redshift of z = 1.752 (Schneider et al., GCN 41363), the burst isotropic energy Eiso (from 1 keV to 10 MeV in its rest frame) is (1.34 +0.07/-0.08)e53 erg, which makes this GRB consistent with type II GRBs in the 'Amati' relation diagram (Amati et al. 2002). We note that the ECLAIRs results are consistent within the errors with those provided by both SVOM/GRM (Tan et al., GCN 41383) and Fermi/GBM (Smith et al., GCN 41376). All quoted errors are at the 68% confidence level. We note that the calibration of SVOM/ECLAIRs is ongoing thus these results are preliminary. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. The SVOM/ECLAIRs point of contact for this GCN circular is Marius Brunet: (marius.brunet AT irap.omp.eu). View this GCN Circular online at https://gcn.nasa.gov/circulars/41412. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39473] GRB 240818A: Fermi GBM Observation
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41411 SUBJECT: GRB 240818A: Fermi GBM Observation DATE: 25/08/18 15:01:48 GMT FROM: Cuán de Barra at UCD <cuan.debarra(a)ucdconnect.ie> C. de Barra (University College Dublin) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 01:31:23.00 UT on 18 August 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250818A (trigger 777173487/250818063) which was also detected by Swift BAT (S. B. Cenko et al. 2025, GCN 41403). The Fermi GBM on-ground location is consistent with the Swift BAT position. The angle from the Fermi LAT boresight is 146 degrees. The GBM light curve consists of multiple emission episodes with a duration (T90) of about 101 s (50-300 keV). The time-averaged spectrum from T0-9.2 to T0+94.2 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.83 +/- 0.14 and the cutoff energy, parameterized as Epeak, is 83 +/- 5 keV. The event fluence (10-1000 keV) in this time interval is (1.62 +/- 0.06)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+78 s in the 10-1000 keV band is 8.8 +/- 0.7 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/" View this GCN Circular online at https://gcn.nasa.gov/circulars/41411. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39472] GRB 240418A: Fermi GBM Observation
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41411 SUBJECT: GRB 240418A: Fermi GBM Observation DATE: 25/08/18 15:01:48 GMT FROM: Cuán de Barra at UCD <cuan.debarra(a)ucdconnect.ie> C. de Barra (University College Dublin) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 01:31:23.00 UT on 18 August 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250818A (trigger 777173487/250818063) which was also detected by Swift BAT (S. B. Cenko et al. 2025, GCN 41403). The Fermi GBM on-ground location is consistent with the Swift BAT position. The angle from the Fermi LAT boresight is 146 degrees. The GBM light curve consists of multiple emission episodes with a duration (T90) of about 101 s (50-300 keV). The time-averaged spectrum from T0-9.2 to T0+94.2 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.83 +/- 0.14 and the cutoff energy, parameterized as Epeak, is 83 +/- 5 keV. The event fluence (10-1000 keV) in this time interval is (1.62 +/- 0.06)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+78 s in the 10-1000 keV band is 8.8 +/- 0.7 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/" View this GCN Circular online at https://gcn.nasa.gov/circulars/41411. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39471] LIGO/Virgo/KAGRA S250818k: Coverage and upper limits from MAXI/GSC observations
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41410 SUBJECT: LIGO/Virgo/KAGRA S250818k: Coverage and upper limits from MAXI/GSC observations DATE: 25/08/18 09:38:35 GMT FROM: Satoshi Sugita at Aoyama Gakuin U. <sugita(a)phys.aoyama.ac.jp> M. Nakajima, H. Negoro, K. Takagi (Nihon U.), N. Kawai, T. Mihara, (RIKEN), S. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU) report on behalf of the MAXI team: We examined MAXI/GSC all-sky X-ray images (2-20 keV) after compact binary merger candidate S250818k at 2025-08-18 01:20:06.030 UTC. At the trigger time of S250818k, the high-voltage of MAXI/GSC was on. The instantaneous field of view of GSC at the GW trigger time covered 1% of the 90% credible region of the bayestar sky map, in which we found no significant new X-ray source. The first one-orbit (92 min) scan observation with GSC after the event covered 78% of the 90% credible region of the bayestar skymap from 01:20:06 to 02:51:15 UTC (T0+0 to T0+5469 sec). No significant new source was found in the region in the one-orbit scan observation. A typical 1-sigma averaged upper limit obtained in one scan observation is 20 mCrab at 2-20 keV. If you require information about X-ray flux by MAXI/GSC at specific coordinates, please contact the submitter of this circular by email. View this GCN Circular online at https://gcn.nasa.gov/circulars/41410. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39470] GRB 250818B: SVOM/VT optical observation
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41409 SUBJECT: GRB 250818B: SVOM/VT optical observation DATE: 25/08/18 07:13:51 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> Z. H. Yao, L. P. Xin, H. L. Li, Y. L. Qiu, C. Wu, Y. N. Ma, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), Z. Q. Wang (GXU), Y. F. Liang (PMOC) report on behalf of the SVOM/VT team. SVOM performed an automatic slew on the burst GRB 250818B triggered by SVOM/ECLAIRs (Wang et al., GCN 41405). SVOM/VT start the observations at 2025-08-18T03:32:27.5, 198.5 seconds after the burst, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With X-band data availible, the optical counterpart (GOTO25fzq, Kumar et al., GCN 41406) at the position consistent with the locations of Swift/XRT (Ferro et al., GCN 41407), was clearly detected in VT_B. The magnitudes are: T-T0 (s) | exposure time (s) | band | mag (AB) | mag err ------------------|-------------------|------|----------|-------- 198.5 | 50 | VT_B | 17.61 | 0.03 3963.5 | 50 | VT_B | 19.02 | 0.03 Our photometry was not corrected for Galactic extinction. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. The SVOM point of contact for this burst is Ziqi Wang (ziqi.wang(a)st.gxu.edu.cn) Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst. View this GCN Circular online at https://gcn.nasa.gov/circulars/41409. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39469] GRB 250814A: Further GRAWITA TNG NIR observations
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41408 SUBJECT: GRB 250814A: Further GRAWITA TNG NIR observations DATE: 25/08/18 06:30:03 GMT FROM: Paolo D'Avanzo at INAF - OAB <paolo.davanzo(a)inaf.it> P. D'Avanzo, (INAF-OAB), M.T. Botticella, L. Izzo (INAF - OACn), A. Rossi (INAF-OAS), V. D’Elia (ASI-SSDC), G. Greco (INFN), M.G. Bernardini (INAF-OAB), B. Patricelli (Univ. Pisa), E. Cappellaro (INAF-OAPd), E. Brocato (INAF-OAR), M. Pedani, G Mainella (INAF-TNG), on behalf of GRAWITA report: We observed the field of GRB 250814A detected by Swift/BAT and Fermi/GBM (Caputo et al. GCN Circ. 41358; Fermi team, GCN Circ. 41357) and possibly associated with the sub-threshold GW trigger S250814bg (LVK, GCN Circ. 41364) with the Italian 3.6m TNG telescope, located in Canary Islands (Spain), equipped with the near-infrared camera NICS in imaging mode. A series of images were obtained with the H filter starting on 2025-08-17T00:15:57 UT (i.e. 2.1 days post T0), centered at the position of the X-ray afterglow detected by Swift/XRT (Caputo et al. GCN Circ. 41358; Salvaggio et al., GCN Circ. 41379). The two NIR sources reported by D'Avanzo et al. (GCN Circ. 41391) are clearly detected. From preliminary photometry we find no evidence for significant variability for both sources with respect to our previous epoch of observation (D'Avanzo et al. GCN Circ. 41391). We therefore conclude that these sources are likely unrelated with GRB 250814A. View this GCN Circular online at https://gcn.nasa.gov/circulars/41408. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39468] GRB 250818B: Swift-XRT afterglow detection
by GCN Circulars 18 Aug '25

18 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41407 SUBJECT: GRB 250818B: Swift-XRT afterglow detection DATE: 25/08/18 05:50:58 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> M. Ferro (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M.A. Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of GRB 250818B. We searched for X-ray sources in 604 s of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 604 s, obtained between T0+1.7 ks and T0+2.3 ks. An uncatalogued X-ray source is detected and is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. The position of this source is RA, Dec=46.0552, -3.1251 which is equivalent to: RA (J2000): 03h 04m 13.24s Dec(J2000): -03h 07m 30.24s with an uncertainty of 3.6 arcsec (radius, 90% confidence). The light curve is consistent with a constant source of mean count rate 7.1e-01 ct/sec, though only covers a short interval of time. The position is also consistent with the fading optical counterpart candidate GOTO25fzq/AT 2025ukm reported by GOTO (GCN Circ. 41406). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.67 (+0.26, -0.20). The best-fitting absorption column is 8.6 (+8.1, -2.1) x 10^20 cm^-2, consistent with the Galactic value of 6.5 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.0 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 8.6 (+8.1, -2.1) x 10^20 cm^-2 Galactic foreground: 6.5 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.67 (+0.26, -0.20) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/03000030. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00031. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41407. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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