August 2025 - vsnet-grb-info - ooruri.kusastro.kyoto-u.ac.jp

[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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