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

[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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