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

[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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