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

[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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