GRB 250521D
GCN Circular 40516
Subject
GRB 250521D: Fermi GBM Final Real-time Localization
Date
2025-05-21T22:32:26Z (8 days ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 22:21:56 UT on 21 May 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250521D (trigger 769558921.008874 / 250521932).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 281.6, Dec = 21.3 (J2000 degrees, equivalent to J2000 18h 46m, 21d 18'), with a statistical uncertainty of 2.6 degrees.
The angle from the Fermi LAT boresight is 65.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250521932/quicklook/glg_skymap_all_bn250521932.png
The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250521932/quicklook/glg_healpix_all_bn250521932.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250521932/quicklook/glg_lc_medres34_bn250521932.gif
GCN Circular 40520
Subject
GRB 250521D: LAST tentative optical counterpart candidate
Date
2025-05-22T04:07:48Z (8 days ago)
From
Ruslan Konno at Weizmann Institute of Science <ruslankonno@gmail.com>
Via
Web form
R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), S. Spitzer (WIS), and K. Rybicki (WIS) on behalf of the LAST Collaboration
We report observations of GRB 250521D (GCN #40516), detected by Fermi-GBM, with the Large Array Survey Telescope (LAST; Ofek et al. 2023; Ben-Ami et al. 2023).
We observe the field of GRB 250521D using 20 telescopes, each with a FoV of 7.4 deg^2 and no filter (clear - similar to the GAIA Bp band) over several epochs. In each epoch, we coadd 20x20s images. The standard limiting magnitude of a single 20x20s coadd is about 20.5 (AB mag).
We began observations at 2025-05-21 22:47:38 UTC (T-T0 = 25.7 min). The observations are performed by tiling the 90% confidence area, supplied by the glg_healpix_all_bn250521932.fit map, with predefined LAST observation fields. We cover a total area of about 240 deg^2, corresponding to an integrated localization probability of about 56%.
We found one new optical transient candidate, reported as AT 2025kze to TNS, in our automatic transient-detection pipeline (Konno et al., in prep.) which showed a brightening during observations. The candidate coordinates are RA, Dec = 278.4350830, 17.5896389 deg, which is 1.95 deg away from the best-fit position reported in GCN #40516. The associated photometry is
| JD | T-T0 (h) | Mag |
|----------------|----------|-------------------|
| 2460817.45206228 | 0.48 | 20.50 (UL) |
| 2460817.46226134 | 0.73 | 20.50 (UL) |
| 2460817.48502633 | 1.27 | 20.105 ± 0.098 |
| 2460817.50906663 | 1.85 | 19.722 ± 0.083 |
The reported magnitudes are in the AB system, calibrated against the GAIA DR3 catalog, with no filter (clear).
The position shows no LAST detections prior to the GRB trigger time, with the latest observations dating to 2025-04-21 01:33:59 UTC. Neither do we find any evidence of this source prior to the GRB trigger time in the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). There is a faint ambiguous extended underlying source at the candidate position visible in survey images provided by the SDSS SkyServer.
Follow-up observations are encouraged to determine the nature of this candidate or the underlying source present in survey images.
LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/).
GCN Circular 40522
Subject
GRB 250521D: GOTO detects LAST counterpart candidate prior to the GRB
Date
2025-05-22T06:31:47Z (7 days ago)
From
Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk>
Via
email
D. O'Neill, A. Kumar, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration:
We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022) in response to the Fermi/GBM alert for GRB 250521D (GCN 40516, Fermi GBM team).
During targeted follow-up of GRB 250521D (Fermi GBM team, GCN 40516), we detected the counterpart candidate reported by LAST in GCN 40520 (Konno et al., GCN 40520) with L = 19.72 ± 0.10 AB mag (+3.19h post trigger), and L = 19.68 ± 0.19 AB mag (+5.08h post trigger). We note however that we have a pre-trigger 5-sigma detection of the source at 2025-05-21 01:24:41 (-20.95h pre-trigger ) with L = 19.38 ±0.19 AB mags, indicating this is likely an unrelated event. The GOTO-L (400-700nm) and LAST unfiltered (comparable to Gaia Bp) bandpasses are roughly equivalent, and the combined GOTO and LAST observations suggest a variable source.
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).
GCN Circular 40524
Subject
GRB 250521D: DDOTI Optical Upper Limit
Date
2025-05-22T10:30:43Z (7 days ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Roma), Sahil Atri (U Roma), Alan M. Watson (UNAM), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), Margarita Pereyra (UNAM) and Eleonora Troja (U Roma) report:
We observed the field of GRB 250521D detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 40516) with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir (http://ddoti.astroscu.unam.mx) on the night of 2025-05-22 UTC.
DDOTI observed the field of GRB 250521D from 05:34 UTC to 09:09 UTC (from T+7.2 h to T+ 10.8 h after the trigger) and obtained a total exposure of 1.7 hours, alternating with other scientific programs.
Comparing our observations to the USNO-B1 and Pan-STARRS PS1 DR2 catalogues, we
detect no uncatalogued fading sources within the observed field down to a 10-sigma limiting AB magnitude of:
w > 21.0
This value is not corrected for the Galactic extinction.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra of San Pedro Mártir.
GCN Circular 40527
Subject
Fermi GRB 250521D: Global MASTER-Net observations report
Date
2025-05-22T19:15:49Z (7 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
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 250521D ( Fermi GBM team, GCN 40516) errorbox 74439 sec after notice time and 74480 sec after trigger time at 2025-05-22 19:03:16 UT, with upper limit up to 18.2 mag. The observations began at zenith distance = 63 deg. The sun altitude is -20.4 deg.
The galactic latitude b = 10 deg., longitude l = 52 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2877804
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
74510 | 2025-05-22 19:03:16 | MASTER-Kislovodsk | (18h 46m 09.15s , +22d 14m 31.3s) | C | 60 | 17.9 |
74594 | 2025-05-22 19:04:40 | MASTER-Kislovodsk | (18h 54m 44.61s , +22d 15m 50.5s) | C | 60 | 17.4 |
74879 | 2025-05-22 19:09:25 | MASTER-Kislovodsk | (18h 31m 37.29s , +20d 22m 21.4s) | C | 60 | 18.2 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 40530
Subject
GRB 250521D: SVOM/VT optical observations
Date
2025-05-23T00:26:24Z (7 days ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
W. Zheng (UCB), L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, J. Wang (NAOC), Y. F. Liang (PMO), Z. Q. Wang (GXU) report on behalf of the SVOM/VT team:
SVOM performed a Target of Opportunity observation of GRB 250521D detected by Fermi/GBM (Fermi GBM team, GCN 40516). SVOM/VT began observing the position of the counterpart candidate reported by LAST (Konno et al., GCN 40520) at 2025-05-22T05:58:37 UTC, 7.61 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
We clearly detected the candidate in VT both channels. The brightness in AB magnitude was estimated to be:
Mid time (hour) | Band | Exposure Time (second) | Magnitude | Magnitude error
7.819 | VT_B | 15*100 | 19.66 | 0.02
7.819 | VT_R | 15*100 | 19.45 | 0.02
However, considering that there is no significant fading in brightness, and also this candidate was detected by GOTO (O'Neill et al., GCN 40522) before the GBM trigger, we also suggest that this candidate is an unrelated object.
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.
GCN Circular 40561
Subject
GRB 250521D: SVOM/GRM analysis suggests a possible “long” duration type I burst
Date
2025-05-28T10:15:39Z (a day ago)
Edited On
2025-05-28T16:26:56Z (a day ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
SVOM/GRM team: Chen-Wei Wang, Wang-Chen Xue, Wen-Jun Tan, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Marius Brunet (IRAP), Frédéric Piron (LUPM)
report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by a long burst GRB 250521D at 2025-05-21T22:21:56.3 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #40516).
The real-time alert data and light curves of SVOM/GRM were downlinked to the ground through the VHF system with low latency. The GRM light curve shows that this burst consists of multiple episodes, including a possible precursor, a main emission followed by an possible extended emission, with a T90 of 12.1 +1.7/-5.4 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250521D.png
In addition, the position of this burst, as determined by Fermi/GBM (RA= 281.6, Dec= 21.3, GCN #40516), is located at about 80 degrees from the SVOM optical axis.
With this localization, the time-averaged spectrum from T0–2 s to T0+15 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.9 +/-0.1 and the cutoff energy, parameterized as Epeak, is 890 +302/-194 keV. The event fluence (10-1000 keV) in this time interval is (1.02 +0.06/-0.06)E-05 erg/cm^2. Thus GRB 250521D is consistent with Type I GRBs in the 'Amati' relation diagram although the duration is much longer than 2 seconds, as shown at:
https://www.bursthub.cn//admin/static/grb250521D_amati.png
Since the special location of this GRB in 'Amati' relation diagram, as well as the “Type IL” pattern of lightcurve, which is similar with other “long” duration type I GRBs (i.e. GRB 211211A and GRB 230307A), a possible merger origin is suggested [1,2]. Further 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@ihep.ac.cn)
[1] Chen-Wei Wang et al. ApJ 979 73 https://doi.org/10.3847/1538-4357/ad98ec
[2] Wen-Jun Tan et al. arXiv:2504.06616 [astro-ph.HE] https://doi.org/10.48550/arXiv.2504.06616