EP260527a, GRB 260527A
GCN Circular 44838
Subject
GRB 260527A/EP260527a: 6 GHz VLA observations
Date
2026-06-05T10:05:25Z (4 months ago)
From
muskan.yadav@students.uniroma2.eu
Via
Web form
M. Yadav, R. Ricci, E. Troja (U Rome) report:
We observed the field of EP260527a (Yang et al., GCN 44718), likely associated with the short GRB 260527A (Svinkin et al., GCN 44722, Yu et al., GCN 44724; Kawakubo et al., GCN 44726, Wang et al., GCN 44735, Svinkin et al., GCN 44744, Cheung et al., GCN 44757) with Very Large Array in C-band at the centre frequency of 6 GHz with a bandwidth of 4 GHz in two epochs on May 29th 2026 (2.01 days after the trigger) and June 4th 2026 (8.05 days after the trigger).
Data were calibrated and imaged using standard procedures in CASA (6.6.6.18). 1331+305 (3C286) was used for flux density and bandpass calibration and J1224+0330 was used for complex gain calibration.
The radio source reported by Schroeder et al. (GCN 44740) is detected in our first epoch at a consistent position and flux level, and is seen to fade below detection threshold in our second epoch, yielding a preliminary 3-sigma upper limit of ~30 uJy/beam. We therefore confirm this source as the radio counterpart of EP260527a.
We thank the VLA staff for executing the observations.
GCN Circular 44757
Subject
GRB 260527A/EP260527a: Glowbug gamma-ray detection
Date
2026-06-01T12:58:49Z (4 months ago)
From
C.C. Cheung at Naval Research Lab <Teddy.Cheung@nrl.navy.mil>
Via
Web form
The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 260527A, which was detected initially by Einstein Probe as EP260527a (GCN 44718). The GRB was also detected by Konus-Wind (GCN 44722), GECAM-B (GCN 44724), CALET (GCN 44726), Insight-HXMT (GCN 44735), and Mars-Odyssey (HEND) with an IPN triangulation (GCN 44744).
Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2026-05-27 06:16:08.912 with a duration of 0.26 s and a total significance of about 40 sigma. The light curve comprises a primary peak at ~T0+0.1s and a fainter peak at ~T0+0.2s.
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS.
Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12.
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108
Distribution Statement A: Approved for public release. Distribution is unlimited.
GCN Circular 44744
Subject
IPN triangulation of GRB 260527A / EP260527a
Date
2026-05-30T14:35:28Z (4 months ago)
From
Dmitry Svinkin at Ioffe Institute <dmitrysvinkin@gmail.com>
Via
Web form
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. Kawakubo, A. Yoshida, T. Sakamoto, S. Sugita (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii,
Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U),
N. Cannady (GSFC), M. L. Cherry (LSU), S. Ricciarini (U of Florence),
P. S. Marrocchesi (U of Siena),
on behalf of the CALET collaboration,
Chen-Wei Wang (IHEP), Shao-Lin Xiong (IHEP), Shuang-Nan Zhang (IHEP)
on behalf of the Insight-HXMT team,
Zheng-Hang Yu, Chen-Wei Wang, Shao-Lin Xiong, Chao Zheng,
Cheng-Kui Li (IHEP) on behalf of GECAM 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 260527A
(Konus-Wind detection: Svinkin et al., GCN 44722;
GECAM-B detection: Yu et al., GCN 44724;
CALET-GBM detection: Kawakubo et al., GCN 44726;
Insight-HXMT detection: Wang et al., GCN 44735)
was detected by Konus-Wind, CALET (GBM trigger 1463897683),
GECAM-B, Insight-HXMT(HE), and Mars-Odyssey (HEND)
at about 22570 s UT (06:16:10).
We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
---------------------------------------------
RA(2000), deg Dec(2000), deg
---------------------------------------------
Center:
192.924 (12h 51m 42s) +1.318 ( +1d 19' 03")
Corners:
192.308 (12h 49m 14s) +0.301 ( +0d 18' 02")
192.793 (12h 51m 10s) +1.224 ( +1d 13' 25")
193.417 (12h 53m 40s) +2.085 ( +2d 05' 08")
193.052 (12h 52m 13s) +1.406 ( +1d 24' 23")
---------------------------------------------
The error box area is 457 sq. arcmin, and its maximum
dimension is 2.1 deg (the minimum one is 6.4 arcmin).
The Sun distance was 125 deg.
This localization may be improved.
The fast X-ray transient EP260527a (Yang et al., GCN 44718) is inside the box strengthen the association of the transient and GRB 260527A.
A triangulation map is posted at
http://www.ioffe.ru/LEA/GRBs/GRB260527_T22572/IPN/
GCN Circular 44743
Subject
EP260527a/GRB 260527A: further FTW optical and NIR observations
Date
2026-05-30T13:40:09Z (4 months ago)
From
Ziyuan.Zhu@campus.lmu.de
Via
Web form
Ziyuan Zhu (LMU), Malte Busmann (LMU), Brendan O'Connor (CMU), Julius Gassert (LMU/CMU), Daniel Gruen (LMU), Xander Hall (CMU), and Antonella Palmese (CMU) report:
We observed the counterpart of EP260527a/GRB 260527A (Yang et al., GCN 44718; Aguilar-Ruiz et al., GCN 44719; Li et al., GCN 44720; Svinkin et al., GCN 44722; Yu et al., GCN 44724; Yang et al., GCN 44725; Kawakubo et al., GCN 44726; Mo et al., GCN 44727; Busman et al., GCN 44728) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, and J bands for 48x 180s starting at starting at 2026-05-29 20:47 UT (2.6 days after the trigger). In the r band, we do not detect the counterpart down to a limiting magnitude of
r > 24.0 AB mag.
The magnitude is calibrated against the PS1 catalog and not corrected for Galactic extinction.
We thank Christoph Ries from the Wendelstein Observatory for obtaining these observations.
GCN Circular 44740
Subject
GRB 260527A / EP260527a: VLA radio detection
Date
2026-05-29T16:53:30Z (4 months ago)
From
Genevieve Schroeder at Cornell University <genevieveschroeder@u.northwestern.edu>
Via
Web form
G. Schroeder (Cornell), J. Rastinejad (UMD), W. Fong (Northwestern) report:
We observed the location of the short-duration GRB 260527A / EP260527a (Yang et al. GCN 44718, Svinkin et al. GCN 44722, Yu et al. GCN 44724) with the Karl G. Jansky Very Large Array (VLA) in A configuration under program 26A-062 (PI Schroeder) at a mid time of 2026 May 29 at 04:47 UT (1.9 days post-burst) for 0.75 hours at a mean frequency of 6 GHz.
In preliminary analysis, we detect a 3-sigma radio source with a flux density of ~50 microJy at the position:
RA(J2000) = 12:52:13.141
Dec(J2000) = +01:28:56.47
with an uncertainty of ~0.1" in each coordinate. This position is offset by ~0.4" from the optical counterpart (Aguilar-Ruiz et al. GCN 44719). At the proposed event redshift of z=0.8 (Yang et al. GCN 44736), this corresponds to a rest-frame luminosity of ~ 9e29erg/s/Hz, on the bright end of typical short-duration Gamma-ray burst radio afterglow luminosities at a similar rest-frame time (e.g., Laskar et al. 2022, Anderson et al. 2025, Belkin et al. 2026). Further observations are planned to assess the variability of the radio source and its connection to GRB 260527A and EP260527a.
We thank the VLA staff for quickly approving and executing these observations.
GCN Circular 44739
Subject
EP260527a/GRB 260527A: optical upper limit with Kinder observations
Date
2026-05-29T15:40:44Z (4 months ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
K. N.-T. Ho, A. Aryan, T.-W. Chen, W.-J. Hou, H.-Y. Hsiao (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang(HNAS), Y.-H. Lee, A. Sankar.K, M.-H. Lee, A. Dutta, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, C.-S. Lin, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, K. W. Smith, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP260527a (Yang et al., GCN 44718, GCN 44725; Svinkin, GCN 44722; Yu et al., GCN 44724; Kawakubo et al., GCN 44726; Wang et al., GCN 44735) using the 1m LOT at the Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first SLT epoch of observations started at 16:02 UTC on 28th May 2026 (MJD 61188.6683), 33.71 hr after the EP-WXT trigger.
We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. Neither in the individual frames nor in the stacked frame did we detect the reported optical/NIR counterpart candidate (Aguilar-Ruiz et al., GCN 44719; Li et al., GCN 44720; Mo et al., GCN 44727; Busmann et al., GCN 44728; Corcoran et al., GCN 44729; Kabir et al., GCN 44730; O'Connor et al., GCN 44731; Yang et al., GCN 44736).
The details of the observations and measured PSF magnitude with template subtraction (in the AB system) of the possible counterpart of EP260507a are as follows:
--------------------------------------------------------------------------------------------------
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 61188.6683 | 33.71 | 120 * 15 | >21.1 | 1".24 | 2.02
The presented magnitudes are calibrated using the field stars from the Pan-STARRS catalogue. The reported upper limit is not corrected for an expected galactic extinction of A_r = 0.04 mag, in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.
GCN Circular 44736
Subject
GRB 260527A/EP260527a: VLT/X-shooter tentative redshift
Date
2026-05-29T10:15:04Z (4 months ago)
From
Yu-Han Yang at University of Rome Tor Vergata <yyang@roma2.infn.it>
Via
Web form
Y.-H. Yang, E. Troja, M. El Kabir (U Rome) report on behalf of the ERC BHianca team:
We observed the field of EP260527a (Yang et al., GCN 44718), likely associated with the short GRB 260527A (Svinkin et al., GCN 44722, Yu et al., GCN 44724; Kawakubo et al., GCN 44726, Wang et al., GCN 44735) with the X-shooter spectrograph on the ESO Very Large Telescope (VLT). Observations were performed beginning at 41 hours after the GRB trigger, at an average airmass of 1.3 and in excellent conditions. Our spectra cover the wavelength range 3000-25000 AA, and consist of 4x600 s exposures.
Using a nearby bright star for blind offsets, we placed the 1 arcsec slit at the position of the optical counterpart (Aguilar-Ruiz et al., GCN 44719; Li et al., GCN 44720; Busmann et al.44728; Corcoran et al. GCN 44729, El Kabir et al. GCN 44730, O’Connor et al. GCN 44731). Based on a preliminary analysis, we report the identification of weak emission features consistent with the [OII] 3727 doublet at z ~ 0.80, which we propose as the tentative redshift of the EP transient and its underlying host galaxy.
Further observations to confirm this value are planned.
We thank the staff at the VLT for the rapid execution of these observations.
GCN Circular 44735
Subject
GRB 260527A/EP260527a: Insight-HXMT detection
Date
2026-05-29T03:21:07Z (4 months ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
Chen-Wei Wang, Zheng-Hang Yu, Shao-Lin Xiong, Cheng-Kui Li, and Chao Zheng report on behalf of the Insight-HXMT team:
At 2026-05-27T06:16:10.000 (T0), Insight-HXMT/HE detected the short burst GRB 260527A/EP260527a, which is also detected by Konus-Wind (D. Svinkin et al., GCN#44722), GECAM-B (Yu et al., GCN #44724), and EP (Yang et al., GCN # 44718).
The Insight-HXMT/HE light curve mainly consists of multiple pulses with a T90 of 0.50 +0.30/-0.11 s.
The total counts from this burst is 721 counts.
The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb260527A.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.org.
GCN Circular 44731
Subject
GRB 260527A/EP260527a: Gemini GMOS-S Optical Observations
Date
2026-05-29T02:17:48Z (4 months ago)
Edited On
2026-06-01T13:52:40Z (4 months ago)
From
Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>
Via
Web form
Brendan O'Connor (CMU), Malte Busmann (LMU), Julius Gassert (LMU/CMU), Daniel Gruen (LMU), Xander Hall (CMU), and Antonella Palmese (CMU) report:
We observed the counterpart of GRB 260527A/EP260527a (Yang et al., GCN 44718; Aguilar-Ruiz et al., GCN 44719; Li et al., GCN 44720; Svinkin et al., GCN 44722; Yu et al., GCN 44724; Yang et al., GCN 44725; Kawakubo et al., GCN 44726; Mo et al., GCN 44727) using the GMOS-S instrument mounted on the Gemini-South telescope. Our observations were carried out in the r and i filters for 10x90 s exposure in each band. The observations began on 2026-05-29 at 01:34:56 UT, corresponding to 43.25 hr after discovery.
At the location of the optical counterpart (Aguilar-Ruiz et al., GCN 44719), we detect a source with magnitude i = 23.2 +/- 0.1 AB mag, consistent with recent reports (Busmann et al., GCN 44728; Corcoran et al., GCN 44729; El Kabir et al., GCN 44730). The photometry is calibrated against the PS1 catalog and is not corrected for the minimal Galactic extinction along the line of sight.
We thank the Gemini staff, in particular Aleksandar Cikota, for the rapid scheduling and execution of these observations.
GCN Circular 44730
Subject
EP260527a / GRB 260527A: VLT detection of the optical and nIR counterpart
Date
2026-05-29T02:09:56Z (4 months ago)
From
Massine El Kabir <m.elkabir@campus.unimib.it>
Via
Web form
M. El Kabir, Y. Yang and E. Troja (U Rome) report on behalf of the ERC BHianca team:
We observed the field of EP260527a (Yang et al., GCN 44718), likely associated with the short GRB 260527A (Svinkin et al., GCN 44722, Yu et al., GCN 44724; Kawakubo et al., GCN 44726), with the FORS2 and HAWK-I on the ESO Very Large Telescope (VLT). Observations started about 41.4 hrs after the GRB trigger and were performed at an airmass of ~1.2 in excellent conditions. The field was imaged using the r, z, J and K filters.
Photometric calibration was performed using nearby stars from the Pan-STARRS and 2MASS catalogues. Magnitudes are reported in the AB system and are not corrected for Galactic extinction. At the position of the optical counterpart (Aguilar-Ruiz et al., GCN 44719; Li et al., GCN 44720; Busmann et al.44728; Corcoran et al. GCN 44729 ), we detect a source with the following preliminary magnitude:
z = 23.1 +/- 0.1
J = 23.0 +/- 0.1
Compared to earlier observations (Aguilar-Ruiz et al., GCN 44719), our measurements suggest a rapid fading of the afterglow, consistent with the observed X-ray behavior (Yang et al. GCN, 44725).
We thank the staff at the VLT for the rapid execution of these observations.
GCN Circular 44729
Subject
EP260527a / GRB 260527A: NOT optical observations
Date
2026-05-28T23:47:22Z (4 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
G. Corcoran (UCD), J. An (NAOC), A. van Hoof (Radboud), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), Dimple (Birmingham), R. A. J. Eyles-Ferris (Leicester), B. P. Gompertz (Birmingham), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), J. A. Quirola-Vasquez (Radboud), A. M. Kadela (NOT and NBI), report on behalf of a larger collaboration:
We observed the optical afterglow (Aguilar-Ruiz et al., GCN 44719; Li et al. GCN 44720; Busman et al., GCN 44728) of EP260527a (Yang et al., GCN 44718), likely associated with GRB 260527A (Svinkin et al., GCN 44722; Yu et al., GCN 44724; Kawakubo et al., GCN 44726), using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. Observations were secured in both the i (25x120 s) and z (16x120 s) bands.
At mean epochs of 2026 May 28.911 and 28.946 UT (39.59 and 40.44 hr after the Konus/Wind trigger time, respectively), we measure for the counterpart the following magnitudes:
i = 23.17 +- 0.15
z = 23.25 +- 0.31
These magnitudes are calibrated against the Pan-STARRS catalog and are not corrected for Galactic extinction.
We note the presence of a faint, underlying object cataloged in the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) survey (Aihara et al., doi:10.1093/pasj/psab122) at r = 25.72 +/- 0.17, i = 25.44 +/- 0.18 and z = 25.32 +/- 0.29 (all AB). Due to the lack of sufficient high S/N multiband detections, there is no reliable reported photometric redshift for this object.
GCN Circular 44728
Subject
GRB 260527A/EP260527a: FTW optical and NIR observations
Date
2026-05-28T23:03:55Z (4 months ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann (LMU), Brendan O'Connor (CMU), Julius Gassert (LMU/CMU), Daniel Gruen (LMU), Xander Hall (CMU), and Antonella Palmese (CMU) report:
We observed the counterpart of GRB 260527A/EP260527a (Yang et al., GCN 44718; Aguilar-Ruiz et al., GCN 44719; Li et al., GCN 44720; Svinkin et al., GCN 44722; Yu et al., GCN 44724; Yang et al., GCN 44725; Kawakubo et al., GCN 44726; Mo et al., GCN 44727) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J-band simultaneously for 45 x 180 s starting at 2026-05-28T19:58:32 UT (1.57 days after the trigger). We detect the counterpart at
r = (23.1 +/- 0.2) AB mag,
calibrated against the PS1 catalog and not corrected for the Galactic extinction of A_V = 0.050 mag (Schlafly & Finkbeiner 2011).
We note that there is a potential host galaxy with a DESI DR1 (DESI Collaboration et al., 2026, AJ, 171, 285) redshift of z = 0.3656 located ~13" North-East from the afterglow position. At this redshift, the projected physical offset is ~66 kpc, which is at the high end of the distribution of short GRB offsets but not unprecedented (O'Connor 2022, MNRAS, 515, 4890). We derive a probability of chance coincidence of Pcc~0.09, suggesting a possible association. However, as this is a very large offset, deep imaging may reveal additional candidate host galaxies. We do note that based on the available DESI DR1 spectra, there appears to be an overdensity of z~0.36 galaxies along this line-of-sight, suggesting a possible origin within or intersection with a low redshift galaxy cluster (Dichiara et al., 2026, ApJL, 999, L42).
The spectrum can be viewed at: https://www.legacysurvey.org/viewer/desi-spectrum/dr1/targetid39627824205661166.
We thank Christoph Ries for obtaining the observations.
GCN Circular 44727
Subject
EP260527a/GRB 260527A: Infrared J upper limit with MDM/MIRAGE
Date
2026-05-28T22:59:26Z (4 months ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
Web form
Geoffrey Mo (Carnegie/Caltech), K. De, V. Karambelkar, S. Ibrahim, D. Schiminovich (Columbia University) report:
We observed the field of EP260527a/GRB 260527A (Yang et al., GCN 44718; Svinkin et al., GCN 44722; Yu et al., GCN 44724; Yang et al., GCN 44725; Kawakubo et al., GCN 44726) in the near-infrared J band with the MDM InfraRed Astronomy inGaas Experiment (MIRAGE) camera on the MDM 1.3 m telescope.
Observations began at 2026-05-28T05:00:58 (+23 hours after the EP trigger) in the J band, lasting 600 s under cloudy conditions.
We do not detect the optical counterpart (Aguilar-Ruiz et al., GCN 44719; Li et al., GCN 44720), with the following 5-sigma upper limit: J > 18.4 mag AB.
MIRAGE is a new YJHs-band near-infrared imager for the MDM 1.3m telescope. We thank the MDM Observatory staff for supporting the observations.
GCN Circular 44726
Subject
GRB 260527A: CALET Gamma-Ray Burst Monitor detection
Date
2026-05-28T17:22:16Z (4 months ago)
Edited On
2026-06-01T02:15:49Z (4 months ago)
From
Yuta Kawakubo at Aoyama Gakuin University <kawakubo@phys.aoyama.ac.jp>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov>
Via
Web form
Y. Kawakubo, A. Yoshida, T. Sakamoto, S. Sugita (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii,
Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U),
N. Cannady (GSFC), M. L. Cherry (LSU), S. Ricciarini (U of Florence),
P. S. Marrocchesi (U of Siena),
and the CALET collaboration:
The CALET Gamma-ray Burst Monitor (CGBM) detected the short GRB 260527A
(Konus-Wind detection: Svinkin et al., GCN #44722; GECAM-B detection:
Yu et al., GCN #44724) at 06:16:09.76 UTC on 27 May 2026
(https://cgbm.calet.jp/cgbm_trigger/flight/1463897683/index.html).
The burst signal was seen by all CGBM detectors.
The burst light curve shows a multi-peaked structure starting
at T+0.16 s, peaking at T+0.28 s, and ending at T+0.38 s.
The T90 and T50 durations measured with the SGM data are 0.19 +/- 0.04 s
and 0.06 +/- 0.02 s (40-1000 keV), respectively.
Einstein Probe/WXT detected a fast X-ray transient, EP 260527a
(Yang et al., GCN #44718), about 4 minutes after the CGBM trigger,
and Einstein Probe/FXT follow-up observations were also reported by Yang et al.
(GCN Circ. 44725). The position of EP 260527a was observable by CGBM
at the CGBM trigger time, with an incident angle of approximately 40 degrees.
The ground-processed light curve is available at
https://cgbm.calet.jp/cgbm_trigger/ground/1463897683/
The CALET data used in this analysis were provided by
the Waseda CALET Operation Center located at Waseda University.
GCN Circular 44725
Subject
EP260527a/GRB 260527A: EP-FXT follow-up observation
Date
2026-05-28T15:02:24Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
J. Yang (ZZU), B. T. Wang (YNAO, CAS), D. F. Hu (PMO, CAS), J. P. Feng, B. Zhang (USTC), D. Zhu, K. J. Zhang (YNU) and W. D. Zhang (NAO,CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260527a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Yang et al., GCN 44718), was also detected by Konus-Wind and GECAM-B as GRB 260527A (Svinkin et al., GCN 44722, Yu et al., GCN 44724), and was followed by several optical telescopes (Aguilar-Ruiz et al., GCN 44719, Li et al., GCN 44720). EP-FXT performed a follow-up observation of EP260527a about 21 hours after the WXT detection, with an exposure time of 3.4 ks. The FXT telemetry data show that an uncatalogued source was detected within the WXT error circle at R.A. = 193.0548, DEC = 1.4806 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT X-ray spectrum of this uncatalogued source can be fitted with an absorbed powerlaw with a hydrogen column density fixed at the Galactic value of 1.57 x 10^20 cm^-2 and a photon index of 2.6 (+1.2/-0.7). The derived unabsorbed 0.5-10 keV flux is approximately 7.7 (+3.7/-3.9) x 10^-14 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
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).
GCN Circular 44724
Subject
GRB 260527A: GECAM-B detection of a short burst probably associated with EP260527a
Date
2026-05-28T14:41:34Z (4 months ago)
From
yzh807926@163.com
Via
Web form
Zheng-Hang Yu, Chen-Wei Wang, Shao-Lin Xiong, Chao Zheng, Cheng-Kui Li (IHEP) report on behalf of GECAM team:
GECAM-B was triggered in-flight by a short burst GRB 260527A at 2026-05-27T06:16:10.050 UTC (denoted as T0), which is also detected by Konus-Wind (D. Svinkin et al., GCN#44722). According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of multi-pulses with a duration (T90) of 0.40 +0.02/-0.04 s.
The on-ground localization of GECAM-B is:
Ra: 206.1 deg
Dec: 6.7 deg
Err: 4.9 deg (1-sigma, statistical only)
Considering possible systematic uncertainties, this localization is consistent with the EP/WXT localization of EP260257a (Yang et al., GCN#44718). The time coincidence and localization coincidence between GRB 260527A and EP260527a strongly support the association of these two events.
The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb260527A.png
Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).
GCN Circular 44722
Subject
Konus-Wind detection of GRB 260527A (short/hard) ~4 min before EP260527a
Date
2026-05-28T12:11:41Z (4 months ago)
From
Dmitry Svinkin at Ioffe Institute <dmitrysvinkin@gmail.com>
Via
Web form
D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia,
A. Tsvetkova, M. Ulanov, and T. Cline
on behalf of the Konus-Wind team, report:
The short-duration GRB 260527A
triggered Konus-Wind at T0=22572.142 s UT (06:16:12.142).
The burst light curve shows a multi-peaked structure
which starts at ~T0-0.1 s and has a total duration of ~0.2 s.
The emission is seen up to ~10 MeV.
The burst occurred ~4 min before the Einstein Probe detection of EP260527a (Yang et al., GCN 44718),
and also the KW ecliptic latitude response is consistent with the EP260527a position.
This suggests a common origin of two events, which will be further investigated.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260527_T22572/
As observed by Konus-Wind, the burst
had the total fluence of 1.99(-0.46,+0.57)x10^-6 erg/cm2,
and the 16-ms peak flux, measured from T0-0.008 s,
of 3.09(-1.06,+1.28)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst
(measured from T0 to T0+0.064 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.67(-0.36,+0.46),
the high energy photon index beta = -2.13(-0.85,+0.36),
the peak energy Ep = 541(-204,+367) keV
(pgstat = 33/45 dof).
A power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.79(-0.42,+0.49)
and Ep = 670(-240,+1097) keV
fits this spectrum equally well (pgstat = 35/46 dof).
All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.
GCN Circular 44720
Subject
EP260527a: SVOM/VT optical observation
Date
2026-05-28T07:13:15Z (4 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, C. Wu, Y. L. Qiu, L. P. Xin, Y. N. Ma, Z. H. Yao, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC) and J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observations to the field of EP260527a triggered by Einstein Probe (Yang et al., GCN 44718). The observation started at 2026-05-28T02:57:38 UTC, 20.63 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
The optical counterpart (Aguilar-Ruiz et al., GCN 44719) was detected by VT in both channels.
The following measurements are in the AB magnitude without correction for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
21.76 h VT_B 29*50 s 22.32 +/- 0.22 mag
21.81 h VT_R 23*50 s 21.77 +/- 0.19 mag
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 44719
Subject
EP260527a: COLIBRÍ optical detection
Date
2026-05-28T06:36:51Z (4 months ago)
Edited On
2026-06-01T13:51:29Z (4 months ago)
From
Francesco Magnani at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Alan Watson at UNAM <alan@astro.unam.mx>
Via
Web form
Edilberto Aguilar-Ruiz (UNAM), Francesco Magnani (CPPM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Benjamin Schneider (LAM) and report:
We imaged the field of the EP260527a (J. Yang et al, GCN Circ. 44718) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-28 04:13 to 06:05 UTC (from 21.90 to 23.76 hours after the trigger) and obtained 18, 62, and 82 minutes of exposure in the g, r, and z filters.
The data were reduced and coadded with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked images, we detect an uncatalogued source within the FXT uncertainty region at:
RA (J2000) = 12:52:13.13 = 193.05473 degrees
Dec (J2000) = +01:28:56.8 = 1.48244 degrees
The preliminary magnitudes derived for that source is:
g = 22.25 +/- 0.32,
r = 22.19 +/- 0.17,
z = 21.52 +/- 0.15.
The detection in g suggests that it is not a high-redshift event. Further observations are ongoing.
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 44718
Subject
EP260527a: Einstein Probe detection of a fast X-ray transient
Date
2026-05-28T03:21:10Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
J. Yang (ZZU), B. T. Wang (YNAO, CAS), D. F. Hu (PMO, CAS), J. P. Feng, B. Zhang (USTC), D. Zhu, K. J. Zhang (YNU) and W. D. Zhang (NAO,CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260527a. The transient triggered EP-WXT (ID: 01709266119) at 2026-05-27T06:19:54 (UTC). The WXT position of the source is R.A. = 193.057 deg, DEC = 1.473 deg (J2000) with an uncertainty of 2.3 arcmin in radius (90% C.L. statistical and systematic). The preliminary analysis of the WXT data shows that the event started at 2026-05-27T06:16:18 (UTC) and lasted for about 200 seconds. The average WXT 0.5-4.0 keV spectrum can be fitted with an absorbed powerlaw model, with a fixed Galactic hydrogen column density of 1.57 x 10^20 cm^-2 and a photon index of 0.73(+0.44/-0.42). The unabsorbed 0.5-4.0 keV flux is 1.12(+0.33/-0.28) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
This event did not trigger the Follow-up X-ray Telescope (FXT) autonomous follow-up. A follow-up observation with the FXT was performed about 20.5 hours after the WXT detection. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 193.0519 deg, DEC = 1.4832 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 are 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).