Gamma-ray Burst 250908A = EP250908c

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #41754
M. H. Zhang, M. J. Liu (NAO, CAS), Y.-H. I. Yin (NJU), X. X. Sun, T. Y. Liu, H. Y. Ren, H. Sun (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 EP250908c. The transient triggered EP-WXT (ID: 01709240659) at 2025-09-08T23:50:54 (UTC). The WXT position of the source is R.A. = 8.226 deg, DEC = -50.447 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. = 8.2170 deg, DEC = -50.4633 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).

GCN Circular #41756
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-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250908c ( EP Team et al., GCN 41754) errorbox 385 sec after trigger time at 2025-09-08 23:57:19 UT, with upper limit up to 19.4 mag. The observations began at zenith distance = 66 deg. The sun altitude is -21.0 deg.
The galactic latitude b = -67 deg., longitude l = 309 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2987645
We obtain a following upper limits.

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

     426 | 2025-09-08 23:57:19 |         MASTER-OAFA | (00h 32m 33.78s , -50d 34m 25.0s) |   C |    80 | 19.4 |        
     517 | 2025-09-08 23:58:46 |         MASTER-OAFA | (00h 32m 33.61s , -50d 34m 21.6s) |   C |    90 | 19.4 |        
     623 | 2025-09-09 00:00:22 |         MASTER-OAFA | (00h 32m 33.35s , -50d 34m 18.6s) |   C |   110 | 19.4 |        
     704 | 2025-09-09 00:02:17 |         MASTER-OAFA | (00h 32m 33.16s , -50d 34m 16.5s) |   C |    40 | 19.0 |        
     750 | 2025-09-09 00:03:03 |         MASTER-OAFA | (00h 32m 33.01s , -50d 34m 15.4s) |   C |    40 | 19.0 |        
     795 | 2025-09-09 00:03:49 |         MASTER-OAFA | (00h 32m 32.90s , -50d 34m 14.1s) |   C |    40 | 19.0 |        
     842 | 2025-09-09 00:04:35 |         MASTER-OAFA | (00h 32m 32.81s , -50d 34m 12.7s) |   C |    40 | 18.9 |        
     887 | 2025-09-09 00:05:20 |         MASTER-OAFA | (00h 32m 32.71s , -50d 34m 11.3s) |   C |    40 | 19.0 |        
     932 | 2025-09-09 00:06:06 |         MASTER-OAFA | (00h 32m 32.57s , -50d 34m 10.1s) |   C |    40 | 19.0 |        
     978 | 2025-09-09 00:06:51 |         MASTER-OAFA | (00h 32m 32.42s , -50d 34m 09.1s) |   C |    40 | 19.0 |        
    1023 | 2025-09-09 00:07:37 |         MASTER-OAFA | (00h 32m 32.31s , -50d 34m 08.0s) |   C |    40 | 18.9 |        
    1069 | 2025-09-09 00:08:23 |         MASTER-OAFA | (00h 32m 32.23s , -50d 34m 06.6s) |   C |    40 | 18.9 |        
    1116 | 2025-09-09 00:09:09 |         MASTER-OAFA | (00h 32m 32.14s , -50d 34m 05.4s) |   C |    40 | 18.9 |        
    1161 | 2025-09-09 00:09:55 |         MASTER-OAFA | (00h 32m 32.02s , -50d 34m 04.0s) |   C |    40 | 18.9 |        
    1207 | 2025-09-09 00:10:40 |         MASTER-OAFA | (00h 32m 31.90s , -50d 34m 03.0s) |   C |    40 | 19.0 |        
    1253 | 2025-09-09 00:11:26 |         MASTER-OAFA | (00h 32m 31.80s , -50d 34m 01.9s) |   C |    40 | 19.0 |        
    1298 | 2025-09-09 00:12:12 |         MASTER-OAFA | (00h 32m 31.67s , -50d 34m 01.1s) |   C |    40 | 18.9 |        
    1344 | 2025-09-09 00:12:57 |         MASTER-OAFA | (00h 32m 31.57s , -50d 33m 59.7s) |   C |    40 | 18.9 |        
    1390 | 2025-09-09 00:13:43 |         MASTER-OAFA | (00h 32m 31.45s , -50d 33m 58.3s) |   C |    40 | 19.0 |        
    1436 | 2025-09-09 00:14:29 |         MASTER-OAFA | (00h 32m 31.33s , -50d 33m 57.1s) |   C |    40 | 18.9 |        
    1481 | 2025-09-09 00:15:15 |         MASTER-OAFA | (00h 32m 31.23s , -50d 33m 56.1s) |   C |    40 | 19.0 |        
    1528 | 2025-09-09 00:16:01 |         MASTER-OAFA | (00h 32m 31.11s , -50d 33m 54.9s) |   C |    40 | 18.9 |        
    1573 | 2025-09-09 00:16:46 |         MASTER-OAFA | (00h 32m 31.00s , -50d 33m 53.6s) |   C |    40 | 18.9 |        
    1618 | 2025-09-09 00:17:32 |         MASTER-OAFA | (00h 32m 30.90s , -50d 33m 52.2s) |   C |    40 | 18.8 |

Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.

GCN Circular #41760
D. B. Malesani (DAWN/NBI and Radboud), M. Garnichey (LUX-Paris Obs.), D. Xu (NAOC), L. Cotter (UCD), J. A. Chacon (PUC), L. Izzo (INAF/OACn and DARK/NBI), J. An (NAOC), report on behalf the Stargate collaboration:
We observed the field of the fast X-ray transient EP250908c (Zhang et al., GCN 41754) using the ESO VLT UT1 (Antu) equipped with the FORS2 instrument.
In a 150 s I-band observation with mid-time 2.76 hr after the high-energy trigger, we detect an uncatalogued object at J2000 coordinates:
```
RA = 00:32:52.18
Dec = -50:27:39.8
```
This position is consistent with the EP/FXT X-ray location (Zhang et al., GCN 41754) and nothing is visible at this location in archival imaging from the Legacy Survey. We thus conclude that this is the optical afterglow of EP250908c.
Using nearby calibrator stars from the SkyMapper catalog, we measure for the counterpart a magnitude i = 20.40 +/- 0.1 (AB), where most of the error is due to calibration scatter.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Marcela Espinoza, Julien Drevon, Marco Berton and Jonathan Smoker.
GCN Circular #41761
Y. N. Ma, Z. H. Yao, L. P. Xin, Y. L. Qiu, C. Wu, H. L. Li, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM performed a Target of Opportunity observation of EP250908c detected by EP/WXT (Zhang et al., GCN 41754). SVOM/VT began observing the field at 2025-09-09T00:29:02 UTC, 0.636 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data availible, the optical counterpart (Malesani et al., GCN 41760) was clearly detected in both VT_B and VT_R. The magnitudes are:
```
mid time (h) | exposure time (s) | band | mag (AB) | mag err
-------------|-------------------|------|----------|--------
    0.987    |      16*50        | VT_B |  20.73   |  0.05 
    0.987    |      15*50        | VT_R |  19.76   |  0.04 
```
Our photometry was not corrected for Galactic extinction.
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 #41763
M. H. Zhang, X. X. Sun, M. J. Liu, H. Y. Ren, T. Y. Liu (NAO, CAS), Y.-H. I. Yin (NJU), H. Sun (NAO, CAS) report on behalf of the Einstein Probe team:
The X-ray transient EP250908c was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhang et al., GCN 41754). Refined analysis of the WXT data shows that the event started at T0=2025-09-08 23:49:53 (UTC) and lasted for about 62 s before the interruption of the autonomous follow-up. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.6 x 10^20 cm^-2 and a photon index of 0.3 (-0.6/+0.6). The derived average unabsorbed 0.5-4 keV flux is 6.2 (-1.4/+2.2) x 10^(-10) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 277 s after T0. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 8.2167, DEC = -50.4612 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent positionally with the WXT transient. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.6 x 10^20 cm^-2 and a photon index of 1.59 (-0.07/+0.07). The absorption from the host galaxy cannot be constrained. The derived average unabsorbed 0.5-10 keV flux is 8.16 (-0.17/+0.21) x 10^(-12) 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 #41773
Arne Rau (MPE) reports:
We observed the position of EP250908C (Zhang et al., GCN 41754) using the Gamma-Ray burst Optical Near-infrared Detector (GROND, Greiner et al. 2008) mounted to the MPG 2.2m telescope at ESO’s La Silla Observatory on 2025-09-10 07:24 UT, i.e. 31.5 hours after the Einstein Probe trigger.
The candidate optical counterpart reported by Malesani et al. (GCN 41760) was detected with
```
r’= 23.0+/-0.2
i’ = 22.8+/-0.2
```
indicating clear fading. The magnitudes are in the AB magnitude system and calibrated against nearby SkyMapper sources.
We acknowledge the excellent support of the 2.2m staff, in particular Angel Hempel.
GCN Circular #41816
D. B. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF/OACn and DARK/NBI), A. Saccardi (CEA/Irfu), J. An (NAOC), G. Corcoran (UCD), V. D’Elia (ASI/SSDC), M. Garnichey (LUX-Paris Obs.), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), A. Martin-Carrillo (UCD), E. Palazzi (INAF/OAS), G. Pugliese (API), D. Xu (NAOC), report on behalf of a larger collaboration:
We observed the optical counterpart (Malesani et al., GCN 41760; Ma et al., GCN 41761; Rau, GCN 41773) of the fast X-ray transient EP250908c (Zhang et al., GCNs 41754, 41763) using the ESO VLT UT1 equipped with the FORS2 spectrograph.
In the acquisition image (mid time 2025 Sep 9.134 UT, or 3.37 hr after the EP trigger) we measure a magnitude V = 21.4 +- 0.2 (calibrated using archival zeropoints).
A spectrum covering the wavelength range 3800-8600 AA was secured using grism 300V and no order-sorting filter, with a total exposure time of 3x600 s.
A faint trace is detected down to ~4790 AA, where a spectral break is observed. If due to Lyman alpha, this would correspond to a redshift of ~2.9. Consistently, a few, weak absorption features are detected, which can be interpreted as C II 1334, Si IV 1393,1402, Si II 1526, C IV 1548,1550, all at a common redshift of z = 2.87. Given the low S/N of the continuum, we caution that this redshift solution should be considered tentative.
Rectifying what was stated in our previous report (Malesani et al., GCN 41760), we note the presence of an extended object in the Legacy Survey underlying the afterglow position, which is a candidate host galaxy for EP250908c. With a Legacy Survey magnitude of r = 23.1, this corresponds to M(UV) = -22.4 (AB) at z = 2.87. No emission lines are visible in our spectrum to determine the redshift of this object.
We acknowledge excellent support from the observing staff in Paranal, in particular Julien Drevon, Jonathan Smoker and Marcela Espinoza.

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Jochen Greiner, last update: 12-Sep-2025 [Disclaimer]