Gamma-ray Burst 250903A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #41677
Tais Maiolino (LUPM), Claire Guépin-Detrigne (LUPM), Nicolas Dagoneau (CEA/Irfu), Stéphane Schanne (CEA/Irfu), Miguel Moita (CEA/Irfu) report on behalf of the SVOM mission team:
At 2025-09-03T17:24:56 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250903A (SVOM burst-id sb25090304).
The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 12 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 16.17 in the [5-20] keV energy band over a time window of 20.48 seconds starting at 2025-09-03T17:24:43.
The localization of the best alert is R.A., Dec. 356.9689, -75.9460 degrees:
```
R.A. (J2000) = 23h47m52.54s
Dec. (J2000) = -75d56m45.52s
```
with a 90% confidence level (C.L.) radius of 5.17 arcmin (including systematic error of 2 arcmin added in quadrature).
This burst was also detected by SVOM/GRM with a significance of 15.80. The SVOM/GRM light curve showed a multi-peak structure with a T90 duration of about 27 (-11/+4)s.
SVOM slewed to the burst.
No X-ray observation has been performed by SVOM/MXT for the time being. No optical observation has been performed by SVOM/VT for the time being.
We notice the presence of BLLac 2FGL J2351.6-7558 located at 12 arcmin from the ECLAIRs position.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.
The Burst Advocate (BA) on shift for this alert is Tais Maiolino: tais.maiolino@lupm.in2p3.fr. Please contact the BA by email if you require additional information.
GCN Circular #41679
J. An (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Y. Fu (HUST), L.B. He, X. Liu, Z.P. Zhu, S.Q. Jiang, D. Xu (NAOC) report on behalf of a large collaboration:
We observed the field of GRB 250903A detected by SVOM/ECLAIRs (Maiolino et al., GCN 41677), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observations started at 17:34:50 UTC on 2025-09-03, i.e., ~10 min after the SVOM/ECLAIRs trigger. A series of R-band frames were obtained.
An uncatalogued and varying optical source is detected within the SVOM/ECLAIRs error circle (Maiolino et al., GCN 41677), at coordinates
```
R.A. (J2000) = 23:47:11.67
Dec. (J2000) = -75:58:00.21
```
with an uncertainty of ~ 0.8 arcsec.
Preliminary photometry shows the source has R ~ 19.0 mag at 13.35 min post-trigger, calibrated with Legacy Survey DR10 catalog and not corrected for Galactic extinction.
We think the source is the optical counterpart of GRB 250903A.
GCN Circular #41681
D. Götz (CEA), M. Moita (CEA) F. Robinet (IJCLab), P. Maggi (ObAS), H. Goto (Kanazawa Univ./CEA), C. Plasse (CEA) , C. Van Hove (IJCLab) report of behalf of the SVOM/MXT Team:
GRB 250903A (Maiolino et al. GCN# 41677) was observed by SVOM/MXT after an automatic SVOM slew, but due to Earth constraints the observation started at 2025-09-03T18:30:41, about 66 minutes after Tb. Using the on board automatic detection software, MXT detects a faint source at the coordinates
```
R.A. (J2000) = 23h47m21s 
Dec (J2000) =  -75d58m43s
```
with a 90% C.L. radius of 3 arc min (including 25 arc sec systematic error added in quadrature). This position is at 3.1 arc min from the ECLAIRs position and 0.5 arc min from the optical counterpart reported by An et al. (GCN# 41679). We hence identify this source as the X-ray afterglow of GRB 250903A.
Further analysis will be perfromed once the full X-band data set is received.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. MXT was developed jointly by CEA, CNES, University of Leicester, IJCLab and MPE.
The SVOM point of contact for this burst is Tais Maiolino: tais.maiolino@lupm.in2p3.fr. Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.
GCN Circular #41684
L.P. Xin, H.L. Li, C. Wu. Y. L. Qiu, Z.H. Yao, Y.N. Ma, X.H. Han, J. Wang, W.J. Xie, Y. Xu, H.B. Cai, J. Y. Wei (NAOC), J. Palmerio (CEA), Tais Maiolino (LUPM), Claire Guépin-Detrigne (LUPM) report on behalf of the SVOM team.
SVOM/VT performed an automatic slew on the burst triggered by SVOM/ECLAIRs (Maiolino et al. GCN 41677). SVOM/VT began observing the field automatically at 2025-09-03T18:15:57 UTC, about 52 min after the T0, with the slew of the platform triggered on-board, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With downlinked VHF data, the optical counterpart reported (An et al., GCN 41679) within the errorbox of SVOM/MXT (GOTZ et al., GCN 41681) was clear detected with the brightness of VT_B=20.12+/-0.03 and VT_R=19.65 +/-0.03 mag at 54.5 min after the trigger.
More follow-ups are 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. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
GCN Circular #41685
I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), and A. López-Oramas (IAC and ULL)
We observed the field of GRB 250903A detected by SVOM/ECLAIRs (Maiolino et al., GCN 41677) and SVOM/MXT (Götz et al., GCN 41681) with one of the two 1-m telescopes of the Las Cumbres Observatory Global Telescope network (LCO) equipped with Sinistro cameras located at the LCO node at Siding Spring Observatory, Australia. The observation, of 300 seconds in the SDSS-r filter, started at 2025-09-03 19:12:07 UT, about 1.78 hr after the SVOM/ECLAIRs trigger. We clearly detect an uncatalogued source within the SVOM/ECLAIRs (Maiolino et al., GCN 41677) and SVOM/MXT (Götz et al., GCN 41681) error circles at a position consistent with the one reported by An et al. (TRT R band, GCN 41679) and Xin et al. (SVOM/VT VT_B and VT_R, GCN 41684).
We measure a magnitude of r = 20.29 +/- 0.10, calibrated with the Legacy Surveys DR10 catalog and not corrected for Galactic extinction.
This work makes use of observations from the Las Cumbres Observatory global telescope network program IAC2025B-008 (SGLF).
GCN Circular #41686
A. Saccardi (CEA/Irfu), L. P. Xin (NAOC), C. Wu (NAOC), B. Cordier (CEA/Irfu), report on behalf of the SVOM mission team:
We observed the field of the GRB 250903A detected by SVOM/ECLAIRs (Maiolino et al., GCN 41677) with the LCO 1m telescope at South African Astronomical Observatory equipped with the Sinistro instrument.
Our observation started on 2025-09-03 at 23:25:00 UT (6.0 hr after the trigger) and we obtained 4x300 s exposures using the SDSS r filter. In the stacked image, we detected the optical afterglow (An et al., GCN 41679, Xin et al., GCN 41684, Pérez-Fournon et al. GCN 41685) within the errorbox of SVOM/MXT (Gotz et al., GCN 41681).
We derived the following magnitude r = 20.04 +/- 0.11 mag calibrated by the Guide Star Catalog, Version 2.3.2 (GSC2.3) (STScI, 2006) and not corrected for Galactic extinction.
This project is funded by the SVOM collaboration.
GCN Circular #41688
H. Z. Wu (HUST), R.-Z. Li (YNAO, CAS), M. J. Liu and H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250903A (SVOM/sb25090304, Maiolino et al. GCN 41677) at 2025-09-03T18:20:54 (UTC), about 1 hour after the SVOM/ECLAIRs trigger, with an exposure time of 6300s. One uncatalogued source is detected within the ECLAIRs error circle, and the source is spatially consistent with the counterpart reported in optical and X-ray bands (Maiolino et al. GCN 41677, An et al. GCN 41679, Götz et al. GCN 41681, Xin et al. GCN 41683, GCN 41684, Pérez-Fournon et al. GCN 41685). Preliminary analysis on this source are automatically conducted, and details are listed as follows.
Source 1: EPF_J234711.4-755801
```
RA (J2000): 356.7979
Dec (J2000): -75.9668
Flux: 2.41 x 10^-12 erg/s/cm2 (observed, 0.5-10 kev)
Flux_err: 1.56 x 10^-13 erg/s/cm2 (1 sigma)
```
The position uncertainty of the source is about 10 arcsec in radius (90% C.L. statistical and systematic).
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 #41691
A. Saccardi (CEA/Irfu), J. An (NAOC), A. Martin-Carrillo (UCD), G. Corcoran (UCD), N. Habeeb (Leicester), A. J. Levan (Radboud U.), D. B. Malesani (DAWN/NBI and Radboud), G. Pugliese (API), B. Schneider (LAM), S. D. Vergani (LUX-Paris Obs.), D. Xu (NAOC) report on behalf of the Stargate collaboration:
We observed the optical counterpart (An et al., GCN 41679; Xin et al., GCN 41684; Pérez-Fournon et al., GCN 41685; Saccardi et al., GCN 41686) of GRB 250903A detected by SVOM/ECLAIRs (Maiolino et al., GCN 41677), using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
From the 30 s X-Shooter acquisition image at 2025-09-04 01:36 UT (~8.18 hr post-trigger), the optical afterglow is well detected with a magnitude of r ~ 20.5 (AB) calibrated against SkyMapper nearby stars and not corrected by Galactic extinction.
Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 900 s each. Observations started on 2025-09-04 at approximately 01:38 UT (~8.22 hr after the SVOM/ECLAIRs trigger).
In a preliminary reduction of the spectra, we detect a continuum over the entire covered wavelength range. From detection in the blue down to ~3190 AA we set a redshift upper limit z < 1.62. Furthermore, we detect several absorption lines due to C IV, Si IV, Si II, Fe II, Al II, Mg II, Mg I, from which we infer a redshift of z = 1.590.
While the lack of fine-structure lines formally prevents us from securely associating the z = 1.590 system to the GRB, the lack of any unidentified features (and the reasonable S/N of the spectra), coupled with the tight redshift upper limit, indicate that most likely GRB 250903A was indeed at z = 1.590.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Cecilia Bustos, Martina Baratella, and Rob Van Holstein. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).

GCN Circular #41699
P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected source GRB 250903A, collecting 2.4 ks of Photon Counting (PC) mode data between T0+12 ks and T0+24 ks after the trigger. We have detected 2 sources inside the ECLAIRs error region. These have been automatically classified as follows:

  * 0 likely counterparts
  * 0 candidate counterparts
  * 2 uncatalogued X-ray sources
  * 0 known X-ray sources

Uncatalogued X-ray sources
--------------------------
Source 1 (SWIFT J234712.1-755757):
==================================

    RA (J2000.0):   356.8008  =  23 47 12.19
    Dec (J2000.0):  -75.9660  =  -75 57 57.6
    Error:	    4.3 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    2.7 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    0.0387 [+0.0049, -0.0047] ct s^-1
    Mean flux:	    (1.33 [+0.17, -0.16])e-12 erg cm^-2 s^-1
    Peak rate:	    0.052 +/- 0.014 ct s^-1
    Peak flux:	    (1.80 +/- 0.47)e-12 erg cm^-2 s^-1
    ECF:	    3.42e-11 erg cm^-2 ct^-1
		      assuming NH=4.68e+20 cm^-2, gamma=1.95
		      determined from a spectral fit.
    XMM UL:	    2.0e-12 erg cm^-2 s^-1, (0.3-10 keV)
		      so the source is not above this 3-sigma upper limit.
    The source may be fading, at the 0.8-sigma level.

Source 2 (SWIFT J234819.4-755214):
==================================

    RA (J2000.0):   357.0812  =  23 48 19.49
    Dec (J2000.0):  -75.8707  =  -75 52 14.5
    Error:	    6.4 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    4.8 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    (8.6 [+2.9, -2.4])e-3 ct s^-1
    Mean flux:	    (1.06 [+0.36, -0.30])e-11 erg cm^-2 s^-1
    Peak rate:	    (8.6 [+2.9, -2.4])e-3 ct s^-1
    Peak flux:	    (1.06 [+0.36, -0.30])e-11 erg cm^-2 s^-1
    ECF:	    1.23e-09 erg cm^-2 ct^-1
		      assuming NH=4.41e+20 cm^-2, gamma=-2.35
		      determined from a spectral fit.
    XMM UL:	    1.9e-12 erg cm^-2 s^-1, (0.3-10 keV)
		      so the source is 2.7-sigma above this 3-sigma upper limit.
    There is no evidence for fading.
    A SIMBAD object `LEDA  242511' is 4.1" away.
    There is 1 2MASS object within the source's 3-sigma error radius.

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.
Source 1 position is consistent with the optical counterpart discovered by TRT (GCN 41679).
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/SVOM/SVOM_FIELD00034.
This circular is an official product of the Swift-XRT team.

GCN Circular #41720
S. P. R. Shilling (Lancaster U.) reports on behalf of the Swift/UVOT team:
Swift/UVOT has performed follow-up observations of the SVOM/ECLAIRs-detected source GRB 250903A. Swift/UVOT observed the field of GRB 250903A (Maiolino et al., GCN 41677) for 2.3 ks in the U-band starting at 20:57:46 UT, 3.5 hours after the detection by SVOM/ECLAIRs.
A source consistent with the TRT optical counterpart position (An et al., GCN 41679), within the SVOM/ECLAIRs error circle, is detected and is fading. The preliminary detection magnitude reported below is calculated using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373).
```
Filter         T_start(s)   T_stop(s)      Exp(s)        Mag

u              12770        24862          2330          19.71 +/- 0.11
```
GCN Circular #41746
E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-OAR), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Perri (SSDC & INAF-OAR) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of GRB 250903A. We searched for X-ray sources in 2.4 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 6.3 ks, obtained between T0+12.7 ks and T0+344.1 ks.
Four uncatalogued X-ray sources are detected within the estimated 3-sigma SVOM/ECLAIRs error region (509 arcsec), of which one ("Source 1") is fading with >3-sigma significance and its position is consistent with the optical afterglow (An et al., GCN 41679). Therefore, this is the GRB afterglow. Using 2370 s of PC mode data and 3 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 356.79798, -75.96644 which is equivalent to:
```
RA (J2000): 23h 47m 11.52s
Dec(J2000): -75d 57' 59.2"
```
with an uncertainty of 3.0 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay index of alpha=0.9 (+/-0.9).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.95 (+0.73, -0.21). The best-fitting absorption column is consistent with the Galactic value of 4.7 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     4 (+/-10) x 10^20 cm^-2
Galactic foreground: 4.7 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.95 (+0.73, -0.21)
```
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/03000070. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00034.
This circular is an official product of the Swift-XRT team.
GCN Circular #41785
Nicolas Dagoneau (CEA), Jean-Luc Atteia (IRAP), Floriane Cangemi (APC), Tais Maiolino (LUPM), Claire Guépin-Detrigne (LUPM)
Using the event-by-event data downloaded through the X-band ground station, we report further analysis of ECLAIRs observations of GRB 250903A (SVOM burst-id sb25090304).
The burst that triggered ECLAIRs onboard (GCN 41677) was detected in the first trigger cycle that followed the SAA exit. Consequently, the precise duration of the event cannot be determined. The exposures in which the burst was detected onboard have durations spanning from 5.12 to 40.96 s starting from T0-13 s to T0+3 s (T0 = 2025-09-03T17:24:56 UTC).
The time-averaged spectrum (from T0-8.7s to T0+7.5s) in the energy range 4-120 keV is best fit by a cutoff power-law with a photon index 1.28 +/- 0.22 and a peak energy Epeak 38 +/- 15 keV. With this model, the total fluence in the 4-120 keV band is (7.66 -5.78/+0.90)e-7 erg/cm^2.
With a redshift of z = 1.59 (GCN 41691), the burst isotropic energy Eiso (from 1 keV to 10 MeV in its rest frame) is 6.4e51 erg. This is only a lower bound since we may have missed the beginning of the burst when SVOM was in the SAA. Still, this value makes this GRB consistent with type II GRBs in the 'Amati' relation diagram (Amati et al. 2002).
The spectrum is also well fit by a simple power-law model with a photon index 1.62 +/- 0.08. With this model, the total fluence in the 4-120 keV band is (9.15 -1.74/+1.06) e-7 erg/cm^2.
All the quoted errors are at the 90% confidence level.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe.
The SVOM/ECLAIRs point of contact for this burst is: Nicolas Dagoneau (CEA) (nicolas.dagoneau at cea.fr).

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