GRB 250901A

Tais Maiolino (LUPM), Frédéric Piron (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 250901A (SVOM burst-id sb25090101). 

The burst that triggered ECLAIRs (GCN #41634
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 ) consists of a single pulse structure with a duration of T90 = 16 +1/-3 s in the 4-120 keV energy band. 

The time-averaged spectrum from T0-8 s to T0+10 s (T0 = 2025-09-01T18:14:22.916) in the energy range 5-120 keV is best fitted by a power-law model with photon index -1.15 +/- 0.09. With this model, the  4-120 keV fluence is (1.4 +/- 0.1) e-06 erg/cm^2. 

All the quoted errors are at the 68% confidence level.

We note that the calibration of SVOM/ECLAIRs is undergoing thus these results are preliminary
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. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC.

The SVOM/ECLAIRs point of contact for this burst is: Tais Maiolino (LUPM) (tais.maiolino at lupm.in2p3.fr)

M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250901A 2.6 ks after the SVOM/ECLAIRs trigger (Piron et al., GCN Circ. 41634
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 ). We do not detect any afterglow consistent with the XRT position (Osborne et al., GCN Circ. 41645
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 ) or optical position (Xin et al., GCN Circ. 41646
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 ; Malesani et al., GCN Circ. 41649
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 ).

Preliminary detections and 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: 

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

u                 2629         4244         1589           >21.01

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.023 in the direction of the burst
(Schlegel et al. 1998).

H. N. Yang (NAO, CAS), Z. Y. Liu (USTC), Y. J. Zhang (THU), C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of GRB 250901A (GCN 41634
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 ) at 2025-09-01T20:03:51 (UTC), about 2 hours after the SVOM/ECLAIRs detection, with an exposure time of 5 ks. The source was also followed by Swift-XRT (GCN 41645
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 ). The optical afterglow was also detected by SVOM/VT (GCN 41646
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 ). FXT detected an uncatalogued source at R.A. = 356.1845, DEC = -15.7418 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), consistent with the position of the source reported by Swift/XRT (offset is 4.0 arcsec). The FXT spectrum of this source can be fitted with an absorbed powerlaw with the absorption fixed at the Galactic value of 1.7x10^20 cm^-2, and a photon index of 1.97(+/-0.16). The derived unabsorbed 0.5-10 keV flux is approximately 2.4(+/-0.3)x10^-12 erg/s/cm2. 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).

D. B. Malesani (DAWN/NBI and Radboud), B. Schneider (LAM), G. Corcoran (UCD), A. Martin-Carrillo (UCD), A. Saccardi (CEA/Irfu), V. Vuolteenaho (NOT and Oulu Univ.), report on behalf of a larger collaboration:

We observed the optical counterpart (Xin et al., GCN 41646
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 ; Osborne et al., GCN 41645
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 ) of GRB 250901A (Piron et al., GCN 41634
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 ) using the Nordic Optical Telescope (NOT), located in the Canary Islands, equipped with the ALFOSC camera. Observations were carried out in the r (3x300 s) and z (5x200 s) bands.

In a preliminary stack of the images, the afterglow is faintly detected in both filters. At a mean epoch of 2025 September 2.00 UT (5.73 hr after the trigger), we measure an AB magnitude

r = 23.1 +- 0.2,

calibrated against nearby stars from the Pan-STARRS catalog, and not corrected for Galactic extinction.

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) , F. Piron (LUPM), C. Guépin (LUPM) report on behalf of the SVOM team.

SVOM/VT performed an automatic slew on the burst triggered by SVOM/ECLAIRs (Piron et al., GCN 41634
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 ). SVOM/VT began observing the field automatically at 2025-09-01T18:20:17.000 UTC,  355 seconds 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, an uncatalogued source is found in stacked images within the errorbox of SVOM/MXT (Piron et al., GCN 41634
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 ) and Swift/XRT (Osborne et al., GCN 41645
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 ) compared to Legacy survey. It is located at R.A. and DEC. 356.18453,  -15.74171 deg or

R.A., (J2000) = 23:44:44.28
DEC.  (J2000) = -15:44:30.16
The error is about 0.5 arcseconds.

The AB magnitudes were derived as follows:
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(T-T0)_mid  exptime   band      mag 
505 sec    300 sec   VT_R      19.66+/-0.03
505 sec    300 sec   VT_B      21.75+/-0.05

Given the color of VT_B-VT_R~2.0 mag, it might be a medium redshift GRB (Wang et al., 2020, RAA).

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.

J.P. Osborne (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo
(INAF-OAB), M.G. Bernardini (INAF-OAB), D.N. Burrows (PSU), S. Lanava
(PSU), S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of GRB 250901A. We
searched for X-ray sources in  754 s of Photon Counting (PC) mode data.
The total exposure at the position of the afterglow (see below) is 754
s, obtained between T0+2.6 ks and T0+3.3 ks.

An uncatalogued X-ray source is detected and is above the LSXPS 3-sigma
upper limit at this position, and is therefore likely the GRB
afterglow.  The position of this source is RA, Dec=356.1837, -15.7410
which is equivalent to:

RA (J2000): 23h 44m 44.08s
Dec(J2000): -15d 44' 27.7"

with an uncertainty of 3.9 arcsec (radius, 90% confidence). 

The light curve can be modelled with a power-law decay with a decay
index of alpha=3.2 (+1.8, -3.0).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+0.5, -0.4). The
best-fitting absorption column is  6 (+12, -4) x 10^20 cm^-2,
consistent with the Galactic value of 1.9 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 3.5 x 10^-11 (4.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6 (+12, -4) x 10^20 cm^-2
Galactic foreground: 1.9 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.9 (+0.5, -0.4)

If the light curve continues to decay with a power-law decay index of
3.2, the count rate at T+24 hours will be 2.9 x 10^-6 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x
10^-16 (1.1 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/03000066.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00033.

This circular is an official product of the Swift-XRT team.

A. Saccardi (CEA/Irfu), B. Schneider (LAM), D. Turpin (CEA/Irfu), B. Cordier (CEA/Irfu), report on behalf of the SVOM mission team:

We observed the field of the GRB 250901A detected by SVOM/ECLAIRs (Piron et al., GCN 41634
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 ) with the LCO 1m telescope at South African Astronomical Observatory equipped with the Sinistro instrument.

Our observation started on 2025-09-01 at 19:22:42 UT (68 minutes after the trigger) and we obtained 3x200 s exposures using the SDSS r filter. In the stacked image, we did not detect any new optical source at the position of the Swift/XRT source #1 (https://www.swift.ac.uk/SVOM/SVOM_FIELD00033/).

We derived the following upper limit r > 21.3 AB mag (3-sigma) calibrated using nearby stars from the PanSTARRS DR1 catalog and not corrected for Galactic extinction.

This project is funded by the SVOM collaboration.

F. Piron (LUPM), C. Guépin (LUPM), S. Schanne (CEA), T. Maiolino (LUPM), C.-W. Wang (IHEP), M. Moita (CEA), report on behalf of the SVOM mission team:

At 2025-09-01T18:14:22 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250901A (SVOM burst-id sb25090101).

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 9 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 10.49 in the [8-120] keV energy band over a time window of 20.48 seconds starting at 2025-09-01T18:14:17.

The localization of the best alert is R.A., Dec. 356.0950, -15.6712 degrees (J2000) with a 90% confidence level (C.L.) radius of 7.61 arcmin (including systematic error of 2 arcmin added in quadrature).

This burst was also detected by SVOM/GRM with a significance of 10. The SVOM/GRM light curve showed a single peak structure with a T90 duration of about 17 seconds.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250901A.png 

SVOM slewed to the burst.

SVOM/MXT began observing the field at 2025-09-01T18:17:20 UTC, 178 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 356.1835, -15.7952 degrees:
R.A. (J2000) = 23h44m44.04s
Dec. (J2000) = -15d47m42.73s
with a 90% C.L. radius of 194 arcseconds (including systematic error).

This location is 9.03 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.

VT began observing the field after the slew. The analysis of the data will be published in a future circular.

We notice that the source SWIFT J2344.7-1573 is located 3.9 arcmin from the MXT position and 5.4 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 Frédéric Piron: piron@in2p3.fr.
Please contact the BA by email if you require additional information.