GRB 250108A

W. Zheng (UCB), Z-Q. Wang (GXU), W.-J. XIE (NAOC, CAS), D.-H. ZHAO (NAOC,
CAS), S. Guillot (IRAP), D. Götz (CEA), H. Goto (CEA), C-W. Wang (IHEP)
report on behalf of the SVOM team.

The SVOM/ECLAIRs telescope triggered and located the long duration GRB
250108A (sb25010802) starting at 2025-01-08T04:37:01.513 UTC (Tb).

The following trigger information was received on the ground with
low-latency by the SVOM VHF Alert Network.

The burst was detected by both the on-board Count-Rate Trigger (CRT) and
Image Trigger (IMT) and 9 alerts were received. The best detection is
obtained by IMT with a signal-to-noise ratio of 10.2 in the 5-8 keV energy
band over a time window of 40.96 s starting at Tb. The light curve in
ECLAIRs with a preliminary T90 duration is about 40 s.

The localization of the best Alert is RA, Dec = 207.946, 26.204 (J2000).
The uncertainty on this position is 7.79 arcminutes at 90% C.L. which
includes 2 arcminutes of systematic uncertainty in quadrature.

The SVOM/GRM light curve can be found here:

https://www.bursthub.cn//admin/static/svgrb250108A.png

SVOM performed an automatic slew and MXT started observing the ECLAIRs
error box at 04:40:49 UTC, i.e. 228 s after Tb. An uncatalogued X-ray
source has been found by the on board software at R.A., Dec. = 207.906,
26.232 (J2000) (13:51:38, +26:13:56, J2000) with an uncertainty of 60 arc
sec (90% C.L., including systematics). This source is located at 2.7 arc
minutes from the ECLAIRs position.

VT results will be reported later.

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.



The SVOM point of contact for this burst is: Weikang Zheng (
weikang@berkeley.edu)

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a ToO observation of the SVOM/ECLAIRs GRB 250108A. 
Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021755

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the SVOM/ECLAIRs event. Any X-ray source
considered to be a probable afterglow candidate will be reported via a 
GCN Circular after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

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

SVOM/VT commissioning team: Y. L. Qiu, H. L. Li (NAOC), W. K. Zheng (UCB), Z. Q. Wang (GXU), X. H. Han, L. P. Xin, C. Wu, J. Wang, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang, Z. H. Yao (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Olivier Godet (IRAP), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Shao-Lin Xiong (IHEP), Bing Zhang (UNLV)

report on behalf of the SVOM team:

The SVOM/VT conducted follow-up observations of the GRB 250108A (Zheng et al., GCN 38845
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 ) in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously. 

With the X band downlinked data, an uncatalogued optical source was detected within the errorbox of MXT (Zheng et al., GCN 38845
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 ) in VT_R stacked image, compared to the DESI catalog. It is the only new source within the error box down to the 3 sigma limit of about 22.7 mag. 

The brightness of the source was estimated to be 22.1+/-0.2 mag in AB magnitude in VT_R image at the mid time of 1.516 hours post the burst, with a total exposure time of 8*100 seconds. The source can not be detected in VT_R 19*100 sec stacked image with a 3 sigma limit of 23.0 mag at the middle time of 3.978 hours. 

The source is located at RA, Dec = 207.89654, 26.23741, which is equivalent to:
RA (J2000) : 13:51:35.17
Dec (J2000): +26:14:14.68
with an uncertainty of 0.5 arcsec.

Additionally, no any new sources were detected within the errorbox of XRT #1 source (Phil Evans et al., GCN 38846
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 ).

We proposed that this source is the candidate for the gamma-ray burst. 

More deep follow-ups are encouraged to confirm the nature of the source. 

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.

E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), J.A. Kennea
(PSU), D.N. Burrows (PSU), M. A. Williams (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 the
SVOM/ECLAIRs-detected burst GRB 250108A. We searched for X-ray sources
in  2.7 ks of Photon Counting (PC) mode data. The total exposure at the
position of the afterglow (see below) is 3.2 ks, obtained between
T0+16.1 ks and T0+34.4 ks.

An uncatalogued X-ray source is detected within the estimated 3-sigma
SVOM/MXTs error region (98 arcsec) and is above the LSXPS 3-sigma upper
limit at this position, and is therefore likely the GRB afterglow.
Using 2670 s of PC mode data and 2 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 = 207.92093, +26.21642 which is
equivalent to:

RA (J2000): 13h 51m 41.02s
Dec(J2000): +26d 12' 59.1"

with an uncertainty of 4.1 arcsec (radius, 90% confidence). This
position is 73 arcsec from the SVOM/MXTs position.  The source has a
mean count rate of 1.7e-02 ct/sec; we cannot determine at the present
time whether it is fading.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021755.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021755.

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

S. De Wet (DTU Space), D. B. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF/OACn and DARK/NBI), D. Xu (NAOC), S. Grund Sorensen (NOT and Aarhus), N. Pyykkinen (NOT and Turku) report on behalf of a larger collaboration:

We observed the field of the SVOM GRB 250108A (Zheng et al., GRB 38845) using the Nordic Optical Telescope (NOT) equipped with ALFOSC. The observations were carried out in the z band, with a mid time 2025 Jan 9.14 UT (22.72 hr after the GRB), and for a total exposure time of 2000 s.

In the stack of our images, we do not detect any source at either the position of the VT optical afterglow candidate (Qiu et al., GCN 38852
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 ), nor within the Swift/XRT localization region of the proposed X-ray counterpart (Ambrosi et al., GCN 38853
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 ). The limiting magnitude is approximately z > 22.1 (AB), calibrated against nearby stars from the Pan-STARRS catalog.

A. A. Breeveld (UCL-MSSL) and N. J. Klingler (GSFC/UMBC/CRESSTII) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of SVOM/ECLAIRs-detected burst GRB 250108A 102468 s after the SVOM trigger (Zheng et al., GCN Circ. 38845
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 ).
No optical afterglow consistent with either the Swift-XRT proposed X-ray counterpart (Ambrosi et al., GCN Circ. 38853
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 ) or the VT optical afterglow candidate (Qiu et al., GCN Circ. 38852
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 ), is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial exposures are:

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

w1              102468       109462         2632         >20.9

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.012 in the direction of the burst (Schlegel et al. 1998).

Tullia Sbarrato (INAF-OAB) & Noel Klinger (GSFC/UMBC/CRESSTII)
report on behalf of the Swift-XRT team:

Swift-XRT has performed further follow-up observations of the
SVOM/ECLAIRs GRB 250108A (Zheng et al., GCN 38845
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 ), 
collecting 6.6 ks of Photon Counting (PC) mode data between 
T0+16.1 ks and T0+192.1 ks. 

The X-ray source detected in the first 2.7 ks of data reported as 
“Source 1” and presented as possible GRB afterglow candidate 
(Ambrosi et al., GCN 38853
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 ) has faded more than 2 sigma 
in the latest observation, and it is thus confirmed as the 
X-ray afterglow of the burst.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021755.

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