TITLE:   GCN CIRCULAR
NUMBER:  38853
SUBJECT: GRB 250108A: Swift-XRT afterglow detection
DATE:    25/01/08 16:18:44 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

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.