GCN Circular 42445

Subject
GRB 251025B: Swift-XRT afterglow detection
Event
GRB 251025B
Date
2025-10-25T19:52:19Z (5 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), B.
Sbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB),
J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans
(U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of GRB 251025B. We
searched for X-ray sources in  1.7 ks of Photon Counting (PC) mode data
obtained between T0+2.8 ks and T0+4.5 ks.

An uncatalogued X-ray source is detected within the estimated 3-sigma
SVOM/ECLAIRs error region (473 arcsec) and is above the RASS 3-sigma
upper limit at this position, and is therefore likely the GRB
afterglow. Using 1712 s of PC mode data and 1 UVOT image, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 29.53107, +30.30373
which is equivalent to:

RA (J2000): 01h 58m 07.46s
Dec(J2000): +30d 18' 13.4"

with an uncertainty of 2.7 arcsec (radius, 90% confidence). This source
is 1.25 arcsec from the SVOM/C-GFT position (Wu et al., GCN Circ.
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 ) and 37 arcsec from the SVOM/MXT position (Hussein et al., GCN
Circ. 42437Loading...
 
 ). At present, the X-ray source shows no evidence of fading.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.00 (+0.27, -0.25). The
best-fitting absorption column is  1.9 (+0.9, -0.8) x 10^21 cm^-2, in
excess of the Galactic value of 5.6 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 (4.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.9 (+0.9, -0.8) x 10^21 cm^-2
Galactic foreground: 5.6 x 10^20 cm^-2
Excess significance: 2.8 sigma
Photon index:	     2.00 (+0.27, -0.25)

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/03000153.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/SVOM/SVOM_FIELD00044.

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