GCN Circular 41108

M.A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), A. Melandri
(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
Fermi/GBM-detected burst GRB 250716A. We searched for X-ray sources in 
2.6 ks of Photon Counting (PC) mode data. The total exposure at the
position of the afterglow (see below) is 2.6 ks, obtained between
T0+79.5 ks and T0+91.7 ks.

An uncatalogued X-ray source is detected within the GOTO error region
and is above the RASS 3-sigma upper limit at this position, and is
therefore likely the GRB afterglow. Using 2790 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 = 66.77171, -46.56649 which is equivalent to:

RA (J2000): 04h 27m 05.21s
Dec(J2000): -46d 33' 59.4"

with an uncertainty of 2.7 arcsec (radius, 90% confidence). This
position is 1.9 arcsec from the GOTO position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.755 (+0.015, -0.755).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.22 (+0.30, -0.27). The
best-fitting absorption column is  6.9 (+6.4, -5.2) x 10^20 cm^-2, in
excess of the Galactic value of 1.2 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 3.0 x 10^-11 (3.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.9 (+6.4, -5.2) x 10^20 cm^-2
Galactic foreground: 1.2 x 10^20 cm^-2
Excess significance: 1.8 sigma
Photon index:	     2.22 (+0.30, -0.27)

The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/ToO_GRBs/00021845/Source1.php.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021845.

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