GCN Circular 40394

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

Swift-XRT has conducted further observations of the field of the
SVOM/MXT-detected burst GRB 250506A. The observations now extend from
T0+63.4 ks to T0+138.3 ks and have a total exposure time of 6.3 ks. The
source previously reported, "Source 1", is fading with 2.8 sigma
significance and thus is believed to be the GRB afterglow. Using 2772 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 = 219.28269, +28.87835 which is equivalent
to:

RA (J2000): 14h 37m 07.85s
Dec(J2000): +28d 52' 42.0"

with an uncertainty of 2.6 arcsec (radius, 90% confidence). This
position is 84 arcsec from the SVOM/MXT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.7 (+0.5, -0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.74 (+0.34, -0.21). The
best-fitting absorption column is  2.1 (+7.8, -0.4) x 10^20 cm^-2,
consistent with the Galactic value of 1.6 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 3.8 x 10^-11 (3.9 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.1 (+7.8, -0.4) x 10^20 cm^-2
Galactic foreground: 1.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.74 (+0.34, -0.21)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021828.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021828.

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