TITLE:   GCN CIRCULAR
NUMBER:  41359
SUBJECT: GRB 250813B: Swift-XRT afterglow detection
DATE:    25/08/14 08:09:11 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>

K.L. Page, A.P. Beardmore and P.A. Evans (U. Leicester) report on behalf
of the Swift-XRT team:

We have analysed 1.6 ks of XRT data for GRB 250813B, from 9.6 to 16.3
ks after the SVOM/ECLAIRs trigger (GCN Circ. 41354). The data are
entirely in Photon Counting (PC) mode. We find an uncatalogued X-ray
source at RA, Dec = 336.7505, +12.4613 which is equivalent to:

RA (J2000): 22 27 00.13
Dec(J2000): +12 27 40.5

with an uncertainty of 3.7 arcsec (radius, 90% confidence).

There is no evidence for fading during this interval, with the X-ray
source showing a mean count rate of 0.17 +/- 0.02 count s^-1. However,
the localisation of this source is consistent with the fading optical
afterglow found by Ondrejov D50 (GCN Circ. 41353, 41355) and
SVOM/COLIBRI (GCN Circ. 41356), and so we consider it the X-ray
afterglow.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.1 (+0.4, -0.3). The
best-fitting absorption column is 2.5 (+1.3, -1.1) × 10^21 cm^-2, in
excess of the Galactic value of 6.2 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 6.2 x 10^-12 (9.3 x
10^-12) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column:        2.5 (+1.3, -1.1) x 10^21 cm^-2
Galactic foreground: 6.2 x 10^20 cm^-2
Photon index:        2.1 (+0.4, -0.3)

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