GCN Circular 38823

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

We have analysed 3.5 ks of XRT data for GRB 250103B, from 144 s to 44.7
ks after the  BAT trigger. The data comprise 31 s in Windowed Timing
(WT) mode with the remainder in Photon Counting (PC) mode. The best
available XRT position	(using the promptly downlinked event data, the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue) is RA, Dec = 54.66413, -33.74945 which is equivalent to:

RA (J2000): 03 38 39.39
Dec(J2000): -33 44 58.0

with an uncertainty of 1.9 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=3.9 (+/-0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 4.2 (+0.7, -0.5). The
best-fitting absorption column is  6.3 (+6.6, -4.5) x 10^20 cm^-2, in
excess of the Galactic value of 1.0 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 2.5 x 10^-11 (4.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     6.3 (+6.6, -4.5) x 10^20 cm^-2
Galactic foreground: 1.0 x 10^20 cm^-2
Excess significance: 2.0 sigma
Photon index:	     4.2 (+0.7, -0.5)

If the light curve continues to decay with a power-law decay index of
3.9, the count rate at T+24 hours will be 2.5 x 10^-10 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.3 x
10^-21 (1.2 x 10^-20) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01278865.

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