GCN Circular 42566

R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), S.
Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 6.8 ks of XRT data for GRB 251103B, from 62 s to 23.6
ks after the   trigger. The data comprise 143 s in Windowed Timing (WT)
mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+10.3 ks) can be modelled with a
power-law decay with a decay index of alpha=1.0 (+/-0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.21 (+/-0.08). The
best-fitting absorption column is  9.0 (+/-0.7) x 10^21 cm^-2, in
excess of the Galactic value of 3.3 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.09 (+0.22, -0.21)
and a best-fitting absorption column of 4.9 (+1.2, -1.1) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.0 x 10^-11 (6.8 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.9 (+1.2, -1.1) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^21 cm^-2
Excess significance: 2.5 sigma
Photon index:	     2.09 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
1.0, the count rate at T+24 hours will be 0.014 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.4 x
10^-13 (9.2 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01409824.

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