GCN Circular 42347

M.G. Bernardini (INAF-OAB), E. Ambrosi	(INAF-IASFPA) , S. Dichiara
(PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester),
A.P. Beardmore (U. Leicester), M. Capalbi (INAF-OAR) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 5.3 ks of XRT data for GRB 251017A, from 96 s to 39.9
ks after the   trigger. The data comprise 75 s in Windowed Timing (WT)
mode (the first 11 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+4.7 ks) can be modelled with a
power-law decay with a decay index of alpha=1.1 (+0.4, -0.3).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.67 (+/-0.15). The
best-fitting absorption column is  1.4 (+/-0.5) x 10^21 cm^-2,
consistent with the Galactic value of 8.9 x 10^20 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 1.9 (+0.8, -0.7) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.5 x 10^-11 (4.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.9 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 8.9 x 10^20 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
1.1, the count rate at T+24 hours will be 7.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.5 x
10^-13 (3.5 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/01404904.

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