GCN Circular 14168

J.A. Kennea (PSU), M.C. Stroh (PSU), D.N. Burrows (PSU), A.P. Beardmore
(U. Leicester), P.A. Evans (U. Leicester), O.M. Littlejohns (U.
Leicester), A. Maselli	(INAF-IASFPA), V. Mangano (INAF-IASFPA), A.
Melandri (INAF-OAB) and D. Grupe report on behalf of the Swift-XRT
team:

We have analysed 9.2 ks of XRT data for GRB 130131A (Grupe  et al. GCN
Circ. 14156
Not found
 
 
),  from 64 s to 18.8 ks after the  BAT trigger. The data
comprise 235 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN. Circ 14160).

The late-time light curve (from T0+5.3 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 2.44 (+/-0.07). The
best-fitting absorption column is  3.17 (+/-0.19) x 10^21 cm^-2, in
excess of the Galactic value of 1.4 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.21 (+0.21, -0.20)
and a best-fitting absorption column of 2.4 (+/-0.6) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (6.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.4 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^20 cm^-2
Excess significance: 6.5 sigma
Photon index:	     2.21 (+0.21, -0.20)

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 2.6 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.3 x
10^-14 (1.6 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00547407.

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