GCN Circular 15311

C. Pagani (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A.
Maselli  (INAF-IASFPA), D.N. Burrows (PSU), J.A. Kennea (PSU), V.
Mangano (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester)
and  report on behalf of the Swift-XRT team: report on behalf of the
Swift-XRT team:

We have analysed 9.1 ks of XRT data for GRB 131004A,  from 79 s to 34.7
ks after the  BAT trigger. The data are entirely in Photon Counting
(PC) mode.  the light curve is consistent with a constant source of
mean count rate 7.4e-02 ct/sec. A power-law fit gives an index of 0.1
(+2.0, -1.6).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.16 (+0.27, -0.26). The
best-fitting absorption column is  3.1 (+1.0, -0.9) x 10^21 cm^-2, in
excess of the Galactic value of 7.0 x 10^20 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.9 x 10^-11 (6.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.1 (+1.0, -0.9) x 10^21 cm^-2
Galactic foreground: 7.0 x 10^20 cm^-2
Excess significance: 4.7 sigma
Photon index:	     2.16 (+0.27, -0.26)

If the light curve continues to decay with a power-law decay index of
0.1, the count rate at T+24 hours will be 0.029 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.9 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00573190.

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