GCN Circular 15154

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

We have analysed 6.8 ks of XRT data for GRB 130831B (Malesani  et al.
GCN Circ. 15141
Not found
 
 
),  from 112 s to 35.4 ks after the  BAT trigger. The
data comprise 8 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. The enhanced
XRT position for this burst was given by Goad et al. (GCN. Circ 15149).

The late-time light curve (from T0+5.1 ks) can be modelled with  a
power-law decay with a decay index of alpha=1.24 (+0.12, -0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.07 (+0.16, -0.15). The
best-fitting absorption column is  1.6 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 6.2 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 3.7 x 10^-11 (5.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+/-0.4) x 10^21 cm^-2
Galactic foreground: 6.2 x 10^20 cm^-2
Excess significance: 4.2 sigma
Photon index:	     2.07 (+0.16, -0.15)

If the light curve continues to decay with a power-law decay index of
1.24, the count rate at T+24 hours will be 9.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.5 x
10^-13 (5.0 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/00568855.

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