GCN Circular 15151

B. Sbarufatti (INAF-OAB/PSU), V. Mangano (PSU), M.C. Stroh (PSU), D.N.
Burrows (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester),
P.A. Evans (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC) and
L.M.Z. Hagen report on behalf of the Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 130831A (Hagen  et al. GCN
Circ. 15139
Not found
 
 
),  from 115 s to 23.9 ks after the	BAT trigger. The data
comprise 73 s in Windowed Timing (WT) mode (the first 10 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Evans et
al. (GCN. Circ 15147).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.0 (+0.4, -0.6). At T+239 s  the decay
flattens to an alpha of -0.1 (+0.3, -0.6) before breaking again at
T+783 s to a final decay with index alpha=1.09 (+0.09, -0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.53 (+0.12, -0.11). The
best-fitting absorption column is  consistent with the Galactic value
of 4.8 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a
photon index of 1.78 (+0.12, -0.08) and a best-fitting absorption
column of 5.2 (+2.8, -0.4) x 10^20 cm^-2. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.9 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.2 (+2.8, -0.4) x 10^20 cm^-2
Galactic foreground: 4.8 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.78 (+0.12, -0.08)

If the light curve continues to decay with a power-law decay index of
1.09, the count rate at T+24 hours will be 0.024 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.5 x
10^-13 (1.1 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/00568849.

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