GCN Circular 12224

O. M. Littlejohns, P. A. Evans, A. P. Beardmore (U. Leicester) and S.
R. Oates (UCL-MSSL) report on behalf of the Swift-XRT team:

We have analysed 8.1 ks of XRT data for GRB 110731A (Oates  et al. GCN
Circ. 12215
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 ), from 56 s to 36.0 ks after the  BAT trigger. The data
comprise 607 s in Windowed Timing (WT) mode (the first 8 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 Beardmore
et al. (GCN. Circ 12219).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.04 (+0.05, -0.10). At T+502 s  the decay
steepens to an alpha of 1.183 (+0.027, -0.024) before breaking again at
T+29.0 ks to a final decay with index alpha=4.2 (+5.8, -2.0). We note
that the uncertainty in this final decay is large due to the small
number of data points after the final break.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.08 (+/-0.05). The
best-fitting absorption column is  2.05 (+0.14, -0.13) x 10^21 cm^-2,
in excess of the Galactic value of 1.0 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.92 (+/-0.12) and a
best-fitting absorption column of 1.7 (+0.4, -0.3) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.0 x 10^-11 (5.5 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.7 (+0.4, -0.3) x 10^21 cm^-2
Galactic foreground: 1.0 x 10^21 cm^-2
Excess significance: 8.4 sigma
Photon index:	     1.92 (+/-0.12)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00458448.

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