GCN Circular 13963

A. Melandri (INAF-OAB) and T. N. Ukwatta (MSU) report on behalf of the
Swift-XRT team:

We have analysed 24 ks of XRT data for GRB 121108A (Ukwatta  et al. GCN
Circ. 13952
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 ), from 47 s to 82.6 ks after the  BAT trigger. The data
comprise 129 s in Windowed Timing (WT) mode (the first 6 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 13959).

The light curve (from T0+0.8 ks) can be modelled with a power-law decay
with a decay index of alpha=1.0 (+/-0.1). Flare activity is detected
from T0+100 and T0+200 s after the burst event.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.60 (+/-0.09). The
best-fitting absorption column is  3.6 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 1.9 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.70 (+0.20, -0.18)
and a best-fitting absorption column of 2.3 (+0.7, -0.4) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.6 x 10^-11 (6.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.3 (+0.7, -0.4) x 10^21 cm^-2
Galactic foreground: 1.9 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.70 (+0.20, -0.18)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00537921.

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