GCN Circular 12351

B. Sbarufatti (INAF-OAB/IASFPA) and S. T. Holland (CRESST/USRA/GSFC)
report on behalf of the Swift-XRT team:

We have analysed 3.3 ks of XRT data for GRB 110915A (Holland  et al.
GCN Circ. 12335
Not found
 
 
), from 82 s to 7.7 ks after the  BAT trigger. The data
comprise 1.6 ks in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN. Circ 12336).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=5.6 (+0.6, -0.4). At T+120 s  the decay
flattens to an alpha of 0.49 (+0.15, -1.09) before breaking again at
T+309 s to a final decay with index alpha=0.84 (+0.04, -0.03).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.98 (+/-0.05). The
best-fitting absorption column is  2.69 (+0.18, -0.17) x 10^21 cm^-2,
in excess of the Galactic value of 5.3 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.04 (+0.21, -0.20)
and a best-fitting absorption column of 2.6 (+/-0.7) 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 (6.3 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.6 (+/-0.7) x 10^21 cm^-2
Galactic foreground: 5.3 x 10^20 cm^-2
Excess significance: 6.5 sigma
Photon index:	     2.04 (+0.21, -0.20)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00503219.

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