TITLE:   GCN CIRCULAR
NUMBER:  12384
SUBJECT: GRB 110921A: Swift-XRT refined Analysis
DATE:    11/09/22 08:05:33 GMT
FROM:    Phil Evans at U of Leicester  <pae9@star.le.ac.uk>

P.A. Evans (U. Leicester) and W. H. Baumgartner (GSFC/UMBC) report on
behalf of the Swift-XRT team:

We have analysed 9.7 ks of XRT data for GRB 110921A (Baumgartner  et
al. GCN Circ. 12373), from 134 s to 40.9 ks after the  BAT trigger. The
data comprise 47 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Goad et al. (GCN. Circ 12379).

The light curve can be modelled with  a power-law decay with a decay
index of alpha=0.90 (+/-0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.00 (+0.15, -0.14). The
best-fitting absorption column is  2.0 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 1.4 x 10^21 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.9 x 10^-11 (5.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.0 (+0.5, -0.4) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^21 cm^-2
Excess significance: 7.9 sigma
Photon index:	     2.00 (+0.15, -0.14)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00503652.

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