GCN Circular 13804

A. Melandri (INAF-OAB), D.N. Burrows (PSU), J.A. Kennea (PSU), M.C.
Stroh (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester),
P.A. Evans (U. Leicester), B. Sbarufatti (INAF-OAB/PSU), G. Stratta
(ASDC) and V.N. Yershov report on behalf of the Swift-XRT team:

We have analysed 9.3 ks of XRT data for GRB 120922A (Yershov  et al.
GCN Circ. 13793
Not found
 
 
), from 120 s to 19.4 ks after the  BAT trigger. The
data comprise 484 s 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 13794).

The light curve can be modelled with a series of power-law decays. The
light curve initially rises, with an index alpha=-1.5 (+0.0, -1.3). At
T+130 s it breaks to an alpha of 3.09 (+0.09, -0.08). The light curve
breaks again at T+243 s to a decay with alpha=3.83 (+0.09, -0.10), 
before a final break at T+737 s s after which the decay index is 0.85
(+0.04, -0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.33 (+/-0.04). The
best-fitting absorption column is  2.15 (+/-0.09) x 10^21 cm^-2, in
excess of the Galactic value of 7.7 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.11 (+0.13, -0.12)
and a best-fitting absorption column of 1.06 (+0.28, -0.27) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.5 x 10^-11 (4.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.06 (+0.28, -0.27) x 10^21 cm^-2
Galactic foreground: 7.7 x 10^20 cm^-2
Excess significance: 1.8 sigma
Photon index:	     2.11 (+0.13, -0.12)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00534394.

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