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GCN Circular 12643

Subject
GRB 111209A: Swift-XRT refined Analysis
Date
2011-12-09T19:34:18Z (12 years ago)
From
Dirk Grupe at PSU/Swift-XRT <dxg35@psu.edu>
D. Grupe (PSU) and E. A. Hoversten (PSU) reports on behalf of the
Swift-XRT team:

We have analyzed 11 ks of XRT data for GRB 111209A (Hoversten  et al.
GCN Circ. 12632), from 425 s to 25.2 ks after the  BAT trigger. The
data comprise 9.4 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 12639).

The light curve can be modeled with a series of power-law decays. The
light curve initially rises, with an index alpha=-0.21 (+/-0.04). At
T+864 s it breaks to an alpha of 7.3 (+0.7, -2.6). The light curve
breaks again at T+899 s to a decay with alpha=0.679 (+0.012, -0.087), 
and  again at T+5807 s s to alpha=1.179 (+/-0.016),  before a final
break at T+19.4 ks s after which the decay index is 6.3 (+1.2, -0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.333 (+/-0.012). The
best-fitting absorption column is  1.38 (+/-0.04) x 10^21 cm^-2, in
excess of the Galactic value of 1.5 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.83 (+0.17, -0.16)
and a best-fitting absorption column of 1.3 (+0.5, -0.4) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.1 x 10^-11 (5.2 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.3 (+0.5, -0.4) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^20 cm^-2
Excess significance: 4.4 sigma
Photon index:	     1.83 (+0.17, -0.16)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00509336.

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