GCN Circular 12727

B.-B. Zhang and M. H. Siegel (PSU) reports on behalf of the Swift-XRT
team:

We have analysed 10 ks of XRT data for GRB 111225A (Siegel  et al. GCN
Circ. 12720
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 ), from 94 s to 23.3 ks after the  BAT trigger. The data
comprise 556 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 12724).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=0.6 (+0.4, -0.3). At T+114 s  the decay
steepens to an alpha of 1.83 (+0.10, -0.09). The light curve breaks
again at T+353 s to a decay with alpha=3.1 (+/-0.5),  before a final
break at T+590 s s after which the decay index is 1.80 (+0.11, -0.12).
An early time flare (from T+214.6 to T+321.8 s) and a possible late
time flare (from ~T+20000s) are excluded from the fit.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.43 (+/-0.07). The
best-fitting absorption column is  2.09 (+0.16, -0.15) x 10^21 cm^-2,
in excess of the Galactic value of 1.7 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.71 (+0.17, -0.15)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.5 x 10^-11 (5.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     0 (+7.7, -0) x 10^20 cm^-2
Galactic foreground: 1.7 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.71 (+0.17, -0.15)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00510341.

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