GCN Circular 13999

B. Sbarufatti (INAF-OAB/PSU), G. Stratta (ASDC), M.C. Stroh (PSU), D.N.
Burrows (PSU), J.A. Kennea (PSU), P.A. Evans (U. Leicester), O.M.
Littlejohns (U. Leicester), J.P. Osborne (U. Leicester), P. D'Avanzo
(INAF-OAB) and B.N. Barlow report on behalf of the Swift-XRT team:

We have analysed 9.6 ks of XRT data for GRB 121125A (Barlow  et al. GCN
Circ. 13993
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 ), from 73 s to 23.5 ks after the  BAT trigger. The data
comprise 196 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 13998).

The late-time light curve (from T0+4.0 ks) can be modelled with  a
power-law decay with a decay index of alpha=1.28 (+/-0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.02 (+0.10, -0.09). The
best-fitting absorption column is  1.49 (+0.23, -0.22) x 10^21 cm^-2,
in excess of the Galactic value of 1.3 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.14 (+0.13, -0.12)
and a best-fitting absorption column of 1.02 (+0.28, -0.26) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.4 x 10^-11 (4.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.02 (+0.28, -0.26) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^20 cm^-2
Excess significance: 5.6 sigma
Photon index:	     2.14 (+0.13, -0.12)

If the light curve continues to decay with a power-law decay index of
1.28, the count rate at T+24 hours will be 0.013 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.2 x
10^-13 (5.7 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00539563.

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