TITLE:   GCN CIRCULAR
NUMBER:  14124
SUBJECT: GRB 121229A: Swift-XRT refined Analysis
DATE:    12/12/30 04:13:04 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

G. Stratta (ASDC), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), J.A.
Kennea (PSU), M.C. Stroh (PSU), D.N. Burrows (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), C. Pagani (U. Leicester) and E.
Sonbas report on behalf of the Swift-XRT team:

We have analysed 13 ks of XRT data for GRB 121229A (Sonbas  et al. GCN
Circ. 14115),  from 135 s to 68.4 ks after the	BAT trigger. The data
comprise 438 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Goad et al.
(GCN. Circ 14121).

The late-time light curve (from T0+3.9 ks) can be modelled with  a
power-law decay with a decay index of alpha=0.23 (+0.20, -0.22).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.14 (+/-0.05). The
best-fitting absorption column is  1.62 (+0.24, -0.23) x 10^22 cm^-2,
at a redshift of 2.707, in addition to the Galactic value of 1.4 x
10^21 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon
index of 2.14 (+0.33, -0.13) 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 3.4 x
10^-11 (4.8 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.4 x 10^21 cm^-2
Intrinsic column:    0 (+8.4, -0) x 10^21 cm^-2 at z=2.707
Photon index:	     2.14 (+0.33, -0.13)

If the light curve continues to decay with a power-law decay index of
2.00, the count rate at T+24 hours will be 7.6 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 x
10^-13 (3.7 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/00544347.

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