GCN Circular 14053

M.C. Stroh (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), C.
Pagani (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (ASDC),
V. Mangano (INAF-IASFPA), J.A. Kennea (PSU) and A. Maselli report on
behalf of the Swift-XRT team:

We have analysed 18 ks of XRT data for GRB 121209A (Maselli  et al. GCN
Circ. 14045
Not found
 
 
),  from 95 s to 48.0 ks after the  BAT trigger. The data
comprise 28 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 14051).

The late-time light curve (from T0+5.2 ks) can be modelled with  a
power-law decay with a decay index of alpha=1.20 (+/-0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.31 (+/-0.14). The
best-fitting absorption column is  5.3 (+/-0.6) x 10^21 cm^-2, in
excess of the Galactic value of 3.8 x 10^20 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.0 x 10^-11 (9.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.3 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 3.8 x 10^20 cm^-2
Excess significance: 13.4 sigma
Photon index:	     2.31 (+/-0.14)

If the light curve continues to decay with a power-law decay index of
1.20, the count rate at T+24 hours will be 0.014 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.8 x
10^-13 (1.3 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00540964.

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