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GCN Circular 12869

GRB 120118B: Swift-XRT refined Analysis
2012-01-19T09:56:37Z (12 years ago)
Owen Littlejohns at U of Leicester <>
O. M. Littlejohns (U Leicester) reports on behalf of the Swift-XRT

We have analysed 8.6 ks of XRT data for GRB 120118B (Littlejohns  et
al. GCN Circ. 12852), from 96 s to 33.9 ks after the  BAT trigger. The
data comprise 19 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
Osborne et al. (GCN. Circ 12857).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.9 (+0.5, -0.4). At T+312 s  the decay
flattens to an alpha of -0.25 (+0.24, -0.21) before breaking again at
T+2735 s to a final decay with index alpha=0.99 (+/-0.15).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.22 (+0.20, -0.18). The
best-fitting absorption column is  2.7 (+/-0.6) x 10^21 cm^-2, in
excess of the Galactic value of 6.9 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 3.6 x 10^-11 (6.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.7 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 6.9 x 10^20 cm^-2
Excess significance: 6.0 sigma
Photon index:	     2.22 (+0.20, -0.18)

The results of the XRT-team automatic analysis are available at

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