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GCN Circular 26609

Subject
GRB 191228A: Swift-XRT refined Analysis
Date
2019-12-28T19:22:36Z (5 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
J. D. Gropp (PSU), A. Tohuvavohu (U. Toronto), J.A. Kennea (PSU), A.P.
Beardmore (U. Leicester), P.A. Evans (U. Leicester), J.P. Osborne (U.
Leicester), E. Ambrosi	(INAF-IASFPA) , M. Capalbi (INAF-IASFPA) and
M.G. Bernardini report on behalf of the Swift-XRT team:

We have analysed 7.7 ks of XRT data for GRB 191228A (Bernardini et al.
GCN Circ. 26601), from 127 s to 48.0 ks after the  BAT trigger. The
data comprise 216 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. 26606).

The late-time light curve (from T0+5.4 ks) can be modelled with a
power-law decay with a decay index of alpha=1.58 (+0.13, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.96 (+/-0.05). The
best-fitting absorption column is  3.09 (+0.24, -0.23) x 10^21 cm^-2,
in excess of the Galactic value of 3.8 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.14 (+0.16, -0.15)
and a best-fitting absorption column of 4.0 (+/-0.7) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (6.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.0 (+/-0.7) x 10^21 cm^-2
Galactic foreground: 3.8 x 10^20 cm^-2
Excess significance: 8.7 sigma
Photon index:	     2.14 (+0.16, -0.15)

If the light curve continues to decay with a power-law decay index of
1.58, the count rate at T+24 hours will be 8.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.9 x
10^-13 (5.0 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/00946476.

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