GCN Circular 26609

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
Not found
 
 
), 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
Not found
 
 
).

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.