GCN Circular 38686

C. Salvaggio (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M. A.
Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 9.1 ks of XRT data for GRB 241228A, from 82 s to 28.8
ks after the  BAT trigger. The data comprise 83 s in Windowed Timing
(WT) mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+4.8 ks) can be modelled with a
power-law decay with a decay index of alpha=1.54 (+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 2.00 (+0.18, -0.17). The
best-fitting absorption column is  2.2 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 6.7 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.86 (+0.15, -0.14)
and a best-fitting absorption column of 3.6 (+0.8, -0.7) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.2 x 10^-11 (6.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.6 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 6.7 x 10^20 cm^-2
Excess significance: 6.9 sigma
Photon index:	     1.86 (+0.15, -0.14)

If the light curve continues to decay with a power-law decay index of
1.54, the count rate at T+24 hours will be 4.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x
10^-13 (2.7 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/01277277.

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