S. J. LaPorte (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU),
A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), J.P. Osborne
(U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P.
D'Avanzo (INAF-OAB) and M.J. Moss report on behalf of the Swift-XRT
We have analysed 6.2 ks of XRT data for GRB 181228A (Moss et al. GCN
Circ. 23570), from 88 s to 58.6 ks after the BAT trigger. The data
comprise 405 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Beardmore et al. (GCN Circ. 23574).
The late-time light curve (from T0+12.9 ks) can be modelled with a
power-law decay with a decay index of alpha=0.95 (+0.29, -0.28).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.60 (+/-0.05). The
best-fitting absorption column is 3.30 (+0.28, -0.27) x 10^21 cm^-2,
in excess of the Galactic value of 2.9 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.80 (+0.20, -0.19)
and a best-fitting absorption column of 5.3 (+1.4, -1.2) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.6 x 10^-11 (6.8 x 10^-11) erg cm^-2
A summary of the PC-mode spectrum is thus:
Total column: 5.3 (+1.4, -1.2) x 10^21 cm^-2
Galactic foreground: 2.9 x 10^21 cm^-2
Excess significance: 3.2 sigma
Photon index: 1.80 (+0.20, -0.19)
If the light curve continues to decay with a power-law decay index of
0.95, the count rate at T+24 hours will be 0.012 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x
10^-13 (8.4 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
This circular is an official product of the Swift-XRT team.