J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU),
S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo
(INAF-OAB) and J.K. Cannizzo report on behalf of the Swift-XRT team:
We have analysed 4.8 ks of XRT data for GRB 170728A (Cannizzo et al.
GCN Circ. 21367), from 84 s to 22.2 ks after the BAT trigger. The data
are entirely in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Goad et al. (GCN Circ. 21368).
The light curve can be modelled with a power-law decay with a decay
index of alpha=0.89 (+/-0.07).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.22 (+0.32, -0.25). The
best-fitting absorption column is 2.7 (+1.1, -0.4) x 10^21 cm^-2,
consistent with the Galactic value of 2.3 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 3.3 x 10^-11 (5.3 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 2.7 (+1.1, -0.4) x 10^21 cm^-2
Galactic foreground: 2.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index: 2.22 (+0.32, -0.25)
If the light curve continues to decay with a power-law decay index of
0.89, the count rate at T+24 hours will be 2.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.1 x
10^-14 (1.5 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.