GCN Circular 19841

B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), A.
Maselli  (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB),
T.G.R. Roegiers (PSU) and M.H. Siegel report on behalf of the Swift-XRT
team:

We have analysed 9.5 ks of XRT data for GRB 160821B (Siegel et al. GCN
Circ. 19833
Not found
 
 
), from 56 s to 35.9 ks after the  BAT trigger. The data
comprise 198 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 enhanced XRT position for this burst was given by Evans et
al. (GCN Circ. 19837
Not found
 
 
).

The late-time light curve (from T0+4.9 ks) can be modelled with a
power-law decay with a decay index of alpha=0.5 (+/-0.3).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.95 (+0.08, -0.07). The
best-fitting absorption column is  7.5 (+2.0, -1.7) x 10^20 cm^-2,
consistent with the Galactic value of 5.8 x 10^20 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 (3.9 x
10^-11) erg cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     7.5 (+2.0, -1.7) x 10^20 cm^-2
Galactic foreground: 5.8 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.95 (+0.08, -0.07)

If the light curve continues to decay with a power-law decay index of
0.5, the count rate at T+24 hours will be 3.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (1.4 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/00709357.

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