GCN Circular 19591

P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), T.G.R.
Roegiers (PSU), L.M. McCauley (PSU), J.A. Kennea (PSU), S.L. Gibson (U.
Leicester), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and
A. Maselli report on behalf of the Swift-XRT team:

We have analysed 6.4 ks of XRT data for GRB 160625A (Maselli et al. GCN
Circ. 19577
Not found
 
 
), from 1.3 ks to 48.7 ks after the	BAT trigger. The data
comprise 1.7 ks 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. 19583
Not found
 
 
).

The late-time light curve (from T0+7.4 ks) can be modelled with a
power-law decay with a decay index of alpha=0.71 (+0.31, -0.28).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+/-0.5). The
best-fitting absorption column is  2.2 (+0.9, -0.7) x 10^22 cm^-2, in
excess of the Galactic value of 1.0 x 10^22 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.8 x 10^-11 (1.6 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.2 (+0.9, -0.7) x 10^22 cm^-2
Galactic foreground: 1.0 x 10^22 cm^-2
Excess significance: 2.8 sigma
Photon index:	     2.2 (+/-0.5)

If the light curve continues to decay with a power-law decay index of
0.71, the count rate at T+24 hours will be 9.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x
10^-13 (1.6 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00701503.

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