GCN Circular 18147

A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), B.P. Gompertz
(U. Leicester), P. D'Avanzo (INAF-OAB), A. D'ai (INAF-IASFPA), T.G.R.
Roegiers (PSU), L.M. McCauley (PSU), J.A. Kennea (PSU) and V. D'Elia
report on behalf of the Swift-XRT team:

We have analysed 11 ks of XRT data for GRB 150817A (D'Elia et al. GCN
Circ. 18143
Not found
 
 
), from 56 s to 29.2 ks after the  BAT trigger. The data
comprise 32 s in Windowed Timing (WT) mode (the first 7 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 Osborne et
al. (GCN Circ. 18146
Not found
 
 
).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.6 (+/-0.4). At T+104 s  the decay
flattens to an alpha of 0.57 (+0.06, -1.00) before breaking again at
T+4285 s to a final decay with index alpha=1.07 (+/-0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.77 (+/-0.11). The
best-fitting absorption column is  4.5 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 2.7 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 4.5 x 10^-11 (6.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.5 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 2.7 x 10^21 cm^-2
Excess significance: 4.6 sigma
Photon index:	     1.77 (+/-0.11)

If the light curve continues to decay with a power-law decay index of
1.07, the count rate at T+24 hours will be 0.021 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.7 x
10^-13 (1.4 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/00652334.

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