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
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 ), 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
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 ).

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.