GCN Circular 18016

M.G. Bernardini, A. Melandri, P. D'Avanzo (INAF-OAB) and P.A. Evans
(Leicester) report on behalf of the Swift-XRT team:

We have analysed 6.9 ks of XRT data for GRB 150711A (Bernardini et al.
GCN Circ. 18011
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 ), from 96 s to 28.3 ks after the  BAT trigger. The data
comprise 115 s in Windowed Timing (WT) mode (the first 9 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. 18012
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.90 (+0.17, -0.13), followed by a break at T+446 s to
an alpha of 0.62 (+0.07, -0.06). Flaring activity is observed between
T+150 s and T+225 s.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.71 (+/-0.14). The
best-fitting absorption column is  4.2 (+/-0.5) x 10^21 cm^-2, in
excess of the Galactic value of 8.2 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.20 (+0.19, -0.18)
and a best-fitting absorption column of 4.2 (+0.9, -0.8) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.5 x 10^-11 (6.3 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.2 (+0.9, -0.8) x 10^21 cm^-2
Galactic foreground: 8.2 x 10^20 cm^-2
Excess significance: 6.8 sigma
Photon index:	     2.20 (+0.19, -0.18)

If the light curve continues to decay with a power-law decay index of
0.62, the count rate at T+24 hours will be 0.022 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.8 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/00648601.

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

[GCN OPS NOTE(12jul15): Per MGB's request, AM, PDA, and PAE
were added to the author list.]