GCN Circular 18329

A. D'ai (INAF-IASFPA), A. Maselli  (INAF-IASFPA), A. Melandri
(INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N.
Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), 
L. Izzo (URoma/ICRA) and V. D'Elia (ASDC) report on behalf of the
Swift-XRT team:

We have analysed 12 ks of XRT data for the Swift/BAT-detected burst GRB
150915A (D'Elia et al. GCN Circ. 18315
Not found
 
 
), from 118 s to 126.0 ks after
the  Swift/BAT trigger. The data comprise 256 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 refined XRT position is RA,
Dec = 319.6584, -34.9136 which is equivalent to:

RA (J2000): 21 18 38.01
Dec(J2000): -34 54 49.1

with an uncertainty of 3.7 arcsec (radius, 90% confidence). This
position is 5.1 arcmin from the Swift/BAT position. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.5 (+/-0.3), followed by a break at T+170 s to an alpha
of 3.94 (+0.15, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.79 (+/-0.04). The
best-fitting absorption column is  7.8 (+1.7, -1.6) x 10^21 cm^-2, at a
redshift of 1.968, in addition to the Galactic value of 8.6 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 3.7 x
10^-11 (4.5 x 10^-11) erg cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Galactic foreground: 8.6 x 10^20 cm^-2
Intrinsic column:    7.8 (+1.7, -1.6) x 10^21 cm^-2 at z=1.968
Photon index:	     1.79 (+/-0.04)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00655721.

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