GCN Circular 18059

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

We have analysed 11 ks of XRT data for GRB 150724A (Racusin et al. GCN
Circ. 18050
Not found
 
 
), from 91 s to 24.4 ks after the  BAT trigger. The data
comprise 349 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 Goad et al.
(GCN Circ. 18052
Not found
 
 
). The late-time light curve (from T0+4.7 ks) is
consistent with a constant source of mean count rate 2.2e+00 ct/sec. A
power-law fit gives an index of -0.02 (+0.19, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.70 (+/-0.04). The
best-fitting absorption column is  3.70 (+0.21, -0.20) x 10^21 cm^-2,
in excess of the Galactic value of 2.9 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.13 (+0.18, -0.17)
and a best-fitting absorption column of 3.6 (+0.8, -0.7) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (5.8 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.6 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 2.9 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.13 (+0.18, -0.17)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00650141.

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