GCN Circular 17866

A.P. Beardmore (U. Leicester), A. Amaral-Rogers (U. Leicester), P.
D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'a��� (INAF-IASFPA), D.N.
Burrows (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU) and P.A.
Evans (U. Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 150523A collecting 1.4 ks of Photon
Counting (PC) mode data between T0+33.0 ks and T0+34.3 ks. 

An uncatalogued X-ray source is detected and is above the RASS limit,
and is therefore likely the GRB afterglow.  The position of this source
is RA, Dec=115.2859, -45.4209 which is equivalent to:

RA (J2000): 07:41:8.62
Dec(J2000): -45:25:15.1

with an uncertainty of 3.7 arcsec (radius, 90% confidence).  This
position is 3.4 arcmin from the Fermi/LAT position.  The light curve is
consistent with a constant source of mean count rate 1.2e-01 ct/sec. A
power-law fit gives an index of -1.41 (+4.59, -0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.71 (+0.29, -0.21). The
best-fitting absorption column is  consistent with the Galactic value
of 3.5 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 (5.9 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.5 (+/-1.3) x 10^21 cm^-2
Galactic foreground: 3.5 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.71 (+0.29, -0.21)

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00020520/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020520.

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