GCN Circular 17827

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

Swift-XRT has performed follow-up observations of the MAXI-detected
burst GRB 150518A (Kawamuro et al. GCN Circ. 17825
Not found
 
 
) in a series of
observations tiled on the sky. The total exposure time is 2.4 ks,
distributed over 7 tiles; the maximum exposure at a single sky location
was 742 s. The data were collected between T0+10.4 ks and T0+20.2 ks,
and are entirely in Photon Counting (PC) mode. 

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=234.2006, +16.3291 which is equivalent to:

RA (J2000): 15:36:48.14
Dec(J2000): +16:19:44.9

with an uncertainty of 3.8 arcsec (radius, 90% confidence).  This
position is 8.2 arcmin from the MAXI position.	The light curve is
consistent with a constant source of mean count rate 4.0e-01 ct/sec. A
power-law fit gives an index of 0.4 (+0.7, -0.9).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 3.6 (+1.0, -0.8). The
best-fitting absorption column is  8.2 (+4.0, -3.2) x 10^21 cm^-2, in
excess of the Galactic value of 3.4 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 2.2 x 10^-11 (2.1 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.2 (+4.0, -3.2) x 10^21 cm^-2
Galactic foreground: 3.4 x 10^20 cm^-2
Excess significance: 4.1 sigma
Photon index:	     3.6 (+1.0, -0.8)

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/xrt_products/TILED_GRB00044/index_1.php.
The results of the full analysis of the tiled XRT observations are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00044.

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