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GCN Circular 17827

GRB 150518A: Swift-XRT afterglow detection
2015-05-19T10:18:56Z (9 years ago)
Claudio Pagani at U of Leicester <>
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) 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
The results of the full analysis of the tiled XRT observations are
available at

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