TITLE:   GCN CIRCULAR
NUMBER:  19464
SUBJECT: GRB 160521B: Swift-XRT afterglow detection
DATE:    16/05/25 19:12:11 GMT
FROM:    Andrea Melandri at INAF-OAB  <andrea.melandri@brera.inaf.it>

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

Swift-XRT has conducted further observations of the field of the
Fermi/LAT-detected burst GRB 160521B (Axelsson et al. GCN Circ. 19444).
The observations now extend from T0+41.9 ks to T0+346.3 ks. The source
previously reported by Page et al. (GCN Circ. 19448), "Source 1", is
fading with 3-sigma significance, and is therefore likely the GRB
afterglow.  The position of this source is RA, Dec=147.6665, +79.0305
which is equivalent to:

RA (J2000): 09:50:39.96
Dec(J2000): +79:01:49.7

with an uncertainty of 4.2 arcsec (radius, 90% confidence).  This
position is 5.9 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.7 (+0.7, -0.4).

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00020615/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020615.

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