TITLE:   GCN CIRCULAR
NUMBER:  14678
SUBJECT: GRB 130518A: Swift XRT and UVOT possible afterglow
DATE:    13/05/19 16:05:05 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

P.A. Evans (U.Leicester), M. de Pasquale (MSSL/UCL), M.C. Stroh (PSU)
and M. H. Siegel (PSU) report on behalf of the Swift team:

Swift conducted a series of observations, tiled on the sky, of the
Fermi/LAT GRB 130518A. In these observations we find an uncatalogued
source at RA,Dec = 355.6658, +47.4653 degrees (J2000.0) which is
equivalent to:

RA(J2000.0) =    23h 42m 39.79s
Dec (J2000.0) = +47d 27' 55.2"

with an uncertainty of 3.6 arcsec (radius, 90% confidence).

This source shows weak evidence of fading: a power-law fit gives a decay
index of 0.66 (+/- 0.80); at this level we therefore cannot confirm
whether this is the afterglow of GRB 130518A.

The spectrum of this source can be fitted with an absorbed power-law
with a spectral index of 2.5 (+0.8, -0.6). The best-fitting absorption
column is 2.4 (+2.2, -1.4) x 10^21 cm^-2, consistent with the Galactic
value of 1.0 x 10^21 cm^-2 (Kalberla et al., 2005).

UVOT observed the field of the XRT source in u filter between 30.5 ks
and 59.9 ks after the trigger, for a total exposure time of 2.7 ks. We
detect an optical object with magnitude  u = 19.0 ± 0.1 just outside the
3.6 arcsec XRT error circle. The source shows weak indication of  fading
between the beginning and the end of observations.

This circular is an official product of the Swift team.