[vsnet-grb-info 15031] Swift trigger 599860 probably not real

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16325
SUBJECT: Swift trigger 599860 probably not real
DATE:    14/05/24 14:03:58 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien at nasa.gov>

S. D. Barthelmy (GSFC), V. D'Elia (ASDC), N. Gehrels (NASA/GSFC),
L. Izzo (URoma/ICRA), A. Y. Lien (NASA/GSFC/ORAU),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 13:25:47 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 140524A (trigger=599860).  Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 53.405, -36.096 which is
   RA(J2000) = 03h 33m 37s
   Dec(J2000) = -36d 05' 46"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty).  This is a low-threshold trigger due to its position
matching a nearby galaxy in the on-board catalog. As is usual with an image
trigger, the available BAT light curve shows no significant structure.

The XRT began observing the field at 13:28:12.0 UT, 144.2 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 53.4016, -36.1396 which is
equivalent to:
   RA(J2000)  = 03h 33m 36.39s
   Dec(J2000) = -36d 08' 22.5"
with an uncertainty of 2.6 arcseconds (radius, 90% containment). This
location is 157 arcseconds from the BAT onboard position, within the
BAT error circle; and in the nearby AGN NGC1365 in the BAT catalogue. The
XRT position may be improved as more data are received; the latest position is
available at http://www.swift.ac.uk/sper. This position is 3.0 arcseconds from
that of a known X-ray source: 3XMM J033336.4-360825 in the XMM-NEWTON XMMSSC
catalogue which we assume is the AGN X-ray emission from the galaxy.

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.39
x 10^20 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 148 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of
the BAT error circle. The typical 3-sigma upper limit has been about 19.6 mag.
The 8'x8' region for the list of sources generated on-board covers 100% of the
BAT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.02.