GCN Circular 21135

J.G. Gropp (PSU), S. D. Barthelmy (GSFC), C. Gronwall (PSU),
J. A. Kennea (PSU), H. A. Krimm (NSF/USRA), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC) and D. M. Palmer (LANL) report on behalf of the
Swift Team:

At 19:49:15 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170524A (trigger=754322).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 319.456, +48.587, which is 
   RA(J2000)  =  21h 17m 49s
   Dec(J2000) = +48d 35' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 0.25 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 19:50:16.0 UT, 60.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 319.4869, 48.6065 which
is equivalent to:
   RA(J2000)  = 21h 17m 56.86s
   Dec(J2000) = +48d 36' 23.4"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 101 arcseconds from the BAT onboard position, within the
BAT error circle. This position may be improved as more data are
received; the latest position is available at
http://www.swift.ac.uk/sper.  We cannot determine whether the source is
fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data does not constrain the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 67 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 100% of
the XRT 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
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the large, but uncertain extinction expected. 

We note that this is a short burst, close to the Galactic plane 
(lat = -0.5 degrees), although far from the Galactic center
(lon = 91 degrees).  Thus there is a possibility that this
is a previously unknown Soft Gamma Repeater (SGR). 
However, the preliminary 4-energy lightcurve shows more
flux above 100 keV than is typical for an SGR burst. 

Burst Advocate for this burst is J.G. Gropp (jdg44 AT psu.edu). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)