GCN Circular 36536

M. H. Siegel (PSU), J.D. Gropp (PSU), R. Gupta (NASA/GSFC),
J. A. Kennea (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII),
H. A. Krimm (NSF), K. L. Page (U Leicester), D. M. Palmer (LANL) and
M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 14:43:11 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240523A (trigger=1230423).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 33.567, -22.020 which is 
   RA(J2000) = 02h 14m 16s
   Dec(J2000) = -22d 01' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 40 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~-1 sec after the trigger. 

The XRT began observing the field at 14:45:39.6 UT, 147.7 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 33.56813,
-22.03320 which is equivalent to:
   RA(J2000)  = 02h 14m 16.35s
   Dec(J2000) = -22d 01' 59.5"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 47 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 https://www.swift.ac.uk/sper. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 151 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
0.015. 

Burst Advocate for this burst is M. H. Siegel (siegel AT swift.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/)