GCN Circular 33752

R. A. J. Eyles-Ferris (U Leicester), P. A. Evans (U Leicester),
J.D. Gropp (PSU), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), P. Romano (INAF-OAB), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB) and M. A. Williams (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 12:06:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230510A (trigger=1167973).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 318.156, +34.443 which is 
   RA(J2000) = 21h 12m 37s
   Dec(J2000) = +34d 26' 33"
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 60 sec.  The peak count rate
was ~5100 counts/sec (15-350 keV), at ~13 sec after the trigger. 

The XRT began observing the field at 12:07:31.9 UT, 63.4 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 318.1314, 34.4424 which is equivalent to:
   RA(J2000)  = 21h 12m 31.54s
   Dec(J2000) = +34d 26' 32.6"
with an uncertainty of 6.5 arcseconds (radius, 90% containment). This
location is 73 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

The initial flux in the 0.1 s image was 1.19e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 72 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 expected extinction corresponding to E(B-V) of
0.144. 

This trigger was initially marked as matching a source in the BAT
ground catalogue: IGR J21117+3427, a known gamma-ray source. However,
the XRT localisation is more than 9 arcmin from the known position of
IGR J21117+3427, outside its error region of 3.5 arcmin (ATel 873). 
This, together with the BAT transient monitor light-curve showing
little sign of variation suggests that this is in fact a new GRB. 

Burst Advocate for this burst is R. A. J. Eyles-Ferris (raje1 AT leicester.ac.uk). 
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/)