TITLE:   GCN CIRCULAR
NUMBER:  29929
SUBJECT: GRB 210504A: Swift detection of a burst
DATE:    21/05/04 14:39:18 GMT
FROM:    Phil Evans at U of Leicester  <pae9@star.le.ac.uk>

A. P. Beardmore (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
B. Sbarufatti (PSU) and M. H. Siegel (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

Swift Burst Alert Telescope (BAT) triggered and located GRB 210504A
(trigger=1046782), with the trigger time most likely between 13:40
and 13:58 UT. No BAT information is available at this moment. 

XRT began observing the field of the new trigger at 13:58:28 UT. 
In a promptly-downlinked image we find an X-ray source at
RA,Dec = 222.3933, -30.5322 which is equivalent to 

RA (J2000)  = 14:49:34.4
Dec (J2000) = -30:31:56

with an estimated uncertainty of about 10". 

We currently have very limited data products and are awaiting the 
full data after a ground station pass to further characterise
this object. 

UVOT took a finding chart exposure of ​150 seconds with the ​white filter. 
No credible afterglow candidate has been found in the initial data products. 
The 2.7'x2.7' sub-image covers none of the XRT error circle. 
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 extinction in the Milky Way. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.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/)