TITLE:   GCN CIRCULAR
NUMBER:  29933
SUBJECT: GRB 210504A: Swift/UVOT Detection
DATE:    21/05/04 20:42:29 GMT
FROM:    Alice Breeveld at MSSL-UCL  <a.breeveld@ucl.ac.uk>

A. A. Breeveld (UCL-MSSL) and A. P. Beardmore (U. Leicester) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210504A 218 s after the BAT trigger 
(Beardmore et al., GCN Circ. 29929). A fading source is detected in the initial UVOT exposures.

The preliminary UVOT position is:
     RA  (J2000) =  14:49:34.03 = 222.39179 (deg.)
     Dec (J2000) = -30:32:02.2  = -30.53395 (deg.)
with an estimated uncertainty of 0.6 arc sec. (radius, 90% confidence).

Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 
2011, AIP Conf. Proc. 1358, 373) for the early exposures are:

Filter T_start(s) T_stop(s)     Exp(s)    Mag +/-
white     218      368           147      19.75 0.19
white    1168     1877            97      20.08 0.31
v         374      739            58     >18.26
b         473      837            58     >19.29
b        1144     1852            97      19.17 0.25
u         448      812            58     >18.89
uvw1      424      788            58     >18.49
uvm2      399      418            20     >17.24
uvw2      523     1040            59     >18.60