TITLE:   GCN CIRCULAR
NUMBER:  29912
SUBJECT: GRB 210413B: Swift/UVOT Observations
DATE:    21/04/30 00:54:43 GMT
FROM:    Mike Siegel at PSU/Swift MOC  <mhs18@psu.edu>

M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Swift/UVOT team:

Swift/UVOT performed follow-up observations of the Swift/BAT-detected burst GRB 210413B
(Tohuvavohu et al., GCN Circ. 29819) 185 ks after the BAT trigger. We detect a faint source
close to the XRT source reported by Kennea et al. (GCN Circ. 29827).

The preliminary UVOT position is:
   RA  (J2000) =  12:10:07.75 = 182.53230 (deg.)
   Dec (J2000) = +56:02:58.9  = 56.04970 (deg.)
with an estimated uncertainty of 0.4 arc sec. (radius, 90% confidence).

The preliminary detection using the UVOT photometric system (Breeveld et al. 2011, AIP Conf.
Proc. 1358, 373) is wh=19.68+-0.03 mag (Vega) or wh=20.48+-0.03 mag (AB).

We note, however, that the DECaLs survey shows faint source at this position at a g magnitude
of 21.09 mag  (AB).  It is thus not clear if this detection represents a brightening of that source,
which could indicate a host galaxy.