TITLE:   GCN CIRCULAR
NUMBER:  33556
SUBJECT: GRB 230328B: Swift/UVOT Detection
DATE:    23/04/02 23:10:18 GMT
FROM:    Mike Siegel at PSU/Swift MOC  <mhs18@psu.edu>

M. H. Siegel (PSU) and J. D. Gropp (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 230328B
114 s after the BAT trigger (Gropp et al., GCN Circ. 33527).
A source consistent with the XRT position (Osborne et al. GCN Circ. 33529) is
detected in the initial UVOT exposures, confirming the initial
optical detections (Pankov et al., GCN Circ. 33528; Catapano & Izzo, GCN Circ.
33535; Suresh et al., GCN Circ. 33536; Adami et al., GCN Circ. 33537; Komesh
et al., GCN Circ. 33539).

The preliminary UVOT position is:
    RA  (J2000) =  19:24:01.88 = 291.00783 (deg.)
    Dec (J2000) = +80:00:34.6  =  80.00962 (deg.)
with an estimated uncertainty of 0.44 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 (fc)         114          264          147         18.38 +/- 0.05
White             5537         5735          196         20.16 +/- 0.14
white            28147        28879          713         21.88 +/- 0.32
v                  655          849           39        >17.6
b                  582          774           39        >18.3
u (fc)             326          576          246         19.20 +/- 0.16
w1                 705          725           19        >18.5
w2                 805          825           19        >18.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.070 in the direction of the burst
(Schlegel et al. 1998).