TITLE:   GCN CIRCULAR
NUMBER:  34486
SUBJECT: GRB 230818A: Swift/UVOT Detection
DATE:    23/08/19 07:46:57 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:

The Swift/UVOT began settled observations of the field of GRB 230818A
144 s after the BAT trigger (Tohuvavohu et al., GCN Circ. 34479).
A source consistent with the optical position (Lipunov et al., GCN
Circ.  34476) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
   RA  (J2000) =  19:03:33.13 = 285.88806 (deg.)
   Dec (J2000) = +40:53:49.0  =  40.89695 (deg.)
with an estimated uncertainty of 0.48 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              144          294          147         19.09 +/- 0.07
v                  379          572           39         18.40 +/- 0.30
b                  477         1513          117         19.68 +/- 0.20
u                  452         1487          117        >19.5
w1                 428         1463           97        >20.2
w2                1026         4987          255        >20.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.101 in the direction of the burst
(Schlegel et al. 1998).