GCN Circular 18161

L. M. McCauley (PSU), L. M. Z. Hagen (PSU), and V. D'Elia (ASDC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150818A
94 s after the BAT trigger (D'Elia et al., GCN Circ. 18152
Not found
 
 
).
A source consistent with the XRT position (Beardmore et al. GCN Circ. 18159
Not found
 
 
)
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  15:21:25.43 = 230.35594 (deg.)
    Dec (J2000) = +68:20:33.0  =  68.34251 (deg.)
with an estimated uncertainty of 0.66 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            94          121           27         19.39+-0.25
white             3414         4790          344         19.92+-0.17
white            10222        10826          590         19.68+-0.07
v                 3572         5201          393        >19.36
b                 4385         4585          196        >20.16
b                 5821        10217          927         20.56+-0.33
u                 4180         4379          196        >19.66
u                 5616         5816          196         19.63+-0.25
uvw1              3976         4175          195         18.78+-0.20
uvw1              5411         5611          196         18.95+-0.20
uvm2              3776         3976          192         18.81+-0.23
uvm2              5206         5406          196         18.89+-0.24
uvw2              4796         4996          196         18.90+-0.19

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.02 in the direction of the burst
(Schlegel et al. 1998).