GCN Circular 13025

N. P. M. Kuin (MSSL-UCL) and W. H. Baumgartner (GSFC/UMBC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 120308A
158 s after the BAT trigger (Baumgartner et al., GCN Circ. 13017
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 ).
A source consistent with the optical position reported by Virgili et al.
(GCN Circ. 13018
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 ) is detected in the initial UVOT exposures, but only
in the v and b band. In u there is no evidence for a source, which
suggests that the source is at a redshift of z < 4, and in case there
is no contamination of dust, z ~ 3.2.

The best UVOT position is

   RA(J2000)  = 14h 36m 20.05s
   Dec(J2000) = 79d 41' 12.2"

with an uncertainty of 0.55" (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

v                  463          483           20         17.02 +/- 0.17
b                  413          432           19         17.65 +/- 0.13
u                  158         7690         1020        >20.9
w1                 512        13962         1165        >21.3
m2                 636        13409         1415        >21.3
w2                 438         8103          646        >21.5
b                  711          730           19         18.40 +/- 0.25
v                  761         1082           39         18.14 +/- 0.29

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.03 in the direction of the burst
(Schlegel et al. 1998).