TITLE:   GCN CIRCULAR
NUMBER:  9969
SUBJECT: GRB 090929B: Swift/UVOT Observations
DATE:    09/09/29 19:13:57 GMT
FROM:    Peter Curran at MSSL  <pac@mssl.ucl.ac.uk>

P.A. Curran (MSSL-UCL), F. Marshall (GSFC), S. Holland (GSFC) and C.
Pagani (PSU) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 090929B 94s 
after the BAT trigger (Pagani et al., GCN Circ. 9963). No optical 
afterglow is detected in the b, u or uvw2 UVOT exposures. There is a 
detection in the white filter, consistent with the enhanced XRT position 
(Goad et al. GCN 9964, Page et al. GCN 9965), the proposed optical 
counterpart (Wren et al. GCN 9967) and a USNO catalogued star (R2=19.3). 
Since the photometry is contaminated by a bright (R = 9.5), near-by star, 
we cannot determine the nature of the source at this time.


Preliminary magnitudes and 3-sigma upper limits using the UVOT photometric 
system (Poole et al. 2008, MNRAS, 383, 627) for the initial exposures are:

Filter       T_start(s)   T_stop(s)     Exp(s)      Mag
----------------------------------------------------------------
white        94           4032          292         18.7 +/- 0.5
b            3627         3827          197         >20.5
u            3422         3622          197         >20.3
uvw2         4038         4224          183         >20.3

The values quoted above are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.07 in the direction of the burst
(Schlegel et al. 1998).