TITLE:   GCN CIRCULAR
NUMBER:  9877
SUBJECT: GRB 090902B:  Swift/UVOT refined analysis
DATE:    09/09/03 19:55:45 GMT
FROM:    Craig Swenson at PSU/Swift  <cswenson@astro.psu.edu>

C. A. Swenson (PSU) and G. Stratta (ASDC) report on behalf of the
Swift/UVOT team:

The Swift/UVOT began observations of the field of GRB 090902B
approximately 12.5 hours after the Fermi GBM/LAT trigger (de Palma
et al., GCN Circ. 9867; Bissaldi et al., GCN Circ. 9866).  We can
now confirm the detection of the optical afterglow (Swenson et al.,
GCN Circ. 9869; Perley et al., GCN Circ. 9870; Cucchiara et al.,
GCN Circ. 9873; Olivares et al., GCN Circ. 9874; Guidorzi et al.,
GCN Circ. 9875) coincident with the enhanced Swift-XRT position (Evans,
GCN Circ. 9871).  We also observe a decay in the optical afterglow.

The observed magnitudes using the UVOT photometric system (Poole et
al. 2008, MNRAS, 383, 627) are:

Filter       T_start(s)     T_stop(s)      Exp(s)         Mag

u              45096          46188         1075        20.26 ± 0.19
u              50891          58703         3613        20.50 ± 0.11
u              62457          81106         8756        20.85 ± 0.10

The values quoted above are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.04 in the direction of the burst
(Schlegel et al. 1998).