TITLE: GCN CIRCULAR NUMBER: 8179 SUBJECT: GRB 080903: Swift/UVOT Observations DATE: 08/09/03 19:06:16 GMT FROM: Frank Marshall at GSFC F.E. Marshall (NASA/GSFC) and D. Grupe (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT observed the field of GRB 080903 starting 75 s after the BAT trigger (Grupe et al., GCN Circ. 8169). A source is detected in the initial exposure in the V filter at the position of the optical afterglow (de Ugarte Postigo et al., GCN Circ. 8172). There is also a marginal (1.8 sigma) detection in the initial exposures with the U filter. A valid detection in the U filter would indicate a redshift of less than 3.3. The magnitudes and 3-sigma upper limits in the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) for detecting a source in the first finding chart (FC) exposure and subsequent co-added exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag v (FC) 75 475 393 19.8 +/- 0.24 v 595 28705 2596 >21.5 b 555 12429 1307 >21.8 u 530 829 58 20.0 +/- 0.47 u 4906 34528 3049 >21.8 uvw1 506 7268 551 >21.0 uvm2 481 7160 655 >20.7 uvw2 571 6750 452 >21.2 The values quoted above are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.22 in the direction of the burst (Schlegel et al. 1998).