TITLE: GCN CIRCULAR NUMBER: 10563 SUBJECT: GRB 100331B: Swift/UVOT Upper Limits DATE: 10/04/02 02:12:21 GMT FROM: Michele Stark at PSU/Swift M. A. Stark (PSU), G. Stratta (ASDC), E. A. Hoversten (PSU), and M. H. Siegel (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of the SuperAGILE detected GRB 100331B about 17.5 hours after the SuperAGILE detection (Del Monte et al., GCN #10560). No optical afterglow consistent with the XRT enhanced position reported by D'Elia & Stratta (GCN #10562) is detected in the initial UVOT exposure. Observations were conducted using only the UVOT white filter, with a preliminary 3-sigma upper limit using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) for the initial exposure of: Filter T_start(s) T_stop(s) Exp(s) Mag --------------------------------------------------------------- white 63371 64517 650 >21.21 The value quoted above is not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.12 in the direction of the burst (Schlegel et al. 1998).