TITLE: GCN CIRCULAR NUMBER: 10199 SUBJECT: GRB 091127: Swift/UVOT observations DATE: 09/11/28 05:17:52 GMT FROM: Stefan Immler at NASA/GSFC S. Immler (CRESST/UMCP/GSFC) and Troja (NASA/GSFC/ORAU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 091127 3219 s after the BAT trigger (Troja et al., GCN Circ. 10191). We detect the optical afterglow at the refined UVOT position RA, Dec 36.58288, -18.95239, which is RA (J2000) 02:26:19.89 Dec (J2000) -18:57:08.63 with an estimated uncertainty of 0.55 arcsec (radius, 90% confidence). This position is consistent with ground-based optical/IR detections of the afterglows (Smith et al., GCN 10192; Fugazza et al., GCN 10194; Updike et al., GCN 10195; Xu et al., GCN 10196; Gorosabel et al., GCN 10198). Preliminary magnitudes are reported below. Filter T_start(s) T_stop(s) Exposure(s) Mag Err ------------------------------------------------------------------------ white 3219 3369 147.4 16.2 +/- 0.1 v 3375 3575 196.6 17.0 +/- 0.1 uvw1 3785 5600 156.1 15.9 +/- 0.1 uvm2 3580 3780 196.6 15.6 +/- 0.1 ------------------------------------------------------------------------- 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, ApJS, 500, 525). The photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383, 627).