TITLE: GCN CIRCULAR NUMBER: 8559 SUBJECT: GRB 081126: Swift/UVOT Observations DATE: 08/11/27 02:58:03 GMT FROM: Stephen Holland at USRA/NASA/GSFC/SSC S. T. Holland (CRESST/GSFC/USRA) and R. Margutti (Univ Bicocca & OAB) report on the behalf of the Swift UVOT team: The Swift/UVOT observed the field of GRB 081126 starting 56 s after the BAT trigger (Margutti, et al. 2008, GCN Circ. 8554). Settled exposures started at T+75 s. We do not find any new source, relative to the DSS or 2MASS, or any variable source inside the UVOT-enhanced XRT error circle (Osborne, et al. 2008, GCN Circ. 8556). Preliminary 3-sigma upper limits for detecting a source in the co-added images, using a 2.5 arcsecond radius circular aperture, are Filter T_start T_stop Exp(s) Mag ------------------------------------------ white 75 225 147 >21.3 u 289 420 129 >20.1 v 5161 5361 197 >19.7 b 4546 6183 393 >20.9 u 3133 5976 326 >20.7 uvw1 5572 5771 197 >20.0 uvm2 5366 5566 197 >19.8 uvw2 4957 6398 199 >20.0 white 4751 6388 393 >21.8 ------------------------------------------ The quoted upper limits have not been corrected for the expected Galactic extinction along the line of sight corresponding to a reddening of E_{B-V} = 0.78 mag (Schlegel, et al., 1998, ApJS, 500, 525). All photometry is on the UVOT flight system described in Poole et al. (2008, MNRAS, 383, 627). The 2MASS point source 21340333+4842361 is located 3.2 arcsec north of the centre of the XRT error circle. Our photometry is slightly contaminated by an uncatalogued source approximately 2.6 arcsec northwest of the centre of the XRT error circle. This source appears to be extended and may be the host galaxy of GRB 081126. Preliminary magnitudes are Filter Mag Err ---------------------------------------- v 19.2 0.3 b 20.2 0.2 u >20.9 3-sigma upper limit uvw1 >20.1 3-sigma upper limit uvm2 >19.9 3-sigma upper limit uvw2 >20.1 3-sigma upper limit white 20.2 0.2 ---------------------------------------- The detection in the b filter, combined with the lack of a detections in the u and UV filters, is consistent with this source having a redshift of approximately 2.8 < z < 4.5. This corresponds to a physical separation of approximately 18 proper kpc assuming a cosmology with (H_0,Omega_matter,Omega_Lambda) = (70,0.3,0.7).