TITLE: GCN CIRCULAR NUMBER: 8450 SUBJECT: GRB 081029: UVOT Detection of the Optical Afterglow DATE: 08/10/29 16:41:01 GMT FROM: Stephen Holland at USRA/NASA/GSFC/SSC S. T. Holland (CRESST/USRA/GSFC) & T. Sakamoto (NASA/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT observed the field of GRB 081029 starting 2708 s after the BAT trigger (Sakamoto et al., 2008, GCNC 8435). The late start to the follow-up observations is due to an Earth limb constraint at the time of the BAT trigger. We detect the optical afterglow (Rykoff et al., GCNC 8436) in the v, b, and white filters. This is consistent with a redshift of z = 3.8479 (D'Elia, et al., 2008, GCNC 8438). The refined UVOT source position is RA (J2000.0) = 23:07:05.35 = 346.77229 (deg) Dec (J2000.0) = -68:09:19.8 = -68.15549 (deg) with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence, statistical + systematic). Magnitudes and 3-sigma upper limits are reported below. TSTART and TSTOP are the start and stop times of the exposure, in seconds since the BAT trigger. Exposure is the total exposure time, in seconds, during that interval. Filter TSTART TSTOP Exposure Mag Err Comments v 2687 2887 197 19.02 0.20 8557 8757 197 18.66 0.21 b 3072 3272 197 >20.5 3-sigma upper limit 4507 4707 197 20.05 0.20 u 2867 21,932 1601 >21.9 3-sigma upper limit uvw1 4097 21,019 1968 >21.8 3-sigma upper limit uvm2 3892 16,685 1590 >21.7 3-sigma upper limit uvw2 3482 15,250 1279 >21.7 3-sigma upper limit white 2708 7856 147 20.50 0.30 4712 4912 197 19.41 0.10 The above magnitudes are not corrected for the Galactic extinction corresponding to a reddening of E_{B-V} = 0.03 mag (Schlegel et al., 1998, ApJS, 500, 525). The photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383,627). The power-law decay index in the v band between 8557 and 15,864 s is consistent with no decay.