TITLE: GCN CIRCULAR NUMBER: 14003 SUBJECT: GRB 121123A: Swift/UVOT Observations DATE: 12/11/26 22:25:09 GMT FROM: Stephen Holland at STScI S. T. Holland (STScI) and E. A. Helder (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 121123A starting 131 s after the BAT trigger (Helder et al., 2012, GCNC 13982). The refined UVOT position is RA (J2000) 20:29:16.30 = 307.31792 (deg) Dec (J2000) -11:51:35.6 = -11.85989 (deg) with an estimated uncertainty of 0.44 arcsec (radius, 90% confidence, statistical + systematic). We observe a slow decay of the optical afterglow, consistent with that reported by Xu et al., (2012, GCNC 13986). The afterglow is weakly detected in white at approximately 40 ks after the BAT trigger. However, the UVOT photometry is affected by a bright (B ~ 14 mag) star (USNO B1.0 0781-0714667) located approximately 22 arcsec north of the afterglow, so the late-time behavior of the optical afterglow's light curve requires further analysis. Preliminary UVOT photometry, and 3-sigma upper limits, for the afterglow are presented below. ------------------------------------------------------ Filter TSTART TSTOP Exposure Mag Err ------------------------------------------------------ white (fc) 131 281 147 19.43 0.15 u (fc) 289 539 246 19.35 0.23 white (fc) 869 1019 147 19.13 0.12 ------------------------------------------------------ v 619 1070 58 18.29 0.33 b 546 1169 58 19.12 0.29 u 695 1145 39 18.58 0.32 uvw1 669 18,011 2137 >21.5 uvm2 645 17,104 1376 >21.3 uvw2 595 22,980 1376 >21.5 white 570 590 19 >19.7 ------------------------------------------------------ The detection in the UVOT u band, combined with the non-detection in the UVOT uvw1 band, is consistent with GRB 121123A having a redshift of approximately 1.5 < z < 3.4. The quoted magnitudes and upper limits have not been corrected for the Galactic extinction along the line of sight to this burst of E_{B-V} = 0.05 mag (Schlafly et al. 2011, ApJS, 737, 103). The photometry is in the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373).