TITLE: GCN CIRCULAR NUMBER: 12583 SUBJECT: GRB 111121A: Magellan/Megacam observations DATE: 11/11/22 12:58:18 GMT FROM: Edo Berger at Harvard W. Fong (Harvard), A. Seth (U. Utah), R. Chornock and E. Berger (Harvard) report: "We imaged the field of the short GRB 111121A (GCN 12578) with Megacam on the Magellan/Clay 6.5-m telescope at a mid-time of 2011 November 22.27 UT (13.9 hours post-burst). We obtained 25x30-sec exposures in r-band at an airmass of 1.7 in ~0.7" seeing. The location of the latest enhanced XRT error circle (improved from GCN 12579 at http://www.swift.ac.uk/xrt_positions/ with RA(J2000) = 10:19:02.71, Dec(J2000) = -46:40:14.5, 1.4" uncertainty) is contaminated by emission from a nearby bright (R~12.5) star. However, we find a faint source on the southern edge of the XRT circle at (J2000): RA = 10:19:02.7 Dec = -46:40:15.6, with an uncertainty of ~0.5" in each coordinate that includes contributions from the centroiding error of the source and a 0.27" rms uncertainty from the astrometric tie to USNO-B. Due to the location of the source near a diffraction spike from the bright star, it is unclear at present whether it is extended or point-like. Additional analysis to extract a magnitude for the source is on-going, and additional observations to check for variability are planned."