TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4380 SUBJECT: GRB 051221A: RAPTOR Fading Counterpart Constraint DATE: 05/12/22 02:54:22 GMT FROM: James Wren at LANL J. Wren, W.T. Vestrand, R. White, P. Wozniak, and S. Evans report on behalf of the RAPTOR team at Los Alamos National Laboratory: Starting at 02:57:05 UT (1.1 hours after the burst), the RAPTOR-S telescope began a manually initiated response to the short burst identified by Swift (Parsons et al. 4363). Within the XRT error circle (Burrows et al. 4366) at a position consistent with the location of the candidate J-band infrared (Bloom, GCN 4368) and R-band optical (Berger, GCN 4369) counterparts, a stack of 20 30-second unfiltered RAPTOR images yields a marginal detection of a source. Using the USNO-B1 catalog for calibration and ignoring any extinction along the line of sight, our derived 5-sigma upper limit on the brightness of an optical counterpart at that epoch (1.3 hours after the trigger) is R=20.2+/-0.2 magnitude.