[vsnet-grb-info 2543] GRB 051221A: RAPTOR Fading Counterpart Constraint

[GCN Circulars]

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  <jwren at nis.lanl.gov>

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.