TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3406
SUBJECT: GRB 050509c: ROTSE-III Optical Limits 
DATE:    05/05/10 15:19:36 GMT
FROM:    Eli Rykoff at U of Michigan/ROTSE  <erykoff@umich.edu>

E.S. Rykoff (U Mich), H. Swan (U Mich), R. Quimby (U Texas), T.A. McKay 
(U Mich), report on behalf of the ROTSE collaboration:

ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, 
responded to GRB 050509c (HETE trigger 3751), producing images beginning 
6.4 s after the GCN notice time. An automated response took the first 
image at 22:46:19.6 UT, 25.7 s after the burst, under excellent 
conditions. We took 10 5-sec, 10 20-sec and 120 60-sec eposures. These 
unfiltered images are calibrated relative to USNO A2.0 (R).  The final 
error box reported by Prigozhin et al. (GCN 3402) was entirely contained 
within the ROTSE-IIIc field of view of the initial response.

Comparison to the DSS (second epoch) reveals no new sources within the 
SXC/WFC error box reported in GCN 3402; however, we are limited as the 
field is somewhat crowded. Individual images have limiting magnitudes 
ranging from 15.8-17.1. In particular, we set a limit of magnitude 15.9 
in a single 5 s exposure, starting 25.7 s after the burst. Coadding the 
images into sets of 10 reveals no new sources down to limits of 17.0 
(136 s effective length (t_end-t_start), beginning 25.7 s post-burst), 
17.5 (295.4 s effective length, beginning 170.7 s post-burst), and 17.7 
(695 s effective length, beginning 466.1 s post-burst).