TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3604
SUBJECT: GRB 050713A: RAPTOR detection of early optical emission
DATE:    05/07/13 23:15:38 GMT
FROM:    James Wren at LANL  <jwren@nis.lanl.gov>

J. Wren, W.T. Vestrand, P. Wozniak, R. White, S. Evans report
on behalf of the RAPTOR team.

The RAPTOR-S telescope at Los Alamos National Laboratory began
imaging the field of GRB 050713A (Swift trigger 145675) 22.4
seconds after the GRB trigger -- before the end of the interval
of prompt gamma-ray emission (Falcone et al. GCN circ 3581).
At the location of the fading optical and NIR counterpart identified
in later images by Malesani et al. (GCN circ 3582) and Hearty et al.
(GCN circ 3583), respectively, we detected a transient optical
counterpart.  In a stack of eight 10 second unfiltered images
starting at 04:29:24.8 UT (with midpoint at 99.3 s after the GRB
trigger), we measured the optical transient to have a R-band
magnitude of 18.4 (+/- 0.18).  Our preliminary transformation to
R-band magnitudes was based on field stars from the USNO-B1 catalog.