GCN Circular 3604
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.