TITLE: GCN CIRCULAR NUMBER: 6761 SUBJECT: GRB 070810A: ROTSE-III Refined Analysis DATE: 07/08/17 00:24:27 GMT FROM: Fang Yuan at ROTSE F. Yuan, E.S. Rykoff (U Mich), report on behalf of the ROTSE collaboration: ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB 070810A (Swift trigger 287364; Cummings, et al., GCN 6733), and detected the optical counterpart (Swan, et al., GCN 6734). The initial aperture photometry reported in GCN 6734 was biased due to contamination from the nearby star. With a reference image taken 6 days after the burst, we were able to subtract the nearby star and obtain better estimate of the OT brightness with PSF fitting. Our refined analysis shows the OT at 18.0 ± 0.2 mag (unfiltered images, calibrated relative to USNO-B1.0 R) between 1607s and 2290s after the burst. start UT end UT mag ------------------------------------- 02:38:39.5 02:50:02.3 18.0+/-0.2 03:13:15.7 03:24:39.0 18.5+/-0.2 The OT faded with a decay index 0.7 ±0.2 in the next 40 minutes before dropping below our detection threshold. This decay index is significantly shallower than seen at later time in Keck photometry (Perley, et al., GCN 6757), implying a light curve break between 1.2 hr and 4 hr after the burst.