TITLE: GCN CIRCULAR NUMBER: 11073 SUBJECT: GRB 100807A: P60 Optical Afterglow Discovery DATE: 10/08/07 20:55:56 GMT FROM: S. Bradley Cenko at Caltech S. B. Cenko (UC Berkeley) reports on behalf of a larger collaboration: We have imaged the field of GRB 100807A (Grupe et al., GCN 11067) with the automated Palomar 60 inch telescope. Observations were obtained in the Sloan r', i', and z' filters beginning at 9:16 UT on 7 August (~ 3 min after the Swift trigger). At the edge of the enhanced XRT position (Osborne et al., GCN 11070), we detect a fading point source with coordinates (J2000.0): RA: 03:41:12.43 Dec: +67:40:17.4 Using several nearby objects in the USNO-B1 catalog for reference, we report the following photometry for this object: UT (mid-point) t_burst (min) Filter Magnitude --------------------------------------------------------------- 09:16:55 3.7 R 20.47 +/- 0.17 09:18:21 5.1 I 19.80 +/- 0.14 09:47:07 33.9 R 21.57 +/- 0.20 09:49:58 36.8 I 20.96 +/- 0.20 The above magnitudes have not been corrected for the large Galactic extinction along the line of sight. The fading behavior confirms this source is the optical afterglow of GRB 100807A.