GCN Circular 3409
S. Bradley Cenko, B. T. Soifer, Chao Bian, Vandana Desai, S. R. Kulkarni (Caltech), Brian P. Schmidt (ANU), Arjun Dey and Buell T. Jannuzi (NOAO) report on behalf of the Caltech-NRAO-Carnegie GRB collaboration: We have continued to image the field of GRB 050509b with the Low Resolution Imaging Spectrometer mounted on the 10-m Keck I telescope. Our latest observations consisted of 3 x 300 s images simultaneously in the g' and R filters. The mean epoch of these observations was approximately 5:56 UT 10 May (25.8, 25.9, and 26.0 hours after the burst, respectively). We stacked all the R-band images obtained on 9 May (GCNs 3391, 3401) and differenced this from the stack obtained on 10 May. Adopting the value of R = 18.5 for the USNO-A2 star located at RA 12:36:19.970, Dec +29:00:36.31 (J2000.0), we find no variation within the revised XRT error circle (GCN 3395) to a limiting magnitude of R = 25.6. Specifically for the extended source in the XRT error circle (S1 in GCN 3401), we find this object has not varied by more than +/- 0.18 magnitudes from 9 May to 10 May. It is therefore unlikely that any of the 4 sources identified in GCN 3401 is the optical afterglow of GRB 050509b.