GCN Circular 6436
A. Rau, M. M. Kasliwal, and S. B. Cenko (Caltech) report on behalf of a larger collabortation: We obtained R-band photometry of GRB070521 with the Large Format Camera mounted on the Palomar 200-inch telescope. Observations started 29 min post-burst and lasted 3x300s. Calibration was performed relative to the SDSS (Cool et al.; GCN 6432). We find no source in the XRT error circle (Guidorzi et al. 2007; GCN 6431) to a limiting magnitude of R>23.3.