GCN Circular 10750
D. A. Perley, C. R. Klein, and A. N. Morgan (UC Berkeley) report: We began observing the location of the afterglow of GRB 100513A (Baumgartner et al., GCN 10746) at 04:06 UT using the Nickel 40-inch telescope at Lick Observatory. A total of fourteen 300-second images were acquired in the R-band under clear sky conditions. A co-add of the first 9 images (midpoint 04:35 UT) reveals a faint point source at the position reported by Morgan et al (GCN 10747). The source is near the detection limit of the co-add and is not detected in individual images. Photometry of the object relative to three nearby SDSS stars (converted to Rc via the Lupton transformation equation) gives the following magnitude: R = 21.39 +/- 0.25 (t_mid = 2.46 hours) We note that this is significantly fainter than expected from the preliminary JHK magnitudes of Morgan et al (GCN 10749). The IR spectral index is relatively flat (beta~0.5), suggesting little host extinction. Assuming no extinction in addition to the Galactic value, a direct power-law extrapolation of the JHK data to R-band over-predicts the observed flux by >2 magnitudes, strongly suggesting an R-band dropout and likely redshift of 4.6 < z < 6.0. We note also that the X-ray column is consistent with no absorption beyond Galactic (Baumgartner et al.), consistent with a high-redshift origin. We encourage I/z/Y-band follow-up to better constrain the redshift and dust extinction. Spectroscopic observations are planned.