TITLE: GCN CIRCULAR NUMBER: 6756 SUBJECT: GRB 070810B: Keck spectroscopy of possible host galaxies DATE: 07/08/12 03:25:02 GMT FROM: Christina Thoene at Niels Bohr Institute,DARK Cosmo Ctr Christina C. Thoene (DARK, UC Berkeley), Joshua S. Bloom, Nathaniel R. Butler (UCB) and Peter Nugent (LBL) report: We obtained spectra of several sources around the reported XRT positions of GRB70810B (GCN 6754 , Starling et al.) on Aug. 11, 12:30 UT, using Keck I + LRIS. No new source is seen in imaging of the field at the same time. At the position of source 1 ("S1") from Starling et al., there is no optical counterpart visible. From further analysis of the X-ray data, we detect another weak X-ray source at RA 00:35:46.892, DEC 08:51:05.96 ("S3") which includes a weak optical source. Within the BAT error circle, there is an early type galaxy (LEDA 1354367) as noted first in Marshall et. al, GCN 6743, for which we determined a redshift of z=0.0385 derived from Ca H&K absorption lines. Around source 2 ("S2"), there are four galaxies of which two show strong emission lines leading to a redshifts around z=0.49 from OII, OIII and the Balmer series. The two other galaxies have marginal detections of emission lines at the same redshift and these galaxies might therefore be part of a cluster or even interacting. An image indicating the different XRT positions and galaxies observed can be found at: http://lyra.berkeley.edu/~cthoene/GRBs/GRB070810B.png CT wants to thank Keck support astronomer Greg Wirth and his wife for excellent dinner support during the observations. Tusend tak!