TITLE: GCN CIRCULAR NUMBER: 6850 SUBJECT: GRB 071003: Keck spectroscopy DATE: 07/10/04 13:11:24 GMT FROM: Daniel Perley at U.C. Berkeley D. A. Perley, R. Chornock, J. S. Bloom (UC Berkeley), C. Fassnacht, and M. W. Auger (UC Davis) report on behalf of GRAASP: We obtained spectroscopic follow-up of the bright transient associated with GRB 071003 (Schady et al., GCN 6837; Li, GCN 6838) starting at 07:51 UT on the night of 2007-10-04 using Keck I + LRIS (range 3300-8630 Angstroms). Two 10-minute exposures were acquired. The source is well-detected and a preliminary reduction reveals a smooth spectrum consistent with a GRB afterglow with multiple absorption-line systems. We identify a pair of absorption features at 5870 and 5886 Angstroms with the Mg II 2796, 2803 Angstrom doublet at a redshift of 1.100. Other absorption features are consistent with Fe II 2382, 2586, and 2599 Angstroms at this redshift. This absorption system sets a lower limit on the redshift of the GRB of z=1.100. In addition, numerous lines (e.g., Mg II 2796, 2803 Angstroms, Mg I 2852 Angstroms, and Fe II 2599 Angstroms) are present from an intervening absorption system at z=0.372. No strong supernova features or host-galaxy emission lines are seen in the spectrum. This suggests that the transient is a GRB afterglow undergoing a bright late-time optical flare, similar to the afterglow of GRB 070311 (e.g. Halpern & Armstrong, GCN 6203; Guidorzi et al., astroph/0708.1383), rather than a supernova. That flare peaked at R~22 approximately two days after the burst. Improved photometry of the optical transient associated with GRB 071003 gives a magnitude of R = 19.1 +/- 0.3 at 04:49 UT (0.88 days after the trigger) and further rebrightening appears possible. Further monitoring is strongly encouraged.