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GCN Circular 6850

GRB 071003: Keck spectroscopy
2007-10-04T13:11:24Z (17 years ago)
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.
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