GCN Circular 6850
Subject
GRB 071003: Keck spectroscopy
Date
2007-10-04T13:11:24Z (17 years ago)
From
Daniel Perley at U.C. Berkeley <dperley@astro.berkeley.edu>
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.