GCN Circular 6851
Subject
GRB 071003: VLT spectroscopy
Date
2007-10-05T17:30:14Z (17 years ago)
From
Paolo D'Avanzo at INAF-OAB <paolo.davanzo@brera.inaf.it>
D. Fugazza (INAF-OABr), F. Fiore, V. D'Elia (INAF-OAR), P. D'Avanzo
(INAF-OABr), S. Piranomonte, L.A. Antonelli (INAF-OAR), G. Chincarini
(Univ. Bicocca), S. Covino, G. Tagliaferri (INAF-OABr), M. Della Valle
(INAF-OAA), A. Fernandez Soto (Univ. Valencia) report on behalf of the
MISTICI collaboration:
We observed the optical afterglow of GRB 071003 (Schady et al., GCN
6837; Li et al. GCN 6838; Cenko et al. GCN 6839) with the ESO-Very Large
Telescope equipped with the FORS2 camera in spectroscopic mode. We took
a 1800s spectrum with the grism 600B (with a resolution of R = 780) on
Oct 04.055 UT (~ 0.735 days after the burst), under good seeing
conditions (1.0").
The detection of a MgII system at z=1.100 (Perley et al. GCN 6850),
cannot be confirmed by our data, because the 2803 AA component of the
MgII doublet is contamined by the NaI 5891 and 5897 AA doublet of the
nearby bright star.
We also find significant lines at 5417 and 5430 AA, which we interpret
as the MgII absorption doublet at 2797 and 2803 AA at z=0.937. FeII 2382
is also detected at the same redshift.
In addition, we find two more significant absorption lines (5417 and
5446 AA), that can be identified as Ca H and K at z=0.372.
Alternatively, the 5446 AA line can be interpreted as the 2803 AA
component of the MgII doublet at z=0.942, with the 2797 AA component
sharing the absorption feature at 5430 AA with the MgII 2803 of the
system at z=0.937. This can be another line-locking example like that of
the CIV system of GRB 021004 at z=2.3 (Fiore et al. 2005, ApJ, 624, 853;
Starling et al. 2005, MNRAS, 360, 305 ).
Finally, we find another MgII doublet at ~4000 AA (z=0.370), not
consistent with the Ca system at z=0.372. No emission lines are visible.
In summary our analysis of the FORS2 spectrum puts a lower limit on the
redshift of GRB 071003 to z=0.937.
We thank VLT staff for performing the observations, in particular Thomas
Szeifert and Alain Smette.