GCN Circular 9012

C. C. Thoene (INAF-OAB), P. D'Avanzo (Univ. Bicocca & INAF-OAB), D.
Fugazza (INAF-OAB), A. de Ugarte Postigo (ESO), V. D'Elia, S. Piranomonte
(INAF-OAR), S. Covino, S. Campana (INAF-OAB), G. Chincarini (Univ.
Bicocca) report, on behalf of the MISTICI collaboration:

We observed the optical afterglow of GRB 090313 (Chornock et al. GCN 8979
Not found
 
 
;
Mao et al. GCN 8980
Not found
 
 
) with the ESO VLT about 1.9 days after the burst with
the FORS2 camera in spectroscopic mode. We took 4 exposures of 20 min each
with the 600I grism covering the range 6800-9300 Angstrom with a
resolution of R=1500.

While we confirm the redshift reported by Chornok et al. (GCN 8994
Not found
 
 
), we
also report the detection of a broad double absorption feature around
observed lambda 7830 and 7840 angstrom that does not fit any transition at
the GRB redshift (z = 3.375). This double feature can be interpreted as Mg
II 2796,2803, each consisting of 2 velocity components with a separation
of about 300 km/s, and could be a signature of an intervening system at
z ~ 1.8. We cannot confirm the presence of Mg I 2852 at z = 1.8 due to the
possible overlap with Mg I 1827 absorption at the redshift of the GRB.

We acknowledge excellent support from the ESO staff, in particular Yuri
Beletsky.