GCN Circular 17672

V. Pugliese (API/Uva), D. Xu (DARK/NBI), N. R. Tanvir, K. Wiersema (U.
Leicester), J. P. U. Fynbo, B. Milvang-Jensen (DARK/NBI), V. D'Elia
(ASI/ASDC and INAF/Roma) report on behalf of a larger collaboration:

We observed the afterglow of GRB 150403A (Lien et al. GCN 17665
Not found
 
 
; Longo
et al. GCN 17667
Not found
 
 
) with X-shooter at the Very Large Telescope (Paranal
Observatory, Chile). Observations started at 08:44:48 UT on 2015-04-04
(i.e., 10.84 hr after the burst), and consisted of photometry in V-,
r-, z-, and g- bands as well as spectroscopy of 4x600 s exposures
covering the spectral range between 3000 and 25000 AA, equipped with
UVB/VIS/NIR arms.

The optical afterglow is well detected in each photometric image, and
has m(R) ~ 19.1 mag at 10.85 hr post-burst, calibrated with nearby
USNO B1 stars. For the spectroscopy, prominent absorption lines are
detected in a continuum throughout the arms. In a preliminary
reduction we detect absorption features of SII, SiIV, OI, SiII, SiII*,
CII, CII*, CIV, AlII, NiII*, FeII, FeII*, MnI, MgII, MgI at a common
redshift of z = 2.06, which is consistent with a broad Lya trough at
the same redshift. We suggest this is the redshift of the GRB.

We also detect an intervening system at z=1.76, identified with
absorption lines of SiII, CIV, AlII, and so on.

We acknowledge the excellent support provided by Paranal staff, and in
particular Andrea Mehner, and Emanuela Pompei.