GCN Circular 13649

D. Malesani (DARK/NBI), S. Schulze (PUC and MCSS), A. de Ugarte Postigo 
(IAA/CSIC and DARK/NBI), D. Xu (WIS), V. D'Elia (INAF/Roma and 
ASI/ASDC), J. P. U. Fynbo (DARK/NBI), N. R. Tanvir (Univ. Leicester), 
report on behalf of a larger collaboration:

We observed the field of GRB 120815A (Pagani et al., GCN 13645
Not found
 
 
) with the 
VLT/X-shooter spectrograph. We confirm the presence of a bright, 
uncatalogued object consistent with the GROND position (Sudilovsky et 
al., GCN 13648
Not found
 
 
). From the acquisition images, we measure R = 18.9 for 
the afterglow at an epoch of 1.67 hr after the GRB (calibrated to nearby 
USNO stars).

Our spectra cover the wavelength range 3000-25000 AA, and were taken 
starting on 2012 Aug 15.163 UT (1.69 hr after the GRB), for a total of 
40 min exposure. Continuum from the afterglow is detected with high S/N 
in all of the UVB, VIS, and NIR arms.

The spectrum shows a wide trough and a multitude of strong absorption 
lines, which we interpret as due to a DLA and metal transitions of N V, 
S II, Si II, O I, C IV, Si IV, Fe II, Al II, Al III, Mn II, Mg II, Mg I, 
among others, at a common redshift z = 2.358. At the same redshift, we 
also detect fine-structure lines of Ni II and Fe II, thus confirming 
conclusively that this is the GRB redshift. We also detect intervening 
systems at z = 1.539, 1.693, and 2.00. In particular, the system at z = 
1.539 displays exceptionally strong Mg II, Mg I, and Fe II absorption.

We acknowledge the expert support of the VLT observing staff at Paranal, 
in particular Maja Vuckovic, Marcelo Lopez, and Thomas Rivinius.
We also thank Alex Kann for discussion on the afterglow identification.

[GCN OPS NOTE(15aug12): Per author's request, ML and TR were added to the
last paragraph.]