GCN Circular 18426

A. de Ugarte Postigo, (IAA-CSIC, DARK/NBI), D. Malesani (DARK/NBI),
D. Xu (NAOC/CAS) report on behalf of a larger collaboration:

We have observed the field of GRB 151021A (Melandri et al., GCN 18425
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with X-shooter at the VLT (Paranal Observatory, Chile). Observation 
started at 02:13 UT (44 min after the burst) and cover the range between
3000 and 24800 AA.

In the close proximity of the XRT error circle, we detect a bright, uncatalogued 
object in the r'-band acquisition image, at coordinates (J2000, 0.5" error):

RA = 22:30:34.369
Dec = -33:11:49.45

We suggest that this is the optical afterglow of GRB 151021A and we measure 
for it R = 18.2 (Vega), derived from comparison with R-band magnitudes for the 
USNO-B1.0 catalogue. An object at consistent coordinates is also visible in the 
UVOT white-band image.

On a preliminary reduction, the spectrum shows a bright continuum with multiple 
absorption features, including SII, OI, SiII, SiIV, CIV, AlII, ZnII, CrII, FeII, NiII, MnII, 
MgII, MgI, as well as fine-structure lines from FeII* and NiII* and Ly-alpha 
absorption at a common redshift of z = 2.330, which we identify as the redshift of 
the GRB. We also identify an intervening system at z = 1.490 with features of 
FeII, MgII, and MgI.

We acknowledge the excellent support given by the Paranal staff, and in 
particular Andrea Mehner, Julien Mili and Jose Velasquez.