GCN Circular 18187

V. D'Elia (ASI/ASDC and INAF-OAR), T. Kruehler (ESO), K. Wiersema (U. 
Leicester), N. R. Tanvir (U. Leicester), J. Japelj (U. Ljubljana), G. 
Pugliese (API/UvA), D. Malesani (DARK/NBI), B. Milvang-Jensen 
(DARK/NBI), J. P. U. Fynbo (DARK/NBI), report on behalf of a larger 
collaboration:

We observed the optical counterpart of GRB 150821A (Troja et al., GCN 
18186
Not found
 
 
) with the ESO VLT equipped with the X-shooter spectrograph. Four 
spectra with individual exposure time of 600 s were secured covering the 
wavelength range 3,000-25,000 AA. Observations were carried out in 
robotic "rapid response mode" (RRM) and spectroscopy began 12.4 min 
after the BAT trigger (7.7 min after the XRT position, at which point 
the VLT was triggered). The afterglow is well detected in our 
acquisition image, with a magnitude of R ~ 16 (11.1 min after the GRB), 
calibrated to one nearby USNO star.

We detect a number of absorption features at a common redshift of z = 
0.755, including Mg II, Mg I, Fe II, Zn II. We also detect 
fine-structure lines at the same redshift (e.g. FeII* 2396 / 2389, and 
Ni II** 2166 / 2217 / 2223), confirming that the absorption system is 
local to the GRB and not intervening.

We thank the kind assistence of the observing staff at Paranal, in 
particular Claudia Cid and Marcela Espinoza.