VLT/X-Shooter absorption spectroscopy of the GRB 120327a afterglow

Abstract

We analyze two X-shooter spectra of the GRB 120327A afterglow, acquired    2.1 and    30hrs after the trigger. The ISM in the GRB host galaxy at z = 2.8145 isextremely rich in absorption features, with three components contributing to the lineprofiles. The hydrogen column density associated with GRB 120327A islog N_HI / cm² = 22.01 ± 0.09,and the metallicity of the host galaxy is in the range [X/H] =  − 1.7 to  − 1 with respectto the solar values, i.e., a typical value for a GRB host galaxy. Inaddition to the ground state lines, we detect in the first observation several excitedabsorption features, which disappeared in the second observation. Using these features, wederive that component I is at d_I = 200⁺¹⁰⁰_-60 pc from the GRB, while component II is located closer,at d_II = 100⁺⁴⁰₃₀ pc. These values are among the lowest found in GRBs.Comparing the abundances with star formation history models, we find that the GRB 120327Ahost galaxy abundances are compatible with a star formation efficiency of 10Gyr^-1, typical of spheroids (elliptical or bulge).