Spectroscopy of the short-hard GRB 130603B: The host galaxy and
  environment of a compact object merger
  Short duration gamma-ray bursts (SGRBs) are thought to be related to the
violent merger of compact objects, such as neutron stars or black holes, which
makes them promising sources of gravitational waves. The detection of a
'kilonova'-like signature associated to the Swift-detected GRB 130603B has
suggested that this event is the result of a compact object merger. Our
knowledge on SGRB has been, until now, mostly based on the absence of supernova
signatures and the analysis of the host galaxies to which they cannot always be
securely associated. Further progress has been significantly hampered by the
faintness and rapid fading of their optical counterparts (afterglows), which
has so far precluded spectroscopy of such events. Afterglow spectroscopy is the
key tool to firmly determine the distance at which the burst was produced,
crucial to understand its physics, and study its local environment. Here we
present the first spectra of a prototypical SGRB afterglow in which both
absorption and emission features are clearly detected. Together with
multiwavelength photometry we study the host and environment of GRB 130603B.
From these spectra we determine the redshift of the burst to be z =
0.3565+/-0.0002, measure rich dynamics both in absorption and emission, and a
substantial line of sight extinction of A_V = 0.86+/-0.15 mag. The GRB was
located at the edge of a disrupted arm of a moderately star forming galaxy with
near-solar metallicity. Unlike for most long GRBs (LGRBs), N_HX / A_V is
consistent with the Galactic ratio, indicating that the explosion site differs
from those found in LGRBs. The merger is not associated with the most
star-forming region of the galaxy; however, it did occur in a dense region,
implying a rapid merger or a low natal kick velocity for the compact object
binary.
