The Dust Extinction Curves of Gamma-Ray Burst Host Galaxies
  The composition and amount of interstellar dust within gamma-ray burst (GRB)
host galaxies is of key importance when addressing selection effects in the GRB
redshift distribution, and when studying the properties of their host galaxies.
As well as the implications for GRB research, probing the dust within the
high-z hosts of GRBs also contributes to our understanding of the conditions of
the interstellar medium and star-formation in the distant Universe.
Nevertheless, the physical properties of dust within GRB host galaxies
continues to be a highly contended issue. In this paper we explore the mean
extinction properties of dust within the host galaxies of a sample of 17 GRBs
with total host galaxy visual extinction Av<1 (<Av>=0.4), covering a redshift
range z=0.7-3.1. We find the average host extinction curve to have an
ultraviolet slope comparable to that of the LMC, but with little evidence of a
2175Angs dust extinction feature as observed along Milky Way and LMC
sightlines. We cannot at present rule out the presence of a 2175Angs feature,
and both the standard SMC and LMC extinction curves also provide good fits to
our data. However, we can reject an extinction curve that has a UV slope as
flat as the mean Milky Way extinction curve, whilst also having a 2175Angs
feature as prominent as seen in the mean Milky Way extinction curve. This is in
contrast to the clear detection of a 2175Angs bump and the flatter extinction
curves of some more heavily extinguished GRBs (Av>1), which may be indicative
of there being a dependence between dust abundance and the wavelength
dependence of dust extinction, as has been previously speculated.
