VLT/X-shooter GRBs: Individual extinction curves of star-forming regions
  The extinction profiles in Gamma-Ray Burst (GRB) afterglow spectral energy
distributions (SEDs) are usually described by the Small Magellanic Cloud
(SMC)-type extinction curve. In different empirical extinction laws, the
total-to-selective extinction, RV, is an important quantity because of its
relation to dust grain sizes and compositions. We here analyse a sample of 17
GRBs (0.34<z<7.84) where the ultraviolet to near-infrared spectroscopic
observations are available through the VLT/X-shooter instrument, giving us an
opportunity to fit individual extinction curves of GRBs for the first time. Our
sample is compiled on the basis that multi-band photometry is available around
the X-shooter observations. The X-shooter data are combined with the Swift
X-ray data and a single or broken power-law together with a parametric
extinction law is used to model the individual SEDs. We find 10 cases with
significant dust, where the derived extinction, AV, ranges from 0.1-1.0mag. In
four of those, the inferred extinction curves are consistent with the SMC
curve. The GRB individual extinction curves have a flat RV distribution with an
optimal weighted combined value of RV = 2.61+/-0.08 (for seven broad coverage
cases). The 'average GRB extinction curve' is similar to, but slightly steeper
than the typical SMC, and consistent with the SMC Bar extinction curve at ~95%
confidence level. The resultant steeper extinction curves imply populations of
small grains, where large dust grains may be destroyed due to GRB activity.
Another possibility could be that young age and/or lower metallicities of GRBs
environments are responsible for the steeper curves.
