Studies of extinction curves provide insights into the properties of interstellar dust .
Until recently , however , very few extinction curves existed outside the Local Group .
GRB afterglows are well suited to extinction studies due to their brightness , simple power-law spectra and their occurrence in distant star forming galaxies .
In this paper we present results from the SED analysis of a sample of 41 GRB afterglows , from X-ray to NIR wavelengths .
The sample is based on spectra from VLT-FORS , with additional data primarily from Swift .
This is the largest sample of extinction curves outside the Local Group and , to date , the only extragalactic sample of absolute extinction curves based on spectroscopy .
Estimates of the distribution of restframe visual extinctions , the extinction curves , and the intrinsic spectral shapes of GRB afterglows are obtained .
Their correlation with { H \textsc { i } } column density as well as total and gas-phase metal column density are examined .
The line-of-sight gas-to-dust and metals-to-dust ratios are determined and examined as a function of total column density , ISM metallicity and redshift .
The intrinsic SEDs of the afterglows show that approximately half the sample require a cooling break between the optical and X-ray ranges .
The broken power-law SEDs show an average change in the spectral index of \Delta \beta = 0.51 with a very small standard deviation of 0.02 ( excluding the outlier GRB 080210 ) .
This is consistent with the expectations from a simple synchrotron model .
Such a close convergence of values suggests that the X-ray afterglows of GRBs may be used with considerably more confidence to set the absolute flux level and intrinsic spectral indices in the optical and UV .
Of the sample , 63 % are well described by a featureless ( SMC-type ) extinction curve .
Almost a quarter of our sample is consistent with no significant extinction ( typically A _ { V } \lesssim 0.1 ) .
The 2175 Å extinction bump is detected unequivocally in 7 % of our sample ( 3 GRBs ) , which all have A _ { V } > 1.0 , while one afterglow has a very unusual extinction curve with a sharp UV rise .
However , we can only say that the bump is not present in about a quarter of our sample because of low extinction or lack of coverage of the 2200 Å region .
All the afterglows well fit with SMC type curves have moderate or low extinction , with A _ { V } < 0.65 .
This suggests that the SMC extinction curve is not as nearly-universal as previously believed and that extinction curves more similar to those found in the Galaxy and the LMC may be quite prevalent .
We find an anti-correlation between gas-to-dust ratio and metallicity consistent with the Local Group relation ; we find , however , no correlation between the metals-to-dust ratios and the metallicities , redshift and visual extinction ; we find no strong correlation of the extinction column with metallicity either .
Our metals-to-dust ratios derived from the soft X-ray absorption are always larger ( 3–30 times ) than the Local Group value , which may mean that GRB hosts may be less efficient at turning their metals into dust .
However , we find that gas , dust , and metal column densities are all likely to be influenced by photo-ionization and dust destruction effects from the GRB to differing extents and caution must be used in extrapolating the ratios of dust and gas-derived properties from GRB afterglows to the star-forming population in general .