We report the clear detection of the 2175 Å dust absorption feature in the optical afterglow spectrum of the gamma-ray burst ( GRB ) GRB 070802 at a redshift of z = 2.45 .
This is the highest redshift for a detected 2175 Å dust bump to date , and it is the first clear detection of the 2175 Å bump in a GRB host galaxy , while several tens of optical afterglow spectra without the bump have been recorded in the past decade .
The derived extinction curve gives A _ { V } = 0.8 – 1.5 depending on the assumed intrinsic slope .
Of the three local extinction laws , an LMC type extinction gives the best fit to the extinction curve of the host of GRB 070802 .
Besides the 2175 Å bump we find that the spectrum of GRB 070802 is characterized by unusually strong low-ionization metal lines and possibly a high metallicity for a GRB sightline ( [ Si/H ] = -0.46 \pm 0.38 , [ Zn/H ] = -0.50 \pm 0.68 ) .
In particular , the spectrum of GRB 070802 is unique for a GRB spectrum in that it shows clear C i absorption features , leading us to propose a correlation between the presence of the bump and C i .
The gas to dust ratio for the host galaxy is found to be significantly lower than that of other GRB hosts with N ( H i ) / A _ { V } = ( 2.4 \pm 1.0 ) \times 10 ^ { 21 } cm ^ { -2 } mag ^ { -1 } , which lies between typical MW and LMC values .
Our results are in agreement with the tentative conclusion reached by that the shape of the extinction curve , in particular the presence of the bump , is affected by the UV flux density in the environment of the dust .