The properties of the 2175AA extinction feature discovered in GRB
  afterglows
  The unequivocal, spectroscopic detection of the 2175 bump in extinction
curves outside the Local Group is rare. To date, the properties of the bump
have been examined in only two GRB afterglows (GRB 070802 and GRB 080607). In
this work we analyse in detail the detections of the 2175 extinction bump in
the optical spectra of the two further GRB afterglows: GRB 080605 and 080805.
We gather all available optical/NIR photometric, spectroscopic and X-ray data
to construct multi-epoch SEDs for both GRB afterglows. We fit the SEDs with the
Fitzpatrick & Massa (1990) model with a single or broken PL. We also fit a
sample of 38 GRB afterglows, known to prefer a SMC-type extinction curve, with
the same model. We find that the SEDs of GRB 080605 and GRB 080805 at two
epochs are fit well with a single PL with a derived extinction of A_V =
0.52(+0.13 -0.16) and 0.50 (+0.13 -0.10), and 2.1(+0.7-0.6) and 1.5+/-0.2
respectively. While the slope of the extinction curve of GRB 080805 is not
well-constrained, the extinction curve of GRB 080605 has an unusual very steep
far-UV rise together with the 2175 bump. Such an extinction curve has
previously been found in only a small handful of sightlines in the MW. One
possible explanation of such an extinction curve may be dust arising from two
different regions with two separate grain populations, however we cannot
distinguish the origin of the curve. We finally compare the four 2175 bump
sightlines to the larger GRB afterglow sample and to Local Group sightlines. We
find that while the width and central positions of the bumps are consistent
with what is observed in the Local Group, the relative strength of the detected
bump (A_bump) for GRB afterglows is weaker for a given A_V than for almost any
Local Group sightline. Such dilution of the bump strength may offer tentative
support to a dual dust-population scenario.
