The properties of the 2175 å extinction feature discovered in GRB afterglows

Abstract

The unequivocal, spectroscopic detection of the 2175Å bump in extinction curves outside the Local Group is rare. To date, the properties of the bump have been examined in only two gamma-ray burst (GRB) afterglows (GRB070802 and GRB080607). In this work, we analyze in detail the detections of the 2175Å extinction bump in the optical spectra of two further GRB afterglows: GRB080605 and 080805. We gather all available optical/near-infrared photometric, spectroscopic, and X-ray data to construct multi-epoch spectral energy distributions (SEDs) for both GRB afterglows. We fit the SEDs with the Fitzpatrick & Massa model with a single or broken power law. We also fit a sample of 38 GRB afterglows, known to prefer a Small Magellanic Cloud (SMC)-type extinction curve, with the same model. We find that the SEDs of GRB080605 and GRB080805 at two epochs are fit well with a single power law with a derived extinction of AV = 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 GRB080805 is not well constrained, the extinction curve of GRB080605 has an unusual very steep far-UV rise together with the 2175Å bump. Such an extinction curve has previously been found in only a small handful of sightlines in the Milky Way. 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 2175Å 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 AV than for almost any Local Group sightline. Such dilution of the bump strength may offer tentative support to a dual dust-population scenario. © 2012 The American Astronomical Society. All rights reserved.