The 2175 Å extinction feature in the optical afterglow spectrum of GRB 180325A at z=2.25

Abstract

The UV extinction feature at 2175 Å is ubiquitously observed in the Galaxy but is rarely detected at high redshifts. Here we report the spectroscopic detection of the 2175 Å bump on the sightline to the γ-ray burst (GRB) afterglow GRB 180325A at z=2.2486, the only unambiguous detection over the past ten years of GRB follow-up, at four different epochs with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT)/X-shooter. Additional photometric observations of the afterglow are obtained with the Gamma-Ray burst Optical and Near-Infrared Detector (GROND). We construct the near-infrared to X-ray spectral energy distributions (SEDs) at four spectroscopic epochs. The SEDs are well-described by a single power-law and an extinction law with R_V~4.4, A_V~1.5, and the 2175 Å extinction feature. The bump strength and extinction curve are shallower than the average Galactic extinction curve. We determine a metallicity of [Zn/H]>-0.98 from the VLT/X-shooter spectrum. We detect strong neutral carbon associated with the GRB with an equivalent width of Wr(λ1656) = 0.85+/-0.05. We also detect optical emission lines from the host galaxy. Based on the Hαemission line flux, the derived dust-corrected star-formation rate is ~46+/-4 M_sun/yr and the predicted stellar mass is log M*/M_sun~9.3+/-0.4, suggesting the host galaxy is amongst the main-sequence star-forming galaxies.

arXiv comment10 pages, 3 figures, 3 tables, Table 1 is in the journal electronic version, accepted for ApJL

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