The 2175 angstrom Extinction Feature in the Optical Afterglow Spectrum of GRB 180325A at z =2.25

Abstract

The ultraviolet (UV) extinction feature at 2175 is ubiquitously observed in the Galaxy but is rarely detected at high redshifts. Here we report the spectroscopic detection of the 2175 bump on the sightline to the γ-ray burst (GRB) afterglow GRB 180325A at z =2.2486, the only unambiguous detection over the past 10 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 ≈ 4.4, A ≈ 1.5, and the 2175 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 equivalent width of W (λ 1656) =0.85 ±0.05. We also detect optical emission lines from the host galaxy. Based on the Hα emission-line flux, the derived dust-corrected star formation rate is ∼46 ±4 M yr and the predicted stellar mass is log M /M ∼9.3 ±0.4, suggesting that the host galaxy is among the main-sequence star-forming galaxies.© 2018. The American Astronomical Society. All rights reserved.