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

Zafar, T, Heintz, KE, Fynbo, JPU, Malesani, D, Bolmer, J, Ledoux, C, Arabsalmani, M, Kaper, L, Campana, S, Starling, RLC, Selsing, J, Kann, DA, de Ugarte Postigo, A, Schweyer, T, Christensen, L, Moller, P, Japelj, J, Perley, DA, Tanvir, NR, D'Avanzo, P , Hartmann, DH, Hjorth, J, Covino, S, Sbarufatti, B, Jakobsson, P, Izzo, L, Salvaterra, R, D'Elia, V and Xu, D (2018) The 2175 angstrom Extinction Feature in the Optical Afterglow Spectrum of GRB 180325A at z =. 2.25. The Astrophysical Journal Letters, 860 (2). ISSN 2041-8205

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Abstract

The ultraviolet (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 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 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 equivalent width of W r(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−1 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.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical And Space Sciences
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: American Astronomical Society
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Date Deposited: 04 Oct 2018 10:51
Last Modified: 04 Oct 2018 10:57
DOI or Identification number: 10.3847/2041-8213/aaca3f
URI: http://researchonline.ljmu.ac.uk/id/eprint/9418

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