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

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posted on 07.06.2018, 11:20 by T. Zafar, K. E. Heintz, J. P. U. Fynbo, D. Malesani, J. Bolmer, C. Ledoux, M. Arabsalmani, L. Kaper, S. Campana, R. L. C. Starling, J. Selsing, D. A. Kann, A. D. U. Postigo, T. Schweyer, L. Christensen, P. Møller, J. Japelj, D. Perley, N. R. Tanvir, P. D'Avanzo, D. H. Hartmann, J. Hjorth, S. Covino, B. Sbarufatti, P. Jakobsson, L. Izzo, R. Salvaterra, V. D'Elia, D. Xu
The UV extinction feature at 2175 \AA\ is ubiquitously observed in the Galaxy but is rarely detected at high redshifts. Here we report the spectroscopic detection of the 2175 \AA\ bump on the sightline to the \gamma-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 \AA\ 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 log N(CI/cm^{-2}) =14.65+/-0.37. We also detect optical emission lines from the host galaxy. Based on the H\alpha emission 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.


Based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 0100.D−0649(A), PI: Tanvir. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. We acknowledge the use of public data from the Swift data archive. DAK acknowledges support from the Spanish research project AYA 2014-58381-P, and from Juan de la Cierva Incorporaci´on fellowship IJCI-2015-26153. RLCS acknowledges support from STFC. KEH and PJ acknowledge support by a Project Grant (162948–051) from The Icelandic Research Fund. The Cosmic Dawn Center is funded by the DNRF. LC is supported by DFF–4090-00079. SC, PDA and BS acknowledge support from the ASI grant I/004/11/3. AdUP acknowledges support from a Ram´on y Cajal fellowship RyC-2012-09975 and the Spanish research project AYA 2014-58381-P



Astrophysical Journal, 2018, 860(2)

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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American Astronomical Society, IOP Publishing





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