Volume 601, May 2017
|Number of page(s)||10|
|Published online||10 May 2017|
Steep extinction towards GRB 140506A reconciled from host galaxy observations: Evidence that steep reddening laws are local⋆,⋆⋆
1 Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107, Reykjavík, Iceland
2 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
3 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
4 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
5 Institut d’Astrophysique de Paris, CNRS-UPMC, UMR 7095, 98 bis bd Arago, 75014 Paris, France
6 Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
7 Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel
8 University of Leicester, Department of Physics and Astronomy, University Road, Leicester, LE1 7RH, UK
9 European Southern Observatory, Karl-Schwarzschildstrasse 2, 85748 Garching bei München, Germany
10 APC, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, France
11 CAS Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, P.R. China
Received: 27 February 2017
Accepted: 20 March 2017
We present the spectroscopic and photometric late-time follow-up of the host galaxy of the long-duration Swift γ-ray burst GRB 140506A at z = 0.889. The optical and near-infrared afterglow of this GRB had a peculiar spectral energy distribution (SED) with a strong flux-drop at 8000 Å (4000 Å rest-frame) suggesting an unusually steep extinction curve. By analysing the contribution and physical properties of the host galaxy, we here aim at providing additional information on the properties and origin of this steep, non-standard extinction. We find that the strong flux-drop in the GRB afterglow spectrum at <8000 Å and rise at <4000 Å (observers frame) is well explained by the combination of a steep extinction curve along the GRB line of sight and contamination by the host galaxy light at short wavelengths so that the scenario with an extreme 2175 Å extinction bump can be excluded. We localise the GRB to be at a projected distance of approximately 4 kpc from the centre of the host galaxy. Based on emission-line diagnostics of the four detected nebular lines, Hα, Hβ, [O ii] and [O iii], we find the host to be a modestly star forming (SFR = 1.34 ± 0.04 M⊙ yr-1) and relatively metal poor () galaxy with a large dust content, characterised by a measured visual attenuation of AV = 1.74 ± 0.41 mag. We compare the host to other GRB hosts at similar redshifts and find that it is unexceptional in all its physical properties. We model the extinction curve of the host-corrected afterglow and show that the standard dust properties causing the reddening seen in the Local Group are inadequate in describing the steep drop. We thus conclude that the steep extinction curve seen in the afterglow towards the GRB is of exotic origin and issightline-dependent only, further confirming that this type of reddening is present only at very local scales and that it is solely a consequence of the circumburst environment.
Key words: gamma-ray burst: general / gamma-ray burst: individual: GRB 140506A / dust, extinction / galaxies: individual: GRB 140506A host
Based on observations carried out under programme IDs 095.D-0043(A, C) and 095.A-0045(A) with the X-shooter spectrograph and the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) installed at the Cassegrain Very Large Telescope (VLT), Unit 2 – Kueyen and Unit 1 – Antu, respectively, operated by the European Southern Observatory (ESO) on Cerro Paranal, Chile.
The reduced spectra (FITS files) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A83
© ESO, 2017
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