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Steep extinction towards GRB 140506A reconciled from host galaxy observations: Evidence that steep reddening laws are local

Heintz, KE and Fynbo, JPU and Jakobsson, P and Kruehler, T and Christensen, L and Watson, D and Ledoux, C and Noterdaeme, P and Perley, DA and Rhodin, H and Selsing, J and Schulze, S and Tanvir, NR and Moller, P and Goldoni, P and Xu, D and Milvang-Jensen, B (2017) Steep extinction towards GRB 140506A reconciled from host galaxy observations: Evidence that steep reddening laws are local. Astronomy and Astrophysics, 601. ISSN 0004-6361

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Abstract

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.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical And Space Sciences
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Q Science > QD Chemistry
Divisions: Astrophysics Research Institute
Publisher: EDP Sciences
Related URLs:
Date Deposited: 06 Jul 2017 08:55
Last Modified: 06 Jul 2017 08:55
DOI or Identification number: 10.1051/0004-6361/201730702
URI: http://researchonline.ljmu.ac.uk/id/eprint/6787

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