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

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posted on 2017-05-05, 09:27 authored by K. E. Heintz, J. P. U. Fynbo, P. Jakobsson, T. Krühler, L. Christensen, D. Watson, C. Ledoux, P. Noterdaeme, D. A. Perley, H. Rhodin, J. Selsing, S. Schulze, N. R. Tanvir, P. Møller, P. Goldoni, D. Xu, B. Milvang-Jensen
We present the spectroscopic and photometric late-time follow-up of the host galaxy of the long-duration Swift gamma-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 {\AA} (4000 {\AA} rest-frame) suggesting an unusually steep extinction curve. By analyzing 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 {\AA} and rise at < 4000 {\AA} is well explained by the combination of a steep extinction curve along the GRB line of sight and contamination by the host galaxy light so that the scenario with an extreme 2175 {\AA} 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, Halpha, Hbeta, [O II] and [O III], we find the host to be a modestly star forming (SFR = 1.34 +/- 0.04 Msun yr^-1) and relatively metal poor (Z = 0.35^{+0.15}_{-0.11} Zsun) galaxy with a large dust content, characterized by a measured visual attenuation of A_V = 1.74 +/- 0.41 mag, thus 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 conclude that the steep extinction curve seen in the afterglow towards the GRB is of exotic origin, is sightline-dependent only and thus solely a consequence of the circumburst environment.


KEH and PJ acknowledge support by a Project Grant (162948–051) from The Icelandic Research Fund. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Program (FP7/2007–2013)/ERC Grant agreement no. EGGS–278202. TK acknowledges support through the Sofja Kovalevskaja Award to P. Schady from the Alexander von Humboldt Foundation of Germany. LC is supported by YDUN grant DFF – 4090–00079. D.X. acknowledges the support by the One-Hundred-Talent Program of the Chinese Academy of Sciences (CAS), by the Strategic Priority Research Program “Multi-wavelength Gravitational Wave Universe” of the CAS (No. XDB23000000), and by the National Natural Science Foundation of China under grant 11533003.



Astronomy and Astrophysics, 2017

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


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Astronomy and Astrophysics


EDP Sciences for European Southern Observatory (ESO), Springer Verlag





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