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