GRB 071028B, a burst behind large amounts of dust in an unabsorbed galaxy

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
e-mail: cclemens@mpe.mpg.de
² Universe Cluster, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁵ Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978, USA

Received: 1 July 2010
Accepted: 12 January 2011

Abstract

Aims. We report on the discovery and properties of the fading afterglow and underlying host galaxy of GRB 071028B, thereby facilitating a detailed comparison between these two.

Methods. Observations were performed with the Gamma-ray Burst Optical and Near-infrared Detector at the 2.2 m telescope on the La Silla Paranal Observatory in Chile. We conducted five observations from 1.9 d to 227.2 d after the trigger and obtained deep images in the g′r′i′z′ and JHK_s bands.

Results. Based on accurate seven-channel photometry covering the optical to near-infrared wavelength range, we derive a photometric redshift of $\mathit{z}\mathrm{=}\mathrm{0.94}{\hspace{0.17em}}_{-0.10}^{\mathrm{+}\mathrm{0.05}}$ for the unabsorbed host galaxy of GRB 071028B. In contrast, we show that the afterglow with an intrinsic extinction of ${\mathit{A}}_{\mathit{V}}^{\mathrm{SB}}\mathrm{=}\mathrm{(}\mathrm{0.70}\mathrm{\pm }\mathrm{0.11}\mathrm{)}\hspace{0.17em}\mathrm{mag}$ is moderately absorbed and requires a relatively flat extinction curve. According to the reported Swift/BAT observations, the energetics yield an isotropic energy release of ${\mathit{E}}_{\mathit{\gamma ,}\mathrm{iso}}^{\mathrm{rest}}\mathrm{=}\mathrm{\left(}\mathrm{1.4}{\hspace{0.17em}}_{-0.7}^{\mathrm{+}\mathrm{2.4}}\mathrm{\right)}\mathrm{\times }{\mathrm{10}}^{\mathrm{51}}\hspace{0.17em}\mathrm{erg}$.