GRB 160821B is a short duration gamma-ray burst (GRB) detected and localized by the Neil Gehrels Swift Observatory in the outskirts of a spiral galaxy at z=0.1613, at a projected physical offset of 16 kpc from the galaxy's center. We present X-ray, optical/nIR and radio observations of its counterpart and model them with two distinct components of emission: a standard afterglow, arising from the interaction of the relativistic jet with the surrounding medium, and a kilonova, powered by the radioactive decay of the sub-relativistic ejecta. Broadband modeling of the afterglow data reveals a weak reverse shock propagating backward into the jet, and a likely jet-break at 3.5 d. This is consistent with a structured jet seen slightly off-axis while expanding into a low-density medium. Analysis of the kilonova properties suggests a rapid evolution toward red colors, similar to AT2017gfo, and a low nIR luminosity, possibly due to the presence of a long-lived neutron star. The global properties of the environment, the inferred low mass (M_ej < 0.006 Msun) and velocities (v > 0.05 c) of lanthanide-rich ejecta are consistent with a binary neutron star merger progenitor.
@article{arxiv.1905.01290,
title = {The afterglow and kilonova of the short GRB 160821B},
author = {E. Troja and A. J. Castro-Tirado and J. Becerra Gonzalez and Y. Hu and G. S. Ryan and S. B. Cenko and R. Ricci and G. Novara and R. Sanchez-Ramirez and J. A. Acosta-Pulido and M. D. Caballero Garcia and S. Guziy and S. Jeong and A. Y. Lien and I. Marquez and S. B. Pandey and I. H. Park and J. C. Tello and T. Sakamoto and I. V. Sokolov and V. V. Sokolov and A. Tiengo and A. F. Valeev and B. B. Zhang and S. Veilleux},
journal= {arXiv preprint arXiv:1905.01290},
year = {2019}
}
Comments
14 pages, 6 figures, MNRAS, in press. Moderate revision, added Figure 5 and X-ray data to Table 1