The afterglow and kilonova of the short GRB 160821B
Troja, E.; Cenko, S. B.; Sakamoto, T.; Ricci, R.; Veilleux, S.; Tiengo, A.; Pandey, S. B.; Novara, G.; Lien, A. Y.; Acosta-Pulido, J. A.; Castro-Tirado, A. J.; Park, I. H.; Eikenberry, S. S.; Becerra González, J.; Márquez, I.; Hu, Y.; Sokolov, I. V.; Zhang, B. B.; Jeong, S.; Valeev, A. F.; Guziy, S.; Tello, J. C.; Sokolov, V. V.; Ryan, G. S.; Sánchez-Rámirez, R.; Ackley, K. D.; Caballero García, M. D.
United States, Spain, Italy, Australia, Czech Republic, Ukraine, South Korea, India, Japan, Russia, China
Abstract
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 modelling 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 (θview ∼ θcore) while expanding into a low-density medium (n ≈ 10-3 cm-3). Analysis of the kilonova properties suggests a rapid evolution towards red colours, 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 (Mej ≲ 0.006 M⊙) and velocities (vej ≳ 0.05c) of lanthanide-rich ejecta are consistent with a binary neutron star merger progenitor.