The afterglow and kilonova of the short GRB 160821B. Issue 2 (27th August 2019)

• Record Type: Journal Article
• Authors: Troja, E; Castro-Tirado, A J; Becerra González, J; Hu, Y; Ryan, G S; Cenko, S B; Ricci, R; Novara, G; Sánchez-Rámirez, R; Acosta-Pulido, J A; Ackley, K D; Caballero García, M D; Eikenberry, S S; Guziy, S; Jeong, S; Lien, A Y; Márquez, I; Pandey, S B; Park, I H; Sakamoto, T; Tello, J C; Sokolov, I V; Sokolov, V V; Tiengo, A; Valeev, A F; Zhang, B B; Veilleux, S
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 489:Issue 2(2019)
• Page Start: 2104
• Page End: 2116
• Publication Date: 2019-08-27
• Subjects: gravitational waves -- nuclear reactions, nucleosynthesis, abundances -- gamma-ray burst: general -- stars: neutron; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz2255
• Languages: English
• ISSNs: 0035-8711

Abstract

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 ( θ view ∼ θ core ) 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 ( M ej ≲ 0.006 M⊙ ) and velocities ( v ej ≳ 0.05 c ) of lanthanide-rich ejecta are consistent with a binary neutron star merger progenitor.