Bibcode
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.
Bibliographical reference
Monthly Notices of the Royal Astronomical Society
Advertised on:
10
2019
Citations
131
Refereed citations
115
Description
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.
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