Short GRB 160821B: A Reverse Shock, a Refreshed Shock, and a Well-sampled Kilonova
Pian, E.; Greiner, J.; Fruchter, A. S.; Wijers, R. A. M. J.; Watson, D.; Fynbo, J. P. U.; Tanaka, M.; Kann, D. A.; O'Brien, P. T.; van der Horst, A. J.; Perley, D. A.; Corsi, A.; Wiersema, K.; Page, K. L.; Izzo, L.; Levan, A. J.; Starling, R. L. C.; Tanvir, N. R.; Cano, Z.; Evans, P. A.; Hjorth, J.; Jakobsson, P.; Thöne, C. C.; de Ugarte Postigo, A.; Heintz, K. E.; Lamb, G. P.; Malesani, D. B.; Pugliese, G.; Rosswog, S.; Xu, D.; Gompertz, B.; Shibata, M.; Kawaguchi, K.; Higgins, A.
United Kingdom, Netherlands, Spain, Denmark, Japan, United States, Sweden, Germany, Iceland, Italy, China
Abstract
We report our identification of the optical afterglow and host galaxy of the short-duration gamma-ray burst sGRB 160821B. The spectroscopic redshift of the host is z = 0.162, making it one of the lowest redshift short-duration gamma-ray bursts (sGRBs) identified by Swift. Our intensive follow-up campaign using a range of ground-based facilities as well as Hubble Space Telescope, XMM-Newton, and Swift, shows evidence for a late-time excess of optical and near-infrared emission in addition to a complex afterglow. The afterglow light curve at X-ray frequencies reveals a narrow jet, {θ }j∼ {1.9}-0.03+0.10 deg, that is refreshed at >1 day post-burst by a slower outflow with significantly more energy than the initial outflow that produced the main GRB. Observations of the 5 GHz radio afterglow shows a reverse shock into a mildly magnetized shell. The optical and near-infrared excess is fainter than AT2017gfo associated with GW170817, and is well explained by a kilonova with dynamic ejecta mass M dyn = (1.0 ± 0.6) × 10-3 M ⊙ and a secular (post-merger) ejecta mass with M pm = (1.0 ± 0.6) × 10-2 M ⊙, consistent with a binary neutron star merger resulting in a short-lived massive neutron star. This optical and near-infrared data set provides the best-sampled kilonova light curve without a gravitational wave trigger to date.