Short GRB 160821B: A Reverse Shock, a Refreshed Shock, and a Well-sampled Kilonova

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 }<SUB>j</SUB>∼ {1.9}<SUB>-0.03</SUB><SUP>+0.10</SUP> 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 <SUB>dyn</SUB> = (1.0 ± 0.6) × 10<SUP>-3</SUP> M <SUB>☉</SUB> and a secular (post-merger) ejecta mass with M <SUB>pm</SUB> = (1.0 ± 0.6) × 10<SUP>-2</SUP> M <SUB>☉</SUB>, 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.