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Abstract

We report our identification of the optical afterglow and host galaxy of the
short-duration gamma-ray burst sGRB 160821Bb. The spectroscopic redshift of the
host is $z=0.162$, making it one of the lowest redshift sGRBs identified by
Swift. Our intensive follow-up campaign using a range of ground-based
facilities as well as HST, XMM 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,
$\theta_j\sim1.9$ 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 magnetised 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 $M_{\rm dyn}=(2.0\pm1.2)\times10^{-3}$\,M$_{\odot}$
and a secular ejecta mass with $M_{\rm pm}=
1.0\pm0.6)\times10^{-2}$\,M$_\odot$, consistent with a binary neutron star
merger resulting in a short-lived massive neutron star. This optical and
near-infrared dataset provides the best-sampled kilonova light-curve without a
gravitational wave trigger to date.