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 modeling 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 toward red colors, 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.
Highlights
Short duration gamma-ray burst (GRB) were long suspected to be the product of compact binary mergers (Blinnikov et al 1984; Goodman 1986; Paczynski 1986; Eichler et al 1989; Narayan et al 1992), involving either two neutron stars (NSs) or an NS and a solar-mass black hole (BH)
It seems plausible that kilonovae similar to AT2017gfo could have been detected in the optical, not clearly identified prior to GW170817. Whereas this observational evidence suggests that r-process nucleosynthesis is common in the aftermath of a short GRB, it does not inform on the production of the heaviest elements, i.e. those with atomic mass number A 140
The X-ray afterglow is consistent with standard forward shock emission, whereas the radio signal was likely dominated by a weak reverse shock
Summary
Short duration GRBs were long suspected to be the product of compact binary mergers (Blinnikov et al 1984; Goodman 1986; Paczynski 1986; Eichler et al 1989; Narayan et al 1992), involving either two neutron stars (NSs) or an NS and a solar-mass black hole (BH). GRB afterglow probes the jet structure and geometry as well as the properties of the surrounding environment Another long-standing prediction of the NS merger model is the presence of a luminous, short-lived transient arising from the radioactive decay of freshly synthesized r-process elements (Li & Paczynski 1998; Metzger et al 2010; Barnes & Kasen 2013; Tanaka & Hotokezaka 2013). It seems plausible that kilonovae similar to AT2017gfo could have been detected in the optical, not clearly identified prior to GW170817 Whereas this observational evidence suggests that r-process nucleosynthesis is common in the aftermath of a short GRB, it does not inform on the production of the heaviest elements, i.e. those with atomic mass number A 140. The quoted errors are at the 68 per cent confidence level, and upper limits are at the 3σ confidence level
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