The afterglow and kilonova of the short GRB 160821B

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 Xray, 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 (θ view ∼ θ core ) 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 (M ej 0.006 M ⊙ ) and velocities (v ej 0.05c) of lanthanide-rich ejecta are consistent with a binary neutron star merger progenitor.

A luminosity distribution for kilonovae based on short gamma-ray burst afterglows

“…In the Gaussian structured jet case, energy drops as E(θ) = E 0 exp[−θ 2 /(2θ 2 c )] up to a truncating angle θ w , where E 0 is the isotropic equivalent energy and θ c is the opening angle. In the following, we will use the formalism adopted by Troja et al (2018b), where the Gaussian jet is implemented as a series of concentric top hat jets and the cocoon as a decelerating shell model which includes ongoing energy injection. We use the implementation in afterglowpy (Ryan et al 2019).…”

“…Figure 1 shows the observed data superimposed on different fitted models. Here afterglow only denotes the Gaussian afterglow model of Troja et al (2018b), kilonova only the kilonova model of Kasen et al (2017) and kilonova+afterglow the combination between the two models. Adding a kilonova contribution to the afterglow causes an increase in the optical flux at early times and, as a result, predicts less flux from the afterglow at later times (compared to the afterglow only model).…”

“…Lightcurves for the Gaussian afterglow model(Troja et al 2018b), the kilonova modelKasen et al (2017), and a sum of the afterglow and kilonova model for GRB130603B. "Kilonova part" denotes the kilonova contribution to the "after-glow+kilonova" model.…”

Linking extended and plateau emissions of short gamma-ray bursts

“…For example, the ISM density of GRB 130603B is poorly constrained within 4 orders-of-magnitude (Table 1). For GRB 160821B, Lamb et al (2019) and Troja et al (2019) constructed a multi-wavelength model independently, but their interpretations of the light curve such as a jet break do not match with each other. We show both of their results in Table 1 and use their estimated parameters equally.…”

Gamma-ray bursts associating with gravitational waves from compact binary mergers

“…受此启发, 人们对2013年之前观测到的短伽马 暴余辉进行了仔细的检查, 从中又发现了多例疑 似的候选体 [90][91][92] . 此后, 在新的短伽马暴(如GRB 140903A和GRB 160821B)观测中, 类似的候选体仍 然在不断地被发现 [93,94] . 当然, 被完全确认的第 一例千新星无疑是与GW170817引力波事件成协 的AT2017gfo光学暂现源 [95,96] .…”