Radioactivity and Thermalization in the Ejecta of Compact Object Mergers and Their Impact on Kilonova Light Curves

Abstract

One of the most promising electromagnetic signatures of compact object mergers are kilonovae: approximately isotropic radioactively-powered transients that peak days to weeks post-merger. Key uncertainties in modeling kilonova light curves include the emission profiles of the radioactive decay products-non-thermal beta-particles, alpha-particles, fission fragments, and gamma-rays-and the efficiency with which they deposit their energy in the ejecta. The amount of radioactive energy and the efficiency of its thermalization sets the luminosity budget and is therefore necessary for predicting kilonova light curves. We outline the uncertainties in r -process decay, describe the physical processes by which the kinetic energy of the decay products is absorbed in the ejecta, and calculate timedependent thermalization efficiencies for each particle type. We determine the net thermalization efficiency and explore its dependence on r -process nucleosynthetic yields-in particular, the production of translead nuclei that undergo α-decay-and on the ejecta's mass, velocity, composition, and magnetic field configuration. We incorporate our results into new time-dependent, multi-wavelength radiation transport simulations, and calculate updated predictions of kilonova light curves. Thermalization has a substantial effect on kilonova photometry, reducing the luminosity by a factor of roughly 2 at peak, and by an order of magnitude or more at later times (15 days or more after explosion). We present simple analytic fits to time-dependent thermalization efficiencies, which can easily be used to improve light curve models. We briefly revisit the putative kilonova that accompanied gamma ray burst 130603B, and offer new estimates of the mass ejected in that event. We find that later-time kilonova light curves can be significantly impacted by α-decay from translead isotopes; data at these times may therefore be diagnostic of ejecta abundances.

GRB 170817A Associated with GW170817: Multi-frequency Observations and Modeling of Prompt Gamma-Ray Emission

“…It is expected that, first, tidal tails eject 0.01 − 0.03M with outflow speed v ex ≈ 0.1 − 0.3c. This material is likely a site of r-process nucleosynthesis and can be seen as a kilonova -an optical emission of peak luminosity ∼ 10 41 erg s −1 lasting for a few weeks (see Roberts et al 2011;Bauswein et al 2013;Kasen et al 2013;Grossman et al 2014;Wanajo et al 2014;Barnes et al 2016;Wu et al 2017).…”

Follow Up of GW170817 and Its Electromagnetic Counterpart by Australian-Led Observing Programmes

“…Electromagnetically, such mergers are also postulated to generate a relatively rapidly evolving optical/infrared transient-referred to as kilonova or macronova (e.g., Li & Paczyński 1998;Metzger et al 2010;Roberts et al 2011;Barnes & Kasen 2013;Kasen, Fernández, & Metzger 2015;Metzger et al 2015;Barnes et al 2016). The combination of an sGRB and kilonova is considered the 'smoking gun' signature of such mergers.…”

Localization of binary neutron star mergers with second and third generation gravitational-wave detectors

“…Since the gamma ray burst (GRB) emission is likely to be essentially concurrent with the merger, it will be difficult to use GW observations to provide advanced warning to gamma ray burst (GRB) satellites. On the other hand, kilonova emission is expected to last for days or possibly weeks in the optical and near infrared bands [129,130]. Furthermore while gamma ray burst (GRB) are beamed, kilonovae are relatively isotropic, and thus are the more likely counterpart to a typical BNS merger [18].…”

“…There are various models for the kilonova emission, which depend upon the mass of the ejecta as well as its opacity [129,130]. Broadly, the prediction is for an optical or near infrared emission, which will last for days or possibly weeks.…”