Radioactivity and thermalization in the ejecta of compact object mergers and their impact on kilonova light curves

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 kilonovae include the emission profiles of the radioactive decay products---non-thermal beta- and alpha-particles, fission fragments, and gamma-rays---and the efficiency with which they deposit their energy in the ejecta. The total 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 energy of the decay products is absorbed in the ejecta, and present time-dependent thermalization efficiencies for each particle type. We determine the net heating efficiency and explore its dependence on r-process yields---in particular, the production of translead nuclei that undergo alpha-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 net 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 alpha-decay from translead isotopes; data at these times may therefore be diagnostic of ejecta abundances.