Radioactive Heating Rate of rprocess Elements and Macronova Light Curve
Abstract
We study the heating rate of rprocess nuclei and thermalization of decay products in neutron star merger ejecta and macronova (kilonova) light curves. Thermalization of charged decay products, i.e., electrons, αparticles, and fission fragments, is calculated according to their injection energy. The γray thermalization processes are also properly calculated by taking the γray spectrum of each decay into account. We show that the βdecay heating rate at later times approaches a powerlaw decline as ∝t^{2.8}, which agrees with the result of Waxman et al. We present a new analytic model to calculate macronova light curves, in which the density structure of the ejecta is accounted for. We demonstrate that the observed bolometric light curve and temperature evolution of the macronova associated with GW170817 are reproduced well by the βdecay heating rate with the solar rprocess abundance pattern. We interpret the break in the observed bolometric light curve around a week as a result of the diffusion wave crossing a significant part of the ejecta rather than a thermalization break. We also show that the timeweighted integral of the bolometric light curve (Katz integral) is useful to provide an estimate of the total rprocess mass from the observed data, which is independent of the highly uncertain radiative transfer. For the macronova in GW170817, the ejecta mass is robustly estimated as ≍0.05 M_{☉} for A_{min} ≤ 72 and 85 ≤ A_{min} ≤ 130 with the solar rprocess abundance pattern. The code for computation of the heating rate and light curve for given initial nuclear abundances is publicly available.
 Publication:

The Astrophysical Journal
 Pub Date:
 March 2020
 DOI:
 10.3847/15384357/ab6a98
 arXiv:
 arXiv:1909.02581
 Bibcode:
 2020ApJ...891..152H
 Keywords:

 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 16 pages, 11 figures, code available at https://github.com/hotokezaka/HeatingRate