On a Spectral Method for βparticle Bound Excitation Collisions in Kilonovae
Abstract
The interaction of βparticles with the weakly ionized plasma background is an important mechanism for powering the kilonova (KN) transient signal from neutron star mergers. For this purpose, we present an implementation of the approximate fastparticle collision kernel, described by Inokuti following the seminal formulation of Bethe, in a spectral solver of the Vlasov–Maxwell–Boltzmann equation. In particular, we expand the fastparticle planewave atomic excitation kernel into coefficients of the Hermite basis, and derive the relevant discrete spectral system. In this fastparticle limit, the approach permits the direct use of atomic data, including optical oscillator strengths, normally applied to photon–matter interaction. The resulting spectral matrix is implemented in the MASSAPP spectral solver framework, in a way that avoids full matrix storage per spatial zone. We numerically verify aspects of the matrix construction, and present a proofofprinciple 3D simulation of a 2D axisymmetric KN ejecta snapshot. Our preliminary numerical results indicate that a reasonable choice of Hermite basis parameters for βparticles in the KN is a bulk velocity parameter u = 0, a thermal velocity parameter α = 0.5c, and a 9 × 9 × 9 mode velocity basis set (Hermite orders of 0–8 in each dimension). For interiorejecta sample zones, we estimate that the ratio of thermalization from largeangle (≳2.°5) bound excitation scattering to total thermalization is ∼0.002–0.003.
 Publication:

The Astrophysical Journal
 Pub Date:
 May 2024
 DOI:
 10.3847/15384357/ad37f9
 arXiv:
 arXiv:2401.11069
 Bibcode:
 2024ApJ...966..177W
 Keywords:

 Neutron stars;
 Plasma physics;
 1108;
 2089;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 Accepted to ApJ, 26 pages, 8 figures