I will review efforts to model radiation transfer in relativistic spacetimes, describing different methods and approaches. I will discuss the pros and cons of these methods in their application to accreting black hole systems and report on recent results solving the transfer equation directly. I will endeavor to highlight some points of contact between photon radiation transfer applications in accreting systems and neutrino transport in core collapse and compact object mergers.

The 2017 detection of the in-spiral and merger of two neutron stars was a landmark discovery in astrophysics. Through a wealth of multi-messenger data, we now know that the merger of these ultracompact stellar remnants is a central engine of short gamma ray bursts and a site of r-process nucleosynthesis, where the heaviest elements in our universe are formed. The radioactive decay of unstable heavy elements produced in such mergers powers an optical and infra-red transient: The kilonova. One key driver of nucleosynthesis and resultant electromagnetic afterglow is wind driven by an accretion disk formed around the compact remnant. Neutrino transport plays a key role in setting the electron fraction in this outflow, thus controlling nucleosynthesis. Collapsars are black hole accretion disks formed after the core of a massive, rapidly rotating star collapses to a black hole. These dramatic systems rely on much the same physics and modeling as post-merger disks, and can also be a key driver of r-processes nucleosynthesis. I present recent progress in modeling these enigmatic systems, with an emphasis on both the impact and techniques of detailed Monte Carlo neutrino transport.

The linking of physics particle system in mean fields or interacting as multispecies systems can be viewed as a probabilistic model as envisioned by L. Boltzmann and G. Maxwell of dissipative stochastic dynamics of one or more species marking the birth of thermodynamics by expectations of statistical flows. In the early Twentieth Century, the introduction of electron and ions systems and the raise of particle charge and neutral particle dynamics the Boltzmann ideas were extended by Dirac and Fermi to encompassed an accurate model to describe electro-magnetic forces and braking of symmetry, and so the Boltzmann-Dirac-Fermi model arose as an accurate statistical frame work of understanding the hot-electron dynamics in solid states modeled by Boltzmann-Dirac-Fermi in terms of the statistical frameworks of thermodynamics for nanoelectronics. At the same time the raise of astrophysical systems, in the absence of easily observable experimental data become W. Pauli proposed neutral particles that can dissipate relativistic electron dynamics, and labeled them neutrinos and anti-neutrinos as a possible dissipative mechanisms. I will present a natural link between the probabilistic framework from Chapman-Kolmogorov flows the evolution of N-particle systems from different species that under ergodic assumptions that are sufficient for near space homogeneous system at the particle level for model reduction emerge a kinetic systems for neutrino and antineutrino particles couples to equations of relativized matter equations for rest mass density, with temperature and electron fraction rates depending on space-time, all in a spherical coordinate framework. We’ll discuss conservation properties for the system, entropies, and the natural Galerkin-Petrov kinetic scheme as a very efficient tool for fast solvers for this reduced model.

With an increasing demand of extensive parallel computing in numerical simulations addressing various astrophysical problems, codes which can efficiently work on GPUs are the need of the hour. In this talk, I will discuss the salient features of a new open-source general relativistic magnetohydrodynamic (GRMHD) code AsterX, which is built upon CarpetX, a new driver for the Einstein Toolkit. AsterX is based on the flux-conservative Valencia formulation, considering staggered vector potential evolution. It designed to work on GPUs and also takes advantage of the block-structured adaptive mesh refinement provided by CarpetX through the AMReX framework. I will also discuss the various stringent 1D, 2D and 3D GRMHD tests performed with AsterX on the Frontier cluster, and also present scaling results.

The origin of heavy elements synthesized via rapid neutron capture process (r-process) is one of the longstanding questions in astronomy. So far, the only confirmed astrophysical sites of r-process nucleosynthesis are neutron star mergers, thanks to the multimessenger detection in 2017. However, there is a another promising site - dying massive stars, or collapsars, which were first invoked as a model to explain long gamma ray bursts. Here, we are going to present our work in modeling the collapsar explosions using 3-dimensional general relativistic magnetohydrodynamics simulations.