An Extension of the Athena++ Code Framework for Radiation-magnetohydrodynamics in General Relativity Using a Finite-solid-angle Discretization

Abstract

We extend the general-relativistic magnetohydrodynamics (GRMHD) capabilities of Athena++ to incorporate radiation. The intensity field in each finite-volume cell is discretized in angle, with explicit transport in both space and angle properly accounting for the effects of gravity on null geodesics, and with matter and radiation coupled in a locally implicit fashion. Here we describe the numerical procedure in detail, verifying its correctness with a suite of tests. Motivated in particular by black hole accretion in the high-accretion-rate, thin-disk regime, we demonstrate the application of the method to this problem. With excellent scaling on flagship computing clusters, the port of the algorithm to the GPU-enabled AthenaK code now allows the simulation of many previously intractable radiation-GRMHD systems.

Recipes for Jet Feedback and Spin Evolution of Black Holes with Strongly Magnetized Super-Eddington Accretion Disks

“…Since then, the M1 closure scheme has been applied in other GRRMHD codes (McKinney et al 2014;Takahashi et al 2016;Utsumi et al 2022), as well as a GPU-accelerated GRRMHD code (Liska et al 2023). Alternative methods of treating radiation in GRRMHD include directly solving the radiative transfer equations to obtain the Eddington tensor , Monte Carlo methods (Ryan et al 2015), and using a discretized radiation tensor (White et al 2023). The M1 closure scheme allows limited treatment of anisotropic radiation fields.…”

Magnetic Flux Plays an Important Role during a Black Hole X-Ray Binary Outburst in Radiative Two-temperature General Relativistic Magnetohydrodynamic Simulations

“…However, simulating accretion disks in these luminous states is numerically challenging due to the presence of dynamically important radiation fields and thermal decoupling between ions and electrons. Presently, only a handful of GRMHD codes are able to model radiation (e.g., McKinney et al 2013;Saḑowski et al 2013;Fragile et al 2014;Ryan et al 2017;White et al 2023). In addition, since radiative cooling makes such accretion disks thinner, one needs a much higher resolution to resolve them.…”

Black Hole–Disk Interactions in Magnetically Arrested Active Galactic Nuclei: General Relativistic Magnetohydrodynamic Simulations Using a Time-dependent, Binary Metric

“…This is not only because avoiding numerical relativity makes the computation faster, but it will allow the community to take full advantage of the numerous advances made in single black hole GRMHD codes if these are adapted for time-dependent metrics. For instance, in the past couple of decades, the GRMHD accretion community has been able to explore effects such as varying black hole spin (Event Horizon Telescope Collaboration et al 2019;Akiyama et al 2022), varying magnetic flux supply (Tchekhovskoy et al 2011;Narayan et al 2012), varying disk tilt (McKinney et al 2013;Liska et al 2018;White et al 2019a;Chatterjee et al 2020), including radiative effects (Ryan et al 2015;White et al 2023), including nonideal physics (Ressler et al 2015;Chandra et al 2017;Foucart et al 2017;Ripperda et al 2019), and studying a variety of initial conditions informed by larger scales (Ressler et al 2020b(Ressler et al , 2021Cho et al 2023;Kaaz et al 2023;Lalakos et al 2024). Furthermore, since the user base for GRMHD is currently much larger than that for numerical relativity (e.g., Porth et al 2019), it could encourage more researchers to study binary black hole systems.…”

“…As a conservative code using constrained transport for magnetic fields, it conserves mass, energy, momentum, and magnetic flux to machine precision. It also now has full support for radiation, being the first GRMHD code to directly solve the general relativistic Boltzmann transport equation (White et al 2023). Finally, it is widely used, public, and a ported version for use on GPUs will soon be publicly available (AthenaK).…”