arXiv subjects

Martin Luepker

Publications and source records attributed to Martin Luepker.

4 recordsLinked to original sources

arXiv2026

Collisionless Accretion of Finite-Angular-Momentum Plasma onto a Spinning Black Hole

In low-luminosity active galactic nuclei like M87* and Sgr A*, the accretion disk around the central supermassive black hole is tenuous and collisionless. As a result, the usual ideal magnetohydrodynamics (MHD) approximation may not be applicable. In this Letter, we report on the first fully kinetic simulations of the accretion process where the plasma initially has finite angular momentum. The simulated accretion flow behaves remarkably similarly to the magnetically arrested disk (MAD) regime of ideal MHD, reproducing episodes of magnetic flux saturation and eruption typical of MADs. The resemblance to fluid models owes largely to kinetic instabilities, which regulate pressure anisotropy in the disk, allowing fluid terms to dominate the angular momentum transfer. In addition, by handling vacuum regions effectively, our kinetic approach probes the matter supply to the jet funnel. We observe no efficient penetration of the accreting material into this region, which suggests that a pair discharge may be required to sustain the Blandford-Znajek process.

astro-ph.HE↗

arXiv2025

Stable Collisionless Tori Around Kerr Black Holes

In low-luminosity active galactic nuclei like M87$^\ast$ and Sgr A$^\ast$, the accretion flow in the vicinity of the black hole is in the collisionless regime, meaning that the collisional mean free path of charged particles is much larger than the dynamical length scales. To properly model the particle energization and emission from the collisionless accretion flow, a promising approach is to employ the global general-relativistic particle-in-cell simulations$\unicode{x2014}$a newly developed, fully kinetic, first-principles method. However, it has been challenging to set up an initial condition that involves collisionless gas with finite angular momentum. We present, for the first time, a class of analytic kinetic equilibria of collisionless tori around a Kerr black hole. We have successfully implemented the collisionless tori in our GPU-based GRPIC code framework Aperture, and found them to be stable for hundreds to thousands of dynamical times in 2D axisymmetric simulations when there is no initial seed magnetic field. These kinetic equilibria serve as ideal starting points for future studies of the physics of collisionless accretion and jet launching.

astro-ph.HE↗

arXiv2025

Physics of Pair Producing Gaps in Black Hole Magnetospheres: Two Dimensional General Relativistic Particle-in-cell Simulations

Black holes can launch powerful jets through the Blandford-Znajek process. This relies on enough plasma in the jet funnel to conduct the necessary current. However, in some low luminosity active galactic nuclei, the plasma supply near the jet base may be an issue. It has been proposed that spark gaps -- local regions with unscreened electric field -- can form in the magnetosphere, accelerating particles to initiate pair cascades, thus filling the jet funnel with plasma. In this paper, we carry out 2D general relativistic particle-in-cell (GRPIC) simulations of the gap, including self-consistent treatment of inverse Compton scattering and pair production. We observe gap dynamics that is fully consistent with our earlier 1D GRPIC simulations. We find strong dependence of the gap power on the soft photon spectrum and energy density, as well as the strength of the horizon magnetic field. We derive physically motivated scaling relations, and applying to M87, we find that the gap may be energetically viable for the observed TeV flares. For Sgr A$^*$, the energy dissipated in the gap may also be sufficient to power the X-ray flares.

astro-ph.HE↗

arXiv2025

Introducing APERTURE: A GPU-based General Relativistic Particle-in-Cell Simulation Framework

Low-luminosity Active Galactic Nuclei (AGN) are believed to be surrounded by a collisionless, highly magnetized accretion flow. As a result, Particle-in-Cell simulations are the best tools to study the immediate vicinity of the event horizons of these supermassive black holes. We present a GPU-based general relativistic particle-in-cell (GRPIC) code framework called Aperture. Aperture is developed in C++, with compute kernels written in CUDA and HIP to take advantage of the massive acceleration modern GPUs enable. The code is organized in a fully modular way, allowing easy extensions to new physics problems. In this paper, we describe in detail the particle pusher, field solver, and charge-conserving current deposition algorithms employed in Aperture, and present test cases to validate their correctness. Then, we apply the code to study spark gaps and plasma injection in black hole magnetospheres. We find that the apparent location and time-evolution of the gap depend on the observer. Our results reconcile the previous conflicting findings from 1D and 2D simulations in the literature.

astro-ph.HE↗