Conference

Supermassive black hole mergers represent a spectacular cosmic event with immense energy implications, emitting gravitational waves equivalent to the total light output of stars in the entire Universe within a brief timespan. These mergers play a crucial role in shaping the overall mass distribution of supermassive black holes across the cosmos. However, capturing visual evidence of these mergers remains elusive due to uncertainties surrounding the type of light emissions accompanying gravitational waves during these events. To address this challenge, novel General Relativistic Magnetohydrodynamics (GRMHD) simulations are being conducted to gain detailed insights into the astrophysical environments surrounding supermassive black hole binaries as they progress towards merger. By employing sophisticated computational techniques capable of accurately capturing the intricate dynamics of accretion within circumbinary disks and the relativistic flow of magnetized matter around each black hole, these simulations reveal the behavior of gas flows near binary systems, particularly when both black holes exhibit spin. These simulated scenarios provide critical data for predicting the electromagnetic and gravitational wave signatures produced by supermassive binary black holes, guiding future observational strategies utilizing advanced missions like LISA and other upcoming astronomical facilities. Ongoing initiatives are focused on refining computational tools to deepen our understanding of supermassive black hole behavior within binary systems and its interactions.

In this talk, I will explore key open questions in our understanding of neutron star mergers and their multi-messenger emission, highlighting recent progress made by our group. I will then introduce some of the first applications of AthenaK to compact binary mergers and outline our plans for its future development.

With the ongoing transition toward exascale computing to tackle a range of open questions via numerical simulations, the development of GPU-optimized codes has become essential. In this talk, I will highlight the key features of AsterX, a novel open-source, modular, GPU-accelerated general relativistic magnetohydrodynamic (GRMHD) code for fully dynamical spacetimes in 3D Cartesian coordinates. Built for exascale applications, AsterX integrates with CarpetX, the new driver for the Einstein Toolkit, leveraging AMReX for block-structured adaptive mesh refinement (AMR). The code employs the flux-conservative Valencia formulation for GRMHD, and uses high-resolution shock capturing schemes to ensure accurate hydrodynamic modeling. Alongside discussions on the ongoing code development, I will also present the results of comprehensive 1D, 2D, and 3D GRMHD tests conducted on OLCF's Frontier supercomputer, highlighting AsterX's performance gains through subcycling in time and demonstrating its scaling efficiency across thousands of nodes.

In certain scenarios, the accreted angular momentum of plasma onto a black hole could be low; however, how the accretion dynamics depends on the angular momentum content of the plasma is still not fully understood. We present three-dimensional, general relativistic magnetohydrodynamic simulations of low angular momentum accretion flows around rapidly spinning black holes (with spin $a = +0.9$). The initial condition is a Fishbone-Moncrief (FM) torus threaded by a large amount of poloidal magnetic flux, where the angular velocity is a fraction $f$ of the standard value. For $f = 0$, the accretion flow becomes magnetically arrested and launches relativistic jets but only for a very short duration. After that, free-falling plasma breaks through the magnetic barrier, loading the jet with mass and destroying the jet-disk structure. Meanwhile, magnetic flux is lost via giant, asymmetrical magnetic bubbles that float away from the black hole. The accretion then exits the magnetically arrested state. For $f = 0.1$, the dimensionless magnetic flux threading the black hole oscillates quasi-periodically. The jet-disk structure shows concurrent revival and destruction while the gas efficiency at the event horizon changes accordingly. For $f \geq 0.3$, we find that the dynamical behavior of the system starts to approach that of a standard accreting FM torus. Our results thus suggest that the accreted angular momentum is an important parameter that governs the maintenance of a magnetically arrested flow and launching of relativistic jets around black holes.

It has been proposed recently that Inverse Compton scattering of soft photons by pairs acceleration in reconnecting current sheets that form during MAD states, can be the source of the TeV emission detected in M87. In this talk I’ll argue that synchrotron emission by ions accelerated in the current sheet is expected to be the dominant source of the GeV emission observed. The analysis is based on 3D, radiative PIC simulations of magnetic reconnection in pair-ion plasma, under conditions anticipated in M87 during MAD states.

Magnetars produce the brightest detected outbursts in the X-ray and radio bands, offering unique opportunities to probe extreme plasma physics and exotic quantum electrodynamic processes. Magnetospheric reconnection is a suspected mechanism for generating bursts and giant flares. However, modeling the interplay between small-scale reconnection processes and the global structure of magnetar magnetospheres remains a major theoretical and computational challenge. This is due to the multi-scale nature of the problem: resolving localized reconnection while capturing the global dynamics of the star in a single numerical simulation is computationally prohibitive. Global models have primarily used force-free schemes which are unable to capture dissipation, recent advances have made global ideal magnetohydrodynamic models viable but will still lack the explicit resistivity needed to carefully study the reconnection physics. This motivates the use of local resistive relativistic magnetohydrodynamic simulations which probe the details of reconnection in the magnetospheric regime. These local simulations allow for the quantification of magnetic energy dissipation and the role it plays in powering emission due to magnetic reconnection inaccessible to force-free models. Furthermore, the use of ideal global models motivates the careful quantification of numerical dissipation in ideal schemes. We use 1D and 2D tests to quantify the nature of magnetic dissipation in resistive and ideal, relativistic magnetohydrodynamic and force-free-electrodynamic schemes. Our tests, which are agnostic to the form of the effective numerical dissipation operator in ideal schemes, probe both Ohmic dissipation and magnetic reconnection of current structures in the highly magnetized strong guide field regime. These tests characterize exactly how well schemes commonly used to model magnetic dissipation in magnetar and pulsar magnetospheres perform compared to resistive relativistic magnetohydrodynamics. We find Ohmic dissipation in both ideal magnetohydrodynamic and force-free schemes to be subdiffuse, while producing an analogue to the Sweet-Parker regime at low resolutions and an asymptotic reconnection rate at high resolutions. The resistive force-free scheme we test is found to exactly reproduce the Ohmic diffusion and the Sweet-Parker regime, but differs from the full resistive relativistic magnetohydrodynamic result in the asymptotic regime.

Plasmoid-dominated magnetic reconnection is known to convert magnetic energy into heat and kinetic energy and is thought to be closely related to high-energy emission features originating near compact objects. We present preliminary results of high-resolution special-relativistic resistive magnetohydrodynamic simulations of reconnecting 3D current sheets starting from a Harris equilibrium. We focussed on identifying and quantifying flux rope structures and how the properties of produced magnetosonic waves (potentially generating winds around compact objects) rely on the underlying plasma description. We show that while the initial stage does not differ substantially from 2D results, a secondary turbulent reconnection phase can only be studied in 3D.

Observations of luminous black holes in X-ray binaries and Seyfert galaxies show power-law emission, thought to originate from photons that inverse Compton scatter off a hot electron cloud. If the coronal electrons are heated by magnetic dissipation, i.e. reconnection or turbulence, then one might expect to observe direct synchrotron emission in the radio/mm from these electrons. However, because timing studies constrain the X-ray emission to be within ~10 rg of the central black hole, the direct synchrotron emission from this compact volume would be strongly self-absorbed until much further away from BH. In this talk, I will question the de facto definition of the corona as a compact, X-ray-emitting region and shift instead to a paradigm where the corona encompasses multiple layers with distinct spectral components. Motivated by highly-magnetized winds found in GRMHD simulations, I will present a model for such an extended, outflowing corona. I will discuss this model in the context of radio-quiet AGN, where recent observations have demonstrated the presence of compact mm emission.

Tilted magnetically arrested disks (MADs) around black holes—where strong magnetic fields regulate accretion and jets—exhibit striking alignment behavior dictated by black hole spin direction. Using 3D general-relativistic magnetohydrodynamic (GRMHD) simulations of tilted MADs, we find that prograde disks align via a two-stage process: an initial rapid alignment phase, ending at the flux saturation timescale, followed by a slower, spin-independent phase. In contrast, retrograde MADs remain persistently misaligned, with their inner disks precessing four times faster than weakly magnetized systems—a potential explanation for high-frequency quasi-periodic oscillations (QPOs). By analyzing magnetic and hydrodynamic torques within ideal GRMHD, we show that alignment in prograde disks is dominated by electromagnetic stresses from the magnetosphere. However, the same magnetic forces— which always act to align the disk with the black hole spin—are significantly weaker in retrograde disks, allowing opposing hydrodynamic torques to dominate. These results suggest that jets alone may not be sufficient to align MAD disks, instead highlighting the magnetosphere’s crucial role in mediating spin-disk coupling.

What powers the hard, non-thermal X-rays from accreting compact objects has been a longstanding mystery. In my talk, I will address the underlying question of what energizes the particles of the Comptonizing “corona” against the strong inverse Compton (IC) cooling losses with first-principle particle-in-cell simulations of magnetic reconnection subject to IC cooling in magnetically dominated electron-positron plasmas, and in mildly-magnetized electron-ion plasmas. I will also show---using results of global resistive GRMHD simulations of accreting black holes---that the black hole jet sheath is a site of efficient electromagnetic dissipation through processes such as magnetic reconnection and turbulence. The distribution of bulk motions of the radially outflowing plasma along the jet sheath also resembles a Maxwellian distribution with an effective bulk temperature of a few 100 keV, and this could be a candidate for the Comptonizing corona.

A new version of the Athena++ AMR MHD code implemented using the Kokkos library for performance-portability will be described. The code is being used for a variety of problems related to accretion onto (and merger of) compact objects. This talk will focus on results from general relativistic radiation MHD models of accretion in luminous systems such as X-ray binaries and AGN. The models allow observationally important quantities such as the radiative efficiency, variability, and kinetic feedback in winds to be measured over a range of luminosities, from sub-Eddington to highly super-Eddington. A new “cyclic-zoom” approach for modeling such flows over very long time scales will also be briefly introduced.