Session GO08: Extreme Astrophysical Plasma Environments

Mesoscopic Boxes and Subgrid Plasma-Microphysics Prescriptions for Global MHD Modeling of Astrophysical Systems

Recent years have seen remarkable progress in our understanding of large-scale dynamics of high-energy astrophysical systems, such as accreting black holes and neutron-star magnetospheres, thanks largely to global 3D relativistic magnetohydrodynamics (MHD) simulations. However, our confidence in the applicability of these numerical models to collisionless plasmas in many real systems is hampered by the inability of single-fluid MHD to predict observable radiative signatures directly. This motivates the development of subgrid prescriptions for the partitioning of the dissipated magnetic and bulk-flow kinetic energy among various kinetic-level channels (e.g., nonthermal particle acceleration and electron heating fraction).  The recent advent of such prescriptions, based on gyrokinetic and first-principles particle-in-cell (PIC) kinetic simulations, has had a major impact on plasma astrophysics of black holes. However, these subgrid models are usually formulated in terms of single-point local plasma parameters such as plasma-beta or magnetization sigma. I will argue that such approach is too simplistic, and more sophisticated procedures are needed, involving an analysis of MHD-level plasma and field quantities as well as their spatial variation in quasi-local mesoscopic regions. I will outline a broad plan for implementing this strategy for the cases of magnetic reconnection, Kelvin-Helmholtz instability, and plasma turbulence.    

Using Observations of Black-Hole Images to Probe Plasmas in Extreme Gravitational Fields

The ring-like images of the two supermassive black holes captured by the Event Horizon Telescope (EHT) provide powerful probes of the physics of accretion-flow plasmas in the extreme gravitational fields surrounding black holes. Specifically, the brightness asymmetry in the images carries information about the angular velocity profile of the inner accretion flow, owing to the Doppler boosts photons experience at their site of emission. In this talk, I will show that, in order to accommodate the lack of significant brightness asymmetry in the observed images of the black hole at the center of the Milky Way, Sgr A*, the plasma velocities must be significantly sub-Keplerian, i.e., non geodesic, and the black-hole spin cannot be large. Imaging the black hole at different wavelengths with future observations will provide strong constraints not only on the dynamics but also on the emission properties (and hence the thermodynamics) of the plasma in the same strong-field regions.   

A `Leave No Trace' Approach to Initial Conditions in GRMHD Simulations of Accretion

Using observations of black hole systems to test theories of the gravitational force, such as General Relativity (GR), requires an accurate understanding of the turbulent hydrodynamic, radiative, and kinetic processes taking place in the surrounding accretion disk. Material falling into black holes can form organized large-scale structures, dissipate tremendous amounts of energy, and have been linked to the production or accumulation of large-scale magnetic fields. While Gamma Ray Bursts and Tidal Disruption Events may provide supporting empirical evidence that large-scale magnetic dynamos are an intrinsic feature of at least some disks, the exploration of these processes using numerical simulations is particularly sensitive to initial conditions and multi-scale evolution. We present recent progress using a novel approach to initial disk conditions and evolution that is designed to improve the realism of global structure and physicality of the equilibrium disk state reached in 3D-GRMHD simulations. These simulations form the basis of a library that will be used for interpretation of observations, and tests of GR and dynamos with greater fidelity. We will also describe an application of these simulations to understanding systematic error in interpretation of black hole images.   

Reconnection influences jet formation and the active regions in black hole accretion disk coronae

Just like the Sun, black hole (BH) accretion disks can be endowed with a dilute highly magnetized plasma atmosphere called a corona. The magnetic field lines in such an accretion disk corona are anchored, just like in the solar case, to the underlying heavier plasma, which is here the accretion disk. The disk, in turn, can agitate the field line footpoints, leading to reconnection-mediated magnetic relaxation of the corona above and, hence, to particle acceleration and high-energy radiation. Due to a scarcity of first-principles models, many aspects of the corona – including the impact of magnetic field lines connecting the disk to the central black hole – remain poorly understood. In this contribution, we present our recent efforts to model, using general relativistic particle-in-cell simulations, how a black hole couples to and feeds on its accretion disk corona. We find that the coronal magnetic scale height strongly impacts where reconnection occurs (whether mostly on disk-disk or disk-BH field lines), the energy released as radiation, and whether a large-scale relativistic jet is launched. These results may shed light on X-ray binary state transitions and the peculiar changing-look active galactic nucleus 1ES 1927+654.   

The Origin of the Slow-to-Alfvén Wave Cascade Power Ratio and its Implications for Particle Heating in Accretion Flows

The partition of turbulent heating between ions and electrons in radiatively inefficient accretion flows plays a crucial role in determining the observational appearance of accreting black holes. Recent studies of particle heating from collisionless damping of turbulent energy have shown that the partition of energy between ions and electrons is dictated by the ratio of the energy injected into the slow and Alfvén wave cascades. In this talk, I will present the mechanism of the injection of turbulent energy into slow- and Alfvén- wave cascades in magnetized shear flows. I will show that this ratio depends on the particular (rϕ) components of the Maxwell and Reynolds stress tensors that cause the transport of angular momentum, the shearing rate, and the orientation of the mean magnetic field with respect to the shear. I will then use numerical magnetohydrodynamic shearing-box simulations with background conditions relevant to black hole accretion disks to compute the magnitudes of the stress tensors for turbulence driven by the magneto-rotational instability and derive the injection power ratio between slow and Alfvén wave cascades. I use these results to formulate a local subgrid model for the ion-to-electron heating ratio that depends on the macroscopic characteristics of the accretion flow. I will also present methods to infer the heating ratio in global general relativistic magnetohydrodynamic simulations of black hole accretion flows, and discuss its implications on electron heating in these systems.   

Where the Wild Ones Go: Modeling Black-Hole Flares using Hybrid Drift-Kinetic Particle Simulations in GRMHD Backgrounds

The supermassive black hole at the center of our galaxy, Sagittarius A* (Sgr A*), undergoes multiple bright flaring events daily, characterized by significant luminosity increases across various wavelengths. These flares are believed to result from light emitted by non-thermal electrons generated through episodic magnetic reconnection events within the accretion flow of Sgr A*. In this presentation, I will introduce a novel hybrid numerical algorithm that integrates drift-kinetic with general relativistic magnetohydrodynamic (GRMHD) simulations of black hole accretion flows, enabling first-principles simulation of non-thermal electron dynamics. The algorithm leverages recently developed general relativistic covariant guiding-center equations of motion that lead to unprecedented simulation path lengths for charged particles in curved spacetimes. I will show results from simulations of integrating the trajectories of particle ensembles in background GRMHD accretion flows and discuss the implications of these simulations for interpreting Sgr A* flaring events.   

Unified Mean-Field Theory for Turbulent Accretion Disks: Magnetorotational Turbulence and Dynamos

Magnetorotational instability (MRI)-driven turbulence and dynamo phenomena are analyzed using direct statistical simulations. Our approach begins by developing a unified mean-field model that combines the traditionally decoupled problems of the large-scale dynamo and angular-momentum transport in accretion disks. The model consists of a hierarchical set of equations, capturing up to the second-order correlators while employing a statistical closure approximation for third-order correlators. We highlight the web of interactions that connect different components of stress tensors---Maxwell, Reynolds, and Faraday---through shear, rotation, mean fields, and nonlinear terms. We determine the dominant interactions crucial for the development and sustenance of MRI turbulence. Our unified mean-field model allows for a self-consistent construction of the electromotive force, accounting for inhomogeneities and anisotropies. Regarding the large-scale magnetic field dynamo, we identify two key mechanisms: the rotation-shear-current effect and the rotation-shear-vorticity effect, responsible for generating the radial and vertical magnetic fields respectively. We provide explicit expressions for the transport coefficients associated with each of these dynamo effects. Notably, both mechanisms rely on the intrinsic presence of a large-scale vorticity dynamo within MRI turbulence. Finally, I will delve into the fundamental processes associated with the dynamo cycle patterns.   

Exascale simulations of magnetized turbulence driven by supermassive black hole feedback in galaxy clusters

Galaxy clusters are the most massive virialized objects in the universe, with masses hundreds to thousands of times that of our own Milky Way and physical scales extending for megaparsecs. The bulk of the baryons contained within these systems is comprised of a hot (10⁷-10⁸ K), diffuse (with particle number densities of n ~ 10^-6 - 10^-1 cm^-3), and magnetized plasma that glows brightly in X-ray wavelengths (with typical X-ray luminosities of 10⁴⁴-10⁴⁵ erg/s). The energy radiated away by X-rays is replaced by heating from active galactic nuclei, which are relativistic jets powered by accretion onto the supermassive black hole in the cluster's central galaxy, maintaining the system in a dynamic equilibrium. This heating occurs through interactions of the AGN jet with the intracluster medium, which ultimately is transported throughout the highly X-ray luminous cluster core. In this presentation I will present results from exascale magnetohydrodynamics simulations of idealized galaxy clusters with a cold gas accretion-fed, magnetized AGN jet in the center. I will explain the mechanisms by which cold gas triggers the AGN jet, and how the heating from this jet regulates the amount of cold gas in the system.  I will also discuss in detail the generation of turbulence by the magnetized jet, the amplification of the ambient cluster magnetic fields by turbulence-driven dynamos, and I will compare this to more idealized simulations of plasma turbulence.    

Emergence of Double Helix and Organized Plasma Structures from Gravitational Wave Emitters

Helical plasma structures have been shown to form in and propagate away from the circumbinary disks associated with rapidly orbiting Black Hole pairs [1]. These structures are envisioned to extend through very low-density and distant plasma regions up to where they can be disrupted or become dissipated. By now experimental observations and analyses of the morphology of jets have found that they can involve a double-helix magnetic topology in one case and, more recently, single helices and organized plasma structures in general. Thus, structures originating in the plasmas surrounding binary systems are proposed, instead of particle beams emitted by black holes directly, as a possible explanation of the origin of the highly collimated jets associated with a variety of celestial objects that are commonly observed. Theoretically, double-helix structures are found to emerge as non­linearly coupled plasma waves which can propagate independently in either of the two vertical directions. The coupling involves Intrinsic Gravitational Modes originating in the outer circumbinary disk and Inner Gravitational Fluctuations emerging from the Swept (Toroidal) Regions [1] carved, within central area of the disk, by one or both Black Holes.

1. B. Coppi, Fundamental Plasma Physics 4 (2023) 100007.   

Plasma Drift Velocities in the Curved Spacetimes of Black Holes

The gyroradii of charges in the plasmas around black holes are more than ten orders of magnitude smaller than the macroscopic length scales of the systems. Modeling the drift of these charges across magnetic field lines has presented significant challenges arising from the absence of a covariant guiding-center formalism in curved spacetimes. I will discuss how utilizing a novel covariant drift-kinetic formalism for charge trajectories in flat spacetimes has enable the determination of the three primary drift velocities caused by gradients in the magnetic field as well as the presence of electric and gravitational fields. Our findings indicate that, while the electric-field drift velocity remains unaffected by the spacetime curvature, the gradient and gravitational drift velocities are substantially modified, with their magnitudes increasing rapidly near the black-hole horizons.

Research funded by Georgia Institute of Technology.   

Nature of Astrophysical Jets Unraveled

The theoretical finding [1] of robust plasma structures propagating away from circumbinary disks had led to propose [2] that significant astrophysical the jets result from of the emission of these structures. Propagating plasma double-helices emerged from the non-linear interactions [2] of modes generated in the “Swept (Torus) Regions” carved in the circumbinary disk by a pair of (stellar) Black Holes. Successively, the case of a massive black hole paired with a much lighter “sheperd” black hole has been considered. The emitted structures have been associated with the fluctuations from the “Swept Torus” region carved by the “sheperd”. In fact, an analysis of the observed M87 Jet structure [3], that followed the early findings concluded that this was of a double-helix kind. More recent studies of other jets have identified helical [4,5] or different plasmas structures [6].

1. B. Coppi, Invited Papers for the XVI Marcel Grossmann Conference on Relativistic Astrophysics (Session I), July 2021, and for the Asia Pacific Physical Societies Conference on Plasma Physics, (SA-II8), October 2021.

2. B. Coppi, Fundamental Pl. Phys., 100007 (2023).

3. A. Pasetto et al., Ap J. Letters, 923:L5 (2021).

4. G-Y. Zhao, et al.,  Ap J., 932, 732 (2022).

5. I. Issoun, et al.,  Ap J., 934, 145 (2022).

6. A. Fuentes, et al., Nature Astronomy, 7, 1359 (2023).   

Dissipation in astrophysical jets with velocity and magnetic shear: Interaction of Kelvin-Helmholtz and Drift-Kink Instabilities

We present 2D particle-in-cell simulations of a magnetized, collisionless, relativistic pair plasma subjected to a strong combined velocity and magnetic-field shear at a thin vortex- and current-sheet interface, a scenario typical for astrophysical black-hole jet-wind boundaries. By considering a 2D simulation plane formed by the velocity flow and the gradient direction, with magnetic field perpendicular to the plane, we focus on the case where only the Kelvin-Helmholtz (KH) and Drift-Kink (DK) instabilities can develop, while tearing (and hence magnetic reconnection) is forbidden. In addition to a control case where only the velocity shear is present, we analyze a sequence of simulations in which the (out-of-plane) magnetic field is reversed across the interface and the velocity shear is progressively increased. This strategy allows us to explore the general dynamics and dissipation driven synergistically by the interplay of the two shears and compare them to the effects of either of the shears taken in isolation. We thus investigate, for the first time, the nonlinear interaction of the KH and DK instabilities, generating qualitatively new structures with very different dissipative behaviors. We find that DKI can effectively disrupt the cat-eye vortices generated by KHI, creating a turbulent shear layer on the joint DK-KHI timescale. This interplay leads to a significant enhancement of dissipation over the velocity-shear-only case. In addition, we find a special, relatively narrow range in velocity shear where the joint DK-KHI is particularly active, resulting in even stronger dissipation. Finally, we observe efficient nonthermal particle acceleration caused by the alignment of the instability-driven electric fields with Speiser-like motion of particles close to the shear interface. This study highlights the sensitivity of flow structures, dissipation, and particle acceleration to multiple simultaneously operating instabilities, thus providing a strong motivation for further studies of their nonlinear interaction at the kinetic level. Such studies will help elucidate the nature of dissipation and particle acceleration, and in particular explain the observed emission limb-brightening, in relativistic jets from astrophysical supermassive black holes.   

Modeling X-ray emission in radiation-rich magnetar magnetospheres

Neutron star magnetospheres are a source of abundant X-ray activity. They have transients observed in different bands, like the fast radio burst (FRB) and associated hard X-ray flare from the Galactic magnetar SGR 1935+2154. We present global models for magnetar X-ray emission, including a landmark first-principle radiative particle-in-cell simulation of the twisted magnetar magnetosphere with the GPU-PIC code Entity. In one scenario, plasma particles accelerated by surface-motion-induced discharges interact resonantly with thermal background photons. Our GPU-accelerated particle-in-cell simulations track up-scattered high-energy photons that drive secondary pair production and ignite a magnetospheric circuit that persistently generates X-rays. We divulge the plasma properties of such a magnetospheric circuit, including densities and velocities, and give an outlook on alternative ignition scenarios for persistent magnetar X-ray emission.   

Origin of spectral bands in the Crab pulsar radio emission

The model explaining the spectral "zebra'' pattern of the high-frequency interpulse (HFIP) of the Crab pulsar radio emission is proposed. The observed emission bands are diffraction fringes in the spectral domain. The pulsar's own plasma-filled magnetosphere plays a role of a frequency-dependent "diffraction screen''. The observed features such as the proportional band spacing, high polarization, constant position angle, and others are explained. The model is testable and several predictions are made. The two "high-frequency components'' observed at the same frequencies as the HFIP are proposed to be related to HFIP.   

Freezing and Cluster Formation in Dense White Dwarf Plasmas

Recent observational evidence shows that the cooling of white dwarf (WD) stars slows temporarily when the strongly coupled plasma in the star freezes. The energy maintaining the star’s temperature comes from latent heat and gravitational energy from the inward migration of heavier elements. For a small subpopulation of WDs, the cooling delay is anomalously long. The leading explanation for these extra-long delays is neon distillation, which requires Ne-depleted solids to form and float up through the heavier Ne-rich liquid. This explanation relies on the untested assumption that solid clusters form and are stable enough to diffuse through surrounding liquid. We are exploring the formation and properties of clusters in dense single-component Yukawa (screened Coulomb) fluids with different screening lengths. We performed brute-force and seeded molecular dynamics simulations of crystal nucleation in undercooled liquids. With a classical nucleation theory framework, we extract nucleation rates and cluster size distributions at a range of temperatures relevant to crystallizing WDs. This is a first step toward a full description of the size and stability of clusters in the multicomponent fluids that may drive Ne distillation.