Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session NO06: Plasma Astrophysics ILive Streamed
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Chair: Dmitri Uzdensky, Univ. Colorado Room: Ballroom 111 C |
Wednesday, October 19, 2022 9:30AM - 9:42AM |
NO06.00001: Collisionless Black Hole Accretion Alisa Galishnikova, Alexander A Philippov, Eliot Quataert The accretion flows around the black holes in Sgr A*, M87, and other systems are strongly magnetized and collisionless. This in fact makes the usually employed general relativistic (GR) magnetohydrodynamic (MHD) method formally inapplicable. Thus, addressing the BH accretion problem in principle requires a fully kinetic approach. We study axisymmetric accretion of collisionless plasma around the black holes from first principles using GR particle-in-cell simulations (GRPIC). By doing so, we carry out a side-by-side comparison of global dynamics in GRMHD simulations and GRPIC for the same black hole accretion problem. Magnetic reconnection, which is believed to be responsible for particle acceleration and subsequent flares, is accurately captured in our kinetic approach. We directly examine the production of non-thermal particles due to magnetic reconnection. We discuss the implications of our results for event-horizon scale observations of Sgr A* and M87 by GRAVITY and the Event Horizon Telescope. |
Wednesday, October 19, 2022 9:42AM - 9:54AM |
NO06.00002: Flares from black-hole magnetospheres Benjamin Crinquand, Benoît Cerutti, Alexander A Philippov, Kyle Parfrey, Guillaume Dubus, Bart Ripperda A variety of astrophysical phenomena can only be explained as being powered by black holes. In particular, accreting supermassive black holes are responsible for launching relativistic plasma jets and for accelerating ultra-energetic particles. |
Wednesday, October 19, 2022 9:54AM - 10:06AM |
NO06.00003: Nonthermal emission from the plunging region: a model for the high-energy tail of black hole X-ray binary soft states Amelia Hankla, Nicolas Scepi, Jason Dexter X-ray binaries exhibit a soft spectral state comprising thermal blackbody emission at 1 keV and a power-law tail above 10 keV. Empirical models fit the high-energy power-law tail to radiation from a nonthermal electron distribution, but the physical location of the nonthermal electrons and the reason for their power-law index and high-energy cut-off are still largely unknown. Here, we propose that the nonthermal electrons originate from within the black hole's innermost stable circular orbit (the ``plunging region''). Using an analytic model for the plunging region dynamics and electron distribution function properties from particle-in-cell simulations, we outline a steady-state model that can reproduce the observed spectral features. In particular, our model reproduces photon indices of Γ>∼2 and power-law luminosities on the order of a few percent of the disk luminosity for strong magnetic fields, consistent with observations of the soft state. Because the emission originates so close to the black hole, we predict that the power-law luminosity should strongly depend on the system inclination angle and black hole spin. This model could be extended to the power-law tails observed above 400 keV in the hard state of X-ray binaries. |
Wednesday, October 19, 2022 10:06AM - 10:18AM |
NO06.00004: Fully kinetic simulations of magnetorotational turbulence in a three-dimensional shearing box Fabio Bacchini, Lev A Arzamasskiy, Vladimir V Zhdankin, Gregory R Werner, Mitchell C Begelman, Dmitri A Uzdensky The magnetorotational instability (MRI) is a fundamental process driving the dynamics of accretion disks. The MRI amplifies magnetic fields and can promote angular-momentum transport, nonthermal particle acceleration, and differential electron-ion heating by sustaining a turbulent cascade from the macroscopic (fluid) to the microscopic (kinetic) scales of plasmas in such disks. However, a complete understanding of MRI-driven turbulence in the collisionless regime is still missing, due to a lack of fully kinetic, three-dimensional (3D) models capturing the nonlinear MRI dynamics. Here, we present the first large-scale, 3D, Particle-in-Cell (PIC) simulations of MRI-driven turbulence based on a novel shearing-box formulation. Our large-scale simulations reproduce the fluid (i.e. mesoscale) behavior expected from MHD numerical experiments; in addition, our PIC approach allows us to quantify kinetic effects that cannot be captured in MHD, such as particle acceleration, enhanced angular-momentum transport due to pressure anisotropy, collisionless plasma heating, etc. Although here we focus on pair plasmas, our work paves the way for future electron-ion simulations, which will advance our understanding of observationally targeted accretion flows such as those surrounding M87* and SgrA*. |
Wednesday, October 19, 2022 10:18AM - 10:30AM |
NO06.00005: PIC simulations of the magnetorotational instability (MRI) in stratified, collisionless accretion disks Astor Sandoval, Mario A Riquelme, Anatoly Spitkovsky The magnetorotational instability (MRI) is an MHD instability essential for outward transport of angular momentum in various astrophysical accretion disks. In very low-luminosity disks around black holes, such as in Sagittarius A* and M87, particle-particle Coulomb collisions occur very infrequently, which makes these accreting systems effectively "collisionless''. This characteristic gives rise to several kinetic plasma effects that could modify the MRI evolution and generate particle acceleration. We present results of 2D and 3D fully kinetic, particle-in-cell (PIC) plasma simulations of the collisionless MRI. Our simulations are local and stratified, which means that we use the local, shearing box approximation and self-consistently include the vertical structure of the disk. We concentrate on the sub-relativistic plasma regime, relevant at tens of gravitational radii from the central black hole, and quantify the ability of the MRI turbulence to transport angular momentum and to accelerate particles. We find that both angular momentum transport as well as particle acceleration in our stratified simulations are, on average, significantly more efficient than in the case where disk stratification is not included. |
Wednesday, October 19, 2022 10:30AM - 10:42AM |
NO06.00006: Kinetic plasma energization by magnetic Rayleigh-Taylor instability Vladimir V Zhdankin, Bart Ripperda, Alexander A Philippov The magnetic Rayleigh-Taylor instability (RTI) is a fundamental process in many high-energy astrophysical systems, such as pulsar wind nebulae and black-hole accretion flows, caused by an unstable density profile in a gravitational field. In these systems, plasmas are often collisionless and have relativistic components. While the macroscopic RTI dynamics have been well-studied by magnetohydrodynamic models, the kinetic aspects (including particle energization) have not yet been investigated in this regime. We will present results from local kinetic particle-in-cell simulations of the RTI. The simulations consider the unstable equilibrium involving a dense plasma slab on top of a dilute strongly magnetized region, with a magnetic shear across the interface. The ensuing RTI mixes the two plasma domains, driving magnetic reconnection and turbulence. We will describe the dependence of the results on parameters such as the magnetic shear angle, the interface layer thickness, and domain size. The results have implications for black-hole accretion flows, by revealing that the RTI may inject and/or energize a population of nonthermal particles in the disk, explaining observable near-infrared flares. |
Wednesday, October 19, 2022 10:42AM - 10:54AM |
NO06.00007: Entity: new gpu-enabled particle-in-cell code for extreme plasma astrophysics Alexander A Philippov, Hayk Hakobyan, Jens F Mahlmann, Benjamin Crinquand, Alisa Galishnikova Particle-in-cell has been a go-to approach for modeling plasmas in the environments of compact astrophysical objects for the last decade. Yet, there is no single publicly available code that includes all relevant radation-plasma coupling processes and is capable of modeling global systems. In this talk I will describe development of a new-generation PIC code for extreme astrophysical plasmas, Entity. The code is based on the Kokkos framework, which enables efficient implicit multi-architecture portability including GPUs. The code features algorithms for various radiation-plasma coupling processes, such as Compton scattering, production of electron-positron pairs and their annihilation. The code is designed in general coordinate system, defined by the metric functions; this enables the Entity to also efficiently tackle the global (full-system) models of the magnetospheres of compact objects, which require algorithms on non-cartesian (spherical, cubed sphere) non-uniform grids, and even full general relativity. |
Wednesday, October 19, 2022 10:54AM - 11:06AM |
NO06.00008: Radiation from particles energized by pinch/kink instabilities in astrophysical jets Gregory R Werner, José Ortuño-Macías, Krzysztof Nalewajko, Dmitri A Uzdensky, Mitchell C Begelman We use particle-in-cell simulation to investigate whether nonlinear development of magnetohydrodynamic pinch and kink instabilities could explain observed high-energy radiation from astrophysical jets. We start with a simplified jet model, namely a Z-pinch configuration in relativistically-hot collisionless electron-positron pair plasma, that is unstable to pinch/kink modes. We evolve the system from instability onset through nonlinear development, focusing on the release of magnetic energy to plasma heating and nonthermal particle acceleration (NTPA), and the resulting synchrotron and inverse Compton emission from high-energy particles. Our previous work showed this release occurs in two stages: fast and then slow, with the fast stage dominating particle energization. The instability growth rate and NTPA efficiency are greater for the gas-pressure-balanced Z-pinch than for a force-free screw-pinch configuration, leading us to concentrate on the Z-pinch case during the fast stage. We examine the effect of radiative cooling, as well as radiative signatures for different radiation regimes. |
Wednesday, October 19, 2022 11:06AM - 11:18AM |
NO06.00009: Are Reconnection X-points Important for Particle Injection during Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection? Fan Guo Reconnection x-points (with electric field larger than the magnetic field for a vanishing guide field) have been suggested to be important for particle injection that boosts particle energy to the lower energy bound of the power-law energy distribution in relativistic magnetic reconnection. We carry out particle-in-cell kinetic simulations and analyses to elucidate the roles of x-points during nonthermal particle acceleration in relativistic magnetic reconnection. We show that for a vanishing guide field, the x-points can only host particles for a short time, leading to a limited energy gain insufficient for particle injection. Meanwhile, most energy gain for particle injection and nonthermal particle acceleration are through regions outside of the x-points. Interestingly, we also find that some energy gain already occurs before particles interact with the x-points. By evolving a test-particle component in the PIC simulation that does not "see" the x-point electric field, we show that particles can still be efficiently accelerated. We conclude that for the parameter regime we study here, X-points are not important for particle injection or nonthermal particle acceleration. |
Wednesday, October 19, 2022 11:18AM - 11:30AM |
NO06.00010: The universality of particle acceleration in relativistic magnetic reconnection Lorenzo Sironi In the most powerful astrophysical sources, reconnection operates in the "relativistic" regime, where the magnetic field energy exceeds even the rest mass energy of the plasma. Here, reconnection can lead to fast dissipation rates and efficient particle acceleration, thus being a prime candidate for powering the observed fast and bright flares of high-energy non-thermal emission. With fully-kinetic particle-in-cell (PIC) simulations and analytical theory, we investigate three fundamental aspects of the physics of particle acceleration in relativistic reconnection: (1) the injection from thermal energies up to relativistic energies; (2) the physics of power-law formation; (3) the constraints on maximum energy. |
Wednesday, October 19, 2022 11:30AM - 11:42AM |
NO06.00011: Magnetic reconnection in large-scale astrophysical systems Samuel R Totorica, Mami Machida, Amitava Bhattacharjee Magnetic reconnection mediates the conversion of magnetic field energy into plasma kinetic energy through the production of bulk flows, heating, and the acceleration of energetic particles. In high energy astrophysical systems such as pulsars, gamma-ray bursts, and active galactic nuclei jets, reconnection is expected to occur in the relativistic regime where the energy density of the magnetic field exceeds that of the plasma rest mass. Relativistic reconnection thus plays a critical role in the global evolution of astrophysical systems and the production of nonthermal particle distributions that are observed through their radiation. Bridging the gap between the microscopic diffusion regions where magnetic field topology changes occur and the global scales of realistc systems is a major challenge models of reconnection in astrophysical contexts. Understanding the sacrifices in fidelity as the physical model is reduced from a kinetic Vlasov-Maxwell system to a simplified magnetohydrodynamic fluid is critical for the accurate modelling of large-scale realistic systems. Using kinetic particle-in-cell simulations with a fully relativistic Coulomb collision operator, we study the transition from collisionless to collisional regimes of relativistic reconnection for the first time. At sufficiently high collisionalities, we reproduce the relativistic generalization of the Sweet-Parker scaling of reconnection rate with Lundquist number. In intermediate regimes, the collisionality significantly impacts plasmoid formation and particle acceleration. Finally, we discuss the use of two-dimensional kinetic equilibria for modelling compact object magnetospheres. |
Wednesday, October 19, 2022 11:42AM - 11:54AM |
NO06.00012: 3D particle-in-cell simulations of relativistic reconnection with strong synchrotron cooling Alexander Chernoglazov, Hayk Hakobyan, Alexander A Philippov Collisionless relativistic reconnection is believed to power high-energy emission in pulsar magnetospheres and accretion flows around black holes. The observed emission is often associated with the synchrotron radiation of leptons, accelerated in current sheets to relativistic energies. In pulsar magnetospheres, one can also expect the presence of a small fraction of heavy ions which are not affected by synchrotron losses. The dynamics of these ions in relativistic reconnection remain relatively unexplored. In this talk, I will present the results of large three-dimensional particle-in-cell simulations of isolated current sheets in different regimes of synchrotron cooling. I will discuss the structure of the 3D reconnection layer and the properties of the plasma flows in the strong radiative cooling regime. I will describe the mechanism for lepton acceleration to the Lorentz factors comparable to the magnetization parameter, and their further radiation during their encounter with the magnetic field inhomogeneities. I will also outline the acceleration channel for the uncooled ions and compare its efficiency for different regimes of lepton cooling. I will also demonstrate that the highest energy leptons are beamed along the upstream magnetic field. |
Wednesday, October 19, 2022 11:54AM - 12:06PM |
NO06.00013: Radiative reconnection powered TeV flares near the M87 black hole Hayk Hakobyan, Bart Ripperda, Alexander Philippov Active Galactic Nuclei (AGN) in general, and the supermassive black hole in M87 in particular, regularly show bright and rapid gamma-ray flares up to the energies > 100 GeV. Luminosities of these flares are non-negligible compared to the total jet power of the AGN, and the variation timescales are comparable to the dynamical time of the event horizon. However, the emission mechanism for these TeV flares is not well understood. Recent high-resolution GRMHD simulations show clear indications of episodic magnetic reconnection events occurring close to the black hole event horizon. In this work we analyze the radiative properties of the reconnecting current layer under the extreme plasma conditions applicable to the black hole in M87. We show that abundant pair production is expected in the vicinity of the reconnecting sheet, to the extent that the produced secondary pair-plasma dominates the reconnection dynamics. Using analytic estimates backed by 2D particle-in-cell simulations we demonstrate that even in the presence of strong synchrotron cooling, reconnection can still produce a hard power-law distribution of pair plasma imprinted in the outgoing synchrotron (up to few tens of MeV) and the inverse-Compton signal (up to TeV). Our findings provide a viable explanation for the observed TeV flaring activity of M87, and lay a strong basis to predict the lower-energy counterparts (micron-to-keV), as well as the polarization of the flare. |
Wednesday, October 19, 2022 12:06PM - 12:18PM |
NO06.00014: Simulating Relativistic Magnetic Reconnection with a Pseudo-Spectral Maxwell Solver Hannah E Klion, Revathi Jambunathan, Michael Rowan, Remi Lehe, Jean-Luc Vay, Andrew Myers, Weiqun Zhang Magnetic reconnection is an important cause of particle acceleration and heating in astrophysical settings like gamma-ray bursts, magnetars, and pulsars. Using the particle-in-cell code WarpX, we perform large first-principles 2D simulations of relativistic reconnection in plasmas representative of those found in astrophysical environments. These simulations produce particle spectra and plasmoid structures in agreement with results previously reported in the literature. Using this robust baseline case, we compare the accuracy and computational performance of three methods of solving Maxwell's equations, including the commonly-used second-order finite difference time domain (FDTD) method and an ultra-high-order pseudo-spectral analytical time domain (PSATD) method. This is the first time PSATD has been used in simulations of relativistic reconnection. We find that for the reconnection problem, FDTD and PSATD are comparably accurate, but that PSATD is >50% more computationally efficient. These performance gains will make 3D simulations of reconnection more tractable, and complement other efforts to improve simulation efficiency, such as the use of mesh refinement. |
Wednesday, October 19, 2022 12:18PM - 12:30PM |
NO06.00015: Electron-positron cascade in magnetospheres of supermassive black holes Michael C Sitarz, Mikhail V Medvedev Supermassive black holes are known to produce powerful relativistic jets, such as in quasars and AGNs. These jets are thought to be powered by the Blandford-Znajek mechanism, which taps the spin energy of the BH into the electromagnetic Poynting flux. This mechanism requires the presence of plasma to carry current. Theoretical and numerical studies indicate that the plasma is created and replenished in situ via an electron-positron cascade. Here we discuss the cascade mechanism and present the results of the numerical solutions of the system of coupled ODEs describing the particle and photon fluxes. While this semi-analytical study has some limitations, it allows one to explore important scaling relations between the plasma and astrophysical system parameters, which are hard to deduce from PIC simulations. |
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