Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session L08: Astrophysical Particle Acceleration |
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Sponsoring Units: DAP Chair: Louis Strigari, Texas A&M University Room: Sheraton Governor's Square 10 |
Sunday, April 14, 2019 3:30PM - 3:42PM |
L08.00001: 3D magnetic reconnection in relativistic pair plasma with moderate magnetization Gregory R Werner, Dmitri A Uzdensky Magnetic reconnection is a fundamental plasma process that converts magnetic field energy to particle kinetic energy. Relativistic reconnection is of particular interest in astrophysical contexts because it can accelerate particles to high energies where they can emit observable radiation. While reconnection is typically thought of as an essentially 2D process, 3D effects can potentially interfere with reconnection. Using particle-in-cell simulations, we find -- for relativistic pair-plasma reconnection with magnetization σh∼1, where σh is roughly the ratio of magnetic to plasma energy -- that 3D effects (including the drift-kink instability) can substantially disrupt reconnection, e.g., distorting and thickening the current layer, altering the reconnection rate. This contrasts starkly with the σh»1 regime, where 3D effects occur without significantly changing overall reconnection morphology or energetics. Intriguingly, despite 3D processes interfering with reconnection in the σh∼1 regime, nonthermal particle acceleration (NTPA) remains robust, and is even enhanced relative to 2D (although NTPA for σh∼1 is less efficient than for σh»1 in both 2D and 3D). |
Sunday, April 14, 2019 3:42PM - 3:54PM |
L08.00002: Kinetic Beaming of Emission Produced by Radiatively Cooled Relativistic Magnetic Reconnection John M Mehlhaff, Gregory R Werner, Dmitri A Uzdensky Relativistic collisionless magnetic reconnection is often invoked to explain high-energy emission observed in astrophysical sources such as pulsar wind nebulae, gamma-ray bursts, and relativistic blazar jets. Reconnection produces nonthermal spectra, as are commonly observed in these sources. Reconnection may also explain distinctive short-timescale flaring. Such flares could arise because reconnection-accelerated particles—and their emitted photons—tend to be focused into beams with greater collimation at higher energies. This “kinetic beaming” may produce rapid high-energy flares when beams sweep across the observer’s line of sight. Using large-scale 2D particle-in-cell simulations, we systematically investigate the robustness (i.e. existence and duration) of kinetic beaming in emission powered by reconnection subject to external inverse Compton (IC) cooling. We find that only strongly cooled energetic particle beams can radiate most of their energy before they disperse. Thus strong cooling promotes kinetic beaming, while weak cooling suppresses it. Our results support the view that reconnection may power rapid IC gamma-ray flares in sources such as blazar jets. |
Sunday, April 14, 2019 3:54PM - 4:06PM |
L08.00003: Electron and proton energization in relativistic plasma turbulence Vladimir V Zhdankin, Dmitri A Uzdensky, Gregory R Werner, Mitchell Begelman Particle energization is a fundamental, but poorly understood, consequence of plasma turbulence in high-energy astrophysical systems such as black-hole accretion flows. To study this process, we use particle-in-cell simulations of driven turbulence in collisionless, relativistic electron-proton plasmas. We focus on temperatures between the electron and proton rest mass energies, where the plasma consists of sub-relativistic protons and ultra-relativistic electrons. In most of the explored parameter space, we find that protons are preferentially heated, sufficient to establish a two-temperature plasma. We characterize the electron-proton energy partition as a function of plasma parameters, finding that it can be well described as a simple function of the ratio of electron-to-proton characteristic gyroradii. Both particle species exhibit nonthermal acceleration that forms a power-law energy distribution, but the electron acceleration becomes inefficient as the temperature is decreased. Thus, protons generally have a more substantial nonthermal population. Our empirical results have important implications for the establishment of two-temperature plasmas in black-hole accretion flows. |
Sunday, April 14, 2019 4:06PM - 4:18PM |
L08.00004: Direct Numerical Demonstration of Diffusive Nonthermal Particle Acceleration in Particle-in-Cell Simulations of Kinetic Turbulence in Relativistic Pair Plasmas Kai W Wong, Vladimir V Zhdankin, Dmitri A Uzdensky, Gregory R Werner, Mitchell Begelman Nonthermal particle acceleration (NTPA) is ubiquitous in high-energy astrophysics, as evidenced by cosmic rays and the emission spectra of pulsar wind nebulae and active galactic nuclei. Most analytical models of NTPA posit diffusive Fermi acceleration described by a Fokker-Planck (FP) advection-diffusion equation in momentum space. We test the applicability of the FP framework directly in kinetic particle-in-cell simulations of driven magnetized turbulence in relativistic pair plasma. By statistically analyzing the motion of almost a million tracked particles, we confirm the diffusive nature of turbulent NTPA and measure the FP energy diffusion (D) and advection (A) coefficients as functions of particle energy γmec2. We find that D(γ) is proportional to γ2 in the high-energy nonthermal tail, in line with 2nd-order Fermi acceleration theory, but has a much shallower scaling of about γ2/3 at lower energies, while A tends to pull particles towards the peak of the distribution. We also explore the effects of magnetization. Our results provide strong support for the FP picture of turbulent NTPA, thereby enhancing our understanding of space, solar, and astrophysical plasmas. |
Sunday, April 14, 2019 4:18PM - 4:30PM |
L08.00005: Magnetic Helicity and Turbulent Dynamos Ethan T Vishniac, Amir Jafari The generation of large scale magnetic fields in highly conducting, turbulent fluids is a common and important process in astrophysics. Mean field dynamo theory conventionally relies on an intrinsic kinetic helicity in the underlying turbulence. Here we show how differential rotation drives a magnetic helicity flux embedded in small scale structures. This in turn leads to the large scale dynamo. We note that this process always dominates over the kinematic dynamo. Balancing this process against buoyant losses and turbulent mixing, we can naturally predict the dependence stellar magnetic fields on stellar rotation rates. |
Sunday, April 14, 2019 4:30PM - 4:42PM |
L08.00006: Jets from paramagnetic electrons in superstrong fields Paul N. Arendt The assumption that electrons retain their observed anomalous magnetic moment in arbitrarily strong background fields leads to their effective masslessness at a critical field strength of several times 1016 G. We show that this also gives near-critical fields the ability to efficiently transform many forms of energy into tightly collimated beams consisting of electrons, positrons, and photons which are ejected at relativistic speeds along the field lines, equally in both directions. This suggests a model of astrophysical jets as a magnetically catalyzed means of energy extraction from compact sources. The expected properties of the beams qualitatively agree with those of observed jets, and the central engine has a simple and compelling magnetic field geometry, in contrast to the popular Blandford-Znajek jet model. |
Sunday, April 14, 2019 4:42PM - 4:54PM |
L08.00007: Using lab-scale mixing experiments to inform LES of stellar convection Austin F Davis, John J Charonko, Katherine P Prestridge The comparison between Large-eddy simulations (LES) and experimental data is problematic due to the inherent filtering of the LES data. Filtering techniques (Leonard 1974, Germano 1992, Klein 2018) can be applied, allowing for a better comparison between these data sets. The experimental data from the turbulent mixing tunnel (TMT) (Charonko, Prestridge 2017) are velocity and density fields of variable-density mixing conditions in a coflowing jet. This provides an excellent data set for testing filtering techniques as the sample size is large, the samples are high resolution, and variable density effects are tested. Containing 10,000 velocity and density fields per location, a subset of the TMT data is chosen to represent a low sample size data set. Filtering techniques are then applied to the sample set and the averages are compared. These techniques have application to validation of LES simulations and sub grid modeling efforts as the Reynolds stress from filtered vs non-filtered data are important. The jet conditions are relevant to high-shear and mixing regions at the edges of stellar convection zones, so the results can assist in determining the resolution effects on large LES calculations of these flows. |
Sunday, April 14, 2019 4:54PM - 5:06PM |
L08.00008: The X-ray Emissivity of Low-Density Stellar Populations Mario G Ivanov, Craig O Heinke High-density stellar environments allow for close interactions between stars, which can lead to dense stellar remnants being placed into stellar binaries. In close orbits, matter can transfer from a normal star to the stellar remnant, producing radiation up to X-rays - for white dwarfs, these are cataclysmic variables (CVs). Observations of faint X-ray sources indicate CVs are more frequent in denser clusters and link dynamical processes with fainter X-ray binaries. However, previous studies noted that the total X-ray emissivity is lower in denser environments with no unique bright X-ray sources. So, either binaries are destroyed quickly in dense environments or open clusters lose a large fraction of their non-X-ray-emitting mass. We address this by considering the X-ray emissivity in a range of environments and densities. We find that the X-ray emissivity of environments below 10000 solar masses per parsec cubed are not density dominated. We find a significant correlation between X-ray emissivity and binary fraction and less significant correlations with metallicity and age. The available data is limited and sampling via bootstrap gives less significant correlations. |
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