Low-luminosity Accretion Flows Research Articles

Self-organization in collisionless, high-β turbulence

The magnetohydrodynamic (MHD) equations, as a collisional fluid model that remains in local thermodynamic equilibrium (LTE), have long been used to describe turbulence in myriad space and astrophysical plasmas. Yet, the vast majority of these plasmas, from the solar wind to the intracluster medium (ICM) of galaxy clusters, are only weakly collisional at best, meaning that significant deviations from LTE are not only possible but common. Recent studies have demonstrated that the kinetic physics inherent to this weakly collisional regime can fundamentally transform the evolution of such plasmas across a wide range of scales. Here, we explore the consequences of pressure anisotropy and Larmor-scale instabilities for collisionless, $\beta \gg 1$ , turbulence, focusing on the role of a self-organizational effect known as ‘magneto-immutability’. We describe this self-organization analytically through a high- $\beta$ , reduced ordering of the Chew–Goldberger–Low-MHD (CGL-MHD) equations, finding that it is a robust inertial-range effect that dynamically suppresses magnetic-field-strength fluctuations, anisotropic-pressure stresses and dissipation due to heat fluxes. As a result, the turbulent cascade of Alfvénic fluctuations continues below the putative viscous scale to form a robust, nearly conservative, MHD-like inertial range. These findings are confirmed numerically via Landau-fluid CGL-MHD turbulence simulations that employ a collisional closure to mimic the effects of microinstabilities. We find that microinstabilities occupy a small ( ${\sim }5\,\%$ ) volume-filling fraction of the plasma, even when the pressure anisotropy is driven strongly towards its instability thresholds. We discuss these results in the context of recent predictions for ion-vs-electron heating in low-luminosity accretion flows and observations implying suppressed viscosity in ICM turbulence.

Principal-component Interferometric Modeling (PRIMO), an Algorithm for EHT Data. I. Reconstructing Images from Simulated EHT Observations

The sparse interferometric coverage of the Event Horizon Telescope (EHT) poses a significant challenge for both reconstruction and model fitting of black hole images. PRIMO is a new principal components analysis-based algorithm for image reconstruction that uses the results of high-fidelity general relativistic, magnetohydrodynamic simulations of low-luminosity accretion flows as a training set. This allows the reconstruction of images that are consistent with the interferometric data and that live in the space of images that is spanned by the simulations. PRIMO follows Monte Carlo Markov Chains to fit a linear combination of principal components derived from an ensemble of simulated images to interferometric data. We show that PRIMO can efficiently and accurately reconstruct synthetic EHT data sets for several simulated images, even when the simulation parameters are significantly different from those of the image ensemble that was used to generate the principal components. The resulting reconstructions achieve resolution that is consistent with the performance of the array and do not introduce significant biases in image features such as the diameter of the ring of emission.

Yet another modification of relativistic magnetohydrodynamic waves: Electron thermal inertia

This study investigates the properties of waves in relativistic extended magnetohydrodynamics (RXMHD), which includes Hall and electron thermal inertia effects. We focus on the case when the electron temperature is ultrarelativistic, and thus, the electron thermal inertia becomes finite at near the proton inertial scale. We derive the linear dispersion relation of RXMHD and find that the Hall and electron thermal inertia effects couple with the displacement current, giving rise to three superluminous waves in addition to the slow, fast, and Alfvén waves. We also show that the phase- and group-velocity surfaces of fast and Alfvén waves are distorted by the Hall and electron thermal inertia effects. There is a range of scales where the group velocity of fast wave is smaller than that of the Alfvén and slow waves. These findings are applicable to a region near the funnel base of low-luminosity accretion flows where electrons can be ultrarelativistic.

Stochastic Ion Acceleration by the Ion-cyclotron Instability in a Growing Magnetic Field

Abstract Using 1D and 2D particle-in-cell simulations of a plasma with a growing magnetic field , we show that ions can be stochastically accelerated by the ion-cyclotron (IC) instability. As grows, an ion pressure anisotropy arises due to the adiabatic invariance of the ion magnetic moment ( and p ⊥,i are the ion pressures parallel and perpendicular to ). When initially β i = 0.5 ( , where p i is the ion isotropic pressure), the pressure anisotropy is limited mainly by inelastic pitch-angle scattering provided by the IC instability, which in turn produces a nonthermal tail in the ion energy spectrum. After is amplified by a factor of ∼2.7, this tail can be approximated as a power law of index ∼3.4 plus two nonthermal bumps and accounts for 2%–3% of the ions and ∼18% of their kinetic energy. On the contrary, when initially β i = 2, the ion scattering is dominated by the mirror instability, and the acceleration is suppressed. This implies that efficient ion acceleration requires that initially, β i ≲ 1. Although we focus on cases where is amplified by plasma shear, we check that the acceleration occurs similarly if grows due to plasma compression. Our results are valid in a subrelativistic regime where the ion thermal energy is ∼10% of the ion rest-mass energy. This acceleration process can thus be relevant in the inner region of low-luminosity accretion flows around black holes.

Electron and Proton Heating in Transrelativistic Guide Field Reconnection

Abstract The plasma in low-luminosity accretion flows, such as the one around the black hole at the center of M87 or Sgr A* at our Galactic Center, is expected to be collisioness and of two temperatures, with protons hotter than electrons. Here, particle heating is expected to be controlled by magnetic reconnection in the transrelativistic regime , where the magnetization is the ratio of magnetic energy density to plasma enthalpy density. Using large-scale 2D particle-in-cell simulations, we explore for a fiducial how the dissipated magnetic energy is partitioned between electrons and protons as a function of (the ratio of proton thermal pressure to magnetic pressure) and of the strength of a guide field perpendicular to the reversing field B 0. At low , we find that the fraction of initial magnetic energy per particle converted into electron irreversible heat is nearly independent of , whereas protons are heated much less with increasing . As a result, for large , electrons receive the overwhelming majority of irreversible particle heating (∼93% for ). This is significantly different than the antiparallel case , in which irreversible electron heating accounts for only ∼18% of the total particle heating (Rowan et al. 2017). At , when both species start already relativistically hot (for our fiducial ), electrons and protons each receive ∼50% of the irreversible particle heating, regardless of the guide field strength. Our results provide important insights into the plasma physics of electron and proton heating in hot accretion flows around supermassive black holes.

Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field

Abstract We use 2D particle-in-cell simulations to study the effect of the saturated whistler instability on the viscous heating and nonthermal acceleration of electrons in a shearing, collisionless plasma with a growing magnetic field, . In this setup, an electron pressure anisotropy with naturally arises due to the adiabatic invariance of the electron magnetic moment ( and are the pressures parallel and perpendicular to ). If the anisotropy is large enough, then the whistler instability arises, efficiently scattering the electrons and limiting ( ). In this context, taps into the plasma velocity shear, producing electron heating by the so-called anisotropic viscosity. In our simulations, we permanently drive the growth of by externally imposing a plasma shear, allowing us to self-consistently capture the long-term, saturated whistler instability evolution. We find that besides the viscous heating, the scattering by whistler modes can stochastically accelerate electrons to nonthermal energies. This acceleration is most prominent when initially , gradually decreasing its efficiency for larger values of ( ). If initially , then the final electron energy distribution can be approximately described by a thermal component, plus a power-law tail with a spectral index of ∼3.7. In these cases, the nonthermal tail accounts for of the electrons and for of their kinetic energy. We discuss the implications of our results for electron heating and acceleration in low-collisionality astrophysical environments, such as low-luminosity accretion flows.

ELECTRON HEATING BY THE ION CYCLOTRON INSTABILITY IN COLLISIONLESS ACCRETION FLOWS. II. ELECTRON HEATING EFFICIENCY AS A FUNCTION OF FLOW CONDITIONS

In the innermost regions of low-luminosity accretion flows, including Sgr A* at the center of our Galaxy, the frequency of Coulomb collisions is so low that the plasma is two-temperature, with the ions substantially hotter than the electrons. This paradigm assumes that Coulomb collisions are the only channel for transferring the ion energy to the electrons. In this work, the second of a series, we assess the efficiency of electron heating by ion velocity-space instabilities in collisionless accretion flows. The instabilities are seeded by the pressure anisotropy induced by magnetic field amplification, coupled to the adiabatic invariance of the particle magnetic moments. Using two-dimensional (2D) particle-in-cell (PIC) simulations, we showed in Paper I that if the electron-to-ion temperature ratio is < 0.2, the ion cyclotron instability is the dominant mode for values of ion beta_i ~ 5-30 (here, beta_i is the ratio of ion thermal pressure to magnetic pressure), as appropriate for the midplane of low-luminosity accretion flows. In this work, we employ analytical theory and 1D PIC simulations (with the box aligned with the fastest growing wavevector of the ion cyclotron mode) to fully characterize how the electron heating efficiency during the growth of the ion cyclotron instability depends on the electron-to-proton temperature ratio, the plasma beta, the Alfven speed, the amplification rate of the mean field (in units of the ion Larmor frequency) and the proton-to-electron mass ratio. Our findings can be incorporated as a physically-grounded sub-grid model into global fluid simulations of low-luminosity accretion flows, thus helping to assess the validity of the two-temperature assumption.

ELECTRON HEATING BY THE ION CYCLOTRON INSTABILITY IN COLLISIONLESS ACCRETION FLOWS. I. COMPRESSION-DRIVEN INSTABILITIES AND THE ELECTRON HEATING MECHANISM

In systems accreting well below the Eddington rate, the plasma in the innermost regions of the disk is collisionless and two-temperature, with the ions hotter than the electrons. Yet, whether a collisionless faster-than-Coulomb energy transfer mechanism exists in two-temperature accretion flows is still an open question. We study the physics of electron heating during the growth of ion velocity-space instabilities, by means of multi-dimensional particle-in-cell (PIC) simulations. A large-scale compression - embedded in a novel form of the PIC equations - continuously amplifies the field. This constantly drives a pressure anisotropy P_perp > P_parallel, due to the adiabatic invariance of the particle magnetic moments. We find that, for ion plasma beta values beta_i ~ 5-30 appropriate for the midplane of low-luminosity accretion flows, mirror modes dominate if the electron-to-proton temperature ratio is > 0.2, whereas if it is < 0.2 the ion cyclotron instability triggers the growth of strong Alfven-like waves, that pitch-angle scatter the ions to maintain marginal stability. We develop an analytical model of electron heating during the growth of the ion cyclotron instability, which we validate with PIC simulations. We find that for cold electrons (beta_e < m_e/m_i), the electron energy gain is controlled by the magnitude of the E-cross-B velocity induced by the ion cyclotron waves. This term is independent of the initial electron temperature, so it provides a solid energy floor even for electrons starting with extremely low temperatures. On the other hand, the electron energy gain for beta_e > m_e/m_i - governed by the conservation of the magnetic moment in the growing fields of the instability - is proportional to the initial electron temperature. Our results have implications for two-temperature accretion flows as well as the solar wind and intracluster plasmas. [abridged]

The neutron star transient and millisecond pulsar in M28: from sub-luminous accretion to rotation-powered quiescence

The X-ray transient IGR J18245–2452 in the globular cluster M28 contains the first neutron star (NS) seen to switch between rotation-powered and accretion-powered pulsations. We analyse its 2013 March–April 25 d long outburst as observed by Swift, which had a peak bolometric luminosity of ∼6 per cent of the Eddington limit (LEdd), and give detailed properties of the thermonuclear burst observed on 2013 April 7. We also present a detailed analysis of new and archival Chandra data, which we use to study quiescent emission from IGR J18245–2452 between 2002 and 2013. Together, these observations cover almost five orders of magnitude in X-ray luminosity (LX, 0.5–10 keV). The Swift spectrum softens during the outburst decay (photon index Γ from 1.3 above LX/LEdd = 10−2 to ∼2.5 at LX/LEdd = 10−4), similar to other NS and black hole transients. At even lower luminosities, LX/LEdd = [10−4–10−6], deep Chandra observations reveal hard (Γ = 1–1.5), purely non-thermal and highly variable X-ray emission in quiescence. We therefore find evidence for a spectral transition at LX/LEdd ∼ 10−4, where the X-ray spectral softening observed during the outburst decline turns into hardening as the source goes to quiescence. Furthermore, we find a striking variability pattern in the 2008 Chandra light curves: rapid switches between a high-luminosity ‘active’ state (LX ≃ 3.9 × 1033 erg s−1) and a low-luminosity ‘passive’ state (LX ≃ 5.6 × 1032 erg s−1), with no detectable spectral change. We put our results in the context of low-luminosity accretion flows around compact objects and X-ray emission from millisecond radio pulsars. Finally, we discuss possible origins for the observed mode switches in quiescence, and explore a scenario where they are caused by fast transitions between the magnetospheric accretion and pulsar wind shock emission regimes.

Thermally Driven Winds from Radiatively Inefficient Accretion Flows

Abstract Radiatively inefficient accretion flows (RIAFs) are a common feature of low-luminosity accretion flows, including quiescent states of X-ray binaries and low-lunimosity active galactic nuclei. Thermally driven winds are expected from such hot accretion flows. By assuming that the flow has a self-similarity structure in the radial direction, we solved the vertical structure of the wind and accretion flows simultaneously and evaluated the mass loss rates by wind. We have found that the ratio of the outflow rate to the accretion rate is approximately unity for a viscosity parameter of $\alpha $$\lesssim$ 0.1, despite some uncertainties in the angular momentum and temperature distributions. That is, the accretion rate in the RIAFs is roughly proportional to the radius. Moreover, we elucidated the effect of cooling by wind on the underneath accretion flow, finding that this effect could be important for calculating the energy spectrum of the RIAF. Observational implications are briefly discussed in the context of Sgr A$^\ast$.

Observational Constraints on Radiatively Inefficient Accretion Flows in Nearby AGNs

Results of recent X-ray observations of low-luminosity AGNs are summarized. The lack of the signature of X-ray reflection features and big blue bump is in good agreement with the prediction that an optically thick accretion disk is truncated. Comparison of radio and X-ray luminosities indicates that low-luminosity objects tend to be radio loud compared to more luminous AGNs, suggesting efficient jet production in low-luminosity accretion flows.

Reprocessing of radiation by multi-phase gas in low-luminosity accretion flows

We discuss the role that magnetic fields in low-luminosity accretion flows can play in creating and maintaining a multi-phase medium, and show that small clouds or filaments of dense cold gas can dramatically reprocess the ‘primary’ radiation from tori. In particular, radio emission is suppressed by free-free absorption, and an extra (weak) component appears at optical wavelengths. This is expected to be a common process in various environments in the central regions of active galaxies, such as broad-line regions, accretion disc coronae and jets.