# 2019

## May 24: Pavel Abolmasov and Anna Chashkina (University of Turku, Finland)

Talk 1:

Title: Two-dimensional simulations of neutron star spreading layers

Authors: Pavel Abolmasov(U. of Turku, Finland), Joonas Nattila, Juri Poutanen

Abstract: We consider the internal dynamics of a boundary layer formed during accretion onto a non-magnetized neutron star. We use the spreading layer (SL) approximation making the flow essentially two-dimensional on the surface of the star. A system of modified shallow-water equations including viscous heating and radiation losses are solved numerically using spectral methods involving decomposition into spherical harmonics. The method is realized in the form of a free open-source code SLayer (https://github.com/pabolmasov/SLayer). It is found that, during accretion, at least two different instabilities come into play. First is a heating instability capable of moving large chunks of cool, rapidly rotating material from the layer to higher latitudes, simultaneously breaking the axial symmetry of the flow. Second is the shear instability working on smaller scales near the discontinuities in rotation velocity. Development of these instabilities leads to enhanced angular momentum exchange between the newly accreted, rapidly rotating material of the layer and the slowly rotating pre-existing matter.

We compute artificial light curves for spreading layers viewed at different inclination angles. Most of the simulated light curves show oscillations at frequencies close to 1kHz. Oscillations observed in our simulations are most likely inertial modes trapped near the boundary of the SL. Their frequencies, dependence on flux, and amplitude variations closely resemble those of real kHz QPOs observed in the power density spectra of low-mass X-ray binaries.

Talk 2:

Title: Super-Eddington accretion discs with advection and outflows around magnetized neutron stars

Authors: Anna Chashkina (U. of Turku, Finland), Galina Lipunova, Pavel Abolmasov and Juri Poutanen

Abstract: There are strong indications that the brightest off-nuclear X-ray sources in galaxies, the so-called ultraluminous X-ray sources, are compact objects accreting close to or above their Eddington limits. Unexpected discovery of the recent years was that at least some of these objects, and maybe most of them, are in fact neutron stars. Strong magnetic field may increase the actual luminosity limit by about two orders of magnitude due to geometric effects and the decrease in opacity in strong magnetic field. Close to the luminosity limit, radiation pressure plays important role, altering the physical conditions in the magnetosphere and near the inner edge of the accretion disc. We propose a model of an accretion disc interacting with the magnetosphere through a narrow layer near its inner edge. Our model also accounts for radiation pressure from the central source (accretion column). Though the radiation of the disc itself is unlikely to be observed directly, the model allows to find the size of the magnetosphere and, subsequently, to predict equilibrium periods, spin-up rates and possibly other observational properties of both classical and ultra-luminous X-ray pulsars.

## The Structure of Shock Waves in the Heliosphere and the Very Local Interstellar Medium

The observations made by many spacecraft demonstrate that the distant solar wind in the outer heliosphere is indeed mediated by nonthermal energetic particles such as pickup ions (PUIs). They dominate internal pressure in the outer heliosphere and play a fundamental role in the dissipative process at shock waves and in determining their structure. In this talk, I give a general review of the structure of shocks in the outer heliosphere, the heliospheric termination shock (HTS), shocks in the inner heliosheath (IHS), and the very local interstellar medium (VLISM) shocks. We use a two-fluid model to study the structure of shock waves in these regions. I show that a very small percentage of the solar wind flow energy at the upstream of the HTS is converted to downstream thermal heating, as it was observed by Voyager 2 and PUIs provide almost all the dissipative heating of the bulk flow energy. Then, I show that the shocks propagating in the IHS are mediated by the energetic PUIs, and the PUIs represent the primary dissipation mechanism for quasi-perpendicular IHS shocks. Consequently, IHS shocks enhance the IHS PUI temperature. The mediation of the IHS temperature due to the presence of many shocks may result in the more effective production of energetic neutral atoms (ENAs) due to charge exchange between interstellar neutral gas and IHS PUIs, which should be observed in IBEX ENA measurements. I show that the predicted ENA flux matches the observed IBEX ENA flux more closely when shock waves are present in the IHS. Finally, I show that the VLISM is a collisional medium with respect to the thermal gas and the VLISM shock observed by Voyager 1 is controlled by particle collisions and largely not mediated by PUIs since they do not introduce significant dissipation through the shock transition.

## Magneto-immutable plasma turbulence

In weakly collisional plasmas with weak magnetic fields (high beta plasmas), turbulent motions can generate large differences between the thermal pressures parallel and perpendicular to the magnetic field. This pressure anisotropy can have a strong dynamical effect, causing the turbulence to self organise to resist changes to the magnetic-field strength. We call this effect “magneto-immutablity,” by analogy with incompressibility in fluids dominated by thermal pressure. I will discuss the basic physics of magneto-immutable plasma turbulence, illustrated through simulations of simplified models in various physical scenarios. We see a surprising similarity to basic MHD in most statistical measures, despite the flow structures being more constrained.

## The first EHT results: the black hole shadow of M87*

The Event Horizon Telescope Collaboration (EHTC) revealed the first image of the shadow of the black hole in the center of Messier 87. Based on the observed size of the shadow the mass is estimated to be around 6.5 billion solar mass. In this talk I will highlight both the observational and theoretical efforts made by the EHTC to obtain and interpret this spectacular image.

## Hyper-Resistive Model of UHE Cosmic Ray Acceleration by AGNs

Ultra High Energy (UHE) cosmic rays (≈ 1e20 eV) may be produced by known processes of acceleration by plasma turbulence in magnetized jets produced by Active Galactic Nuclei (AGNs). A simple model in which turbulence is represented as hyper-resistivity in Ohm’s Law yields several predictions in sufficient agreement with observations to motivate further investigation. Besides jet dimensions, these predictions include the unique extra-galactic cosmic ray energy spectrum (∝ 1/E^3) and a different interpretation of the synchrotron radiation by which AGN jets are observed. Crucial to the model is a new theory of jet propagation whereby un-collimated jets generated by General Relativistic MHD simulations evolve to a highly collimated structure, finally evolving at speed 0.01c that explains jet dimensions, while relativistic acceleration parallel to field lines yields both cosmic rays and synchrotron radiation.

References:

[1] S. A. Colgate, T. K. Fowler, H. Li & J.Pino, 2014 ApJ 789, 144, on AGN jets

[2] S. A. Colgate, T. K. Fowler, H. Li et al. 2015 ApJ 813, 136, on jet stability

[3] T. K. Fowler & H. Li, 2016 J. Plas. Phys. 82, 595820513, on UHE acceleration

[4] T. K. Fowler, H. Li, R. Anantua, 2019 ArXiV 2615445

## MRI Turbulence in Weakly Collisional Plasmas

In radiatively inefficient accretion flows (RIAFs) onto supermassive black holes (SMBHs), which includes the SMBHs targeted by the Event Horizon Telescope, a large fraction of gravitational energy is converted into thermal energy. This results in a very hot and dilute plasma, in which the collisional mean free path can exceed the system size by orders of magnitude. Because of the low collisionality, fully kinetic simulations are needed to understand the accretion flow in such systems. However, 3D kinetic simulations remain a major computational challenge due to the large separation of scales between the disk rotation frequency and the ion gyrofrequency. In this talk, I will describe new physical insights that can be gained from local, shearing-box simulations in the computationally simpler, weakly-collisional plasma regime. I will overview the evolution of MRI-driven turbulence in the weakly collisional “Braginskii” closure, and discuss similarities and differences relative to ideal-MHD simulations. I will demonstrate that the large anisotropic-pressure forces in Braginskii MHD change the structure of the turbulence, though without significantly affecting overall angular-momentum transport. I will also discuss the importance of weakly-collisional physics for plasma heating and the implications for RIAFs.

## The answer is blowing in the wind: create, move, and observe cold gas around galaxies

Galactic winds are large-scale, multiphase outflows from galaxies, crucial for the galactic ecosystem, and a potent probe for the underlying feedback mechanisms. A common picture is that the cold gas has been accelerated by ram pressure forces due to the hot gas. However, reproducing this ubiquitous observation in hydrodynamical simulations has proven to be challenging - simply because the cold gas is destroyed prior to obtaining the observed velocities.

During my talk, I will show some analytical estimates and results from recent (magneto-)hydrodynamics simulations which suggest a solution to this classical "entrainment problem". I will conclude by discussing potential implications, and observables of cold gas in the surroundings of galaxies. Time provided, I want to show in particular that the Lyman-alpha line is a powerful probe of the (small-scale) structure of neutral hydrogen.

## Fully Kinetic effects in collisionless magnetorotational instability

The magnetorotational instability (MRI) is a crucial mechanism of angular momentum transport in several astrophysical scenarios, like accretion disks around black holes. The MRI has been widely studied using MHD models and simulations, in order to understand the behavior of astrophysical fluids in a state of differential rotation. In particular, the MRI plays a crucial role in the transport of angular momentum and turbulence generation at late nonlinear evolution of the system. In radiatively inefficient accretion flow (RIAF) models for accretion onto compact objects, the accretion proceeds via a hot, low-density plasma with the proton temperature larger than the electron temperature. In order to maintain the two-temperature flow characteristic of RIAF models, the typical collision rate must be much smaller than the accretion rate. This suggests that the standard MHD approach may be insufficient, and a kinetic description is required instead. Leveraging on our recent implementation of the shearing co-rotating framework in OSIRIS 4.0, we present our recent studies on collisionless MRI in high-$\beta$ plasma. On sufficient large simulation domains, we observe the development of an MRI-induced turbulent regime during the late nonlinear stage of the evolution. Increasing the mass ratio of our simulations, we show the development of an enhanced ion heating during the early nonlinear phase of collisionless MRI (channel flows regime). We will explore the mechanism responsible for these effects. The development of a drift-kink instability on large domains, combined with the magnetic reconnection, are responsible for the generation of the turbulent regime observed. The compression of the current sheets during the early nonlinear regime of the MRI acts differently on the two plasma species, inducing the temperature difference observed in our study. We support our assumptions with a theoretical model for kinetic compression of current sheets, giving a quantitative prediction of the electric and magnetic fields acceleration on the trapped particles during the compression phase.

## Particle Acceleration Mechanisms during Magnetic Reconnection

Magnetic reconnection is a fundamental plasma process that drives explosive conversion of magnetic energy into bulk flows, heat, and nonthermal particles. Reconnection is a promising mechanism for particle acceleration in a wide range of astrophysical phenomena including solar and stellar flares, pulsar wind nebulae, and AGN jets. We present results from three-dimensional full-particle kinetic simulations that demonstrate two primary particle energization processes operating during reconnection: (1) a Fermi-type mechanism associated with contracting magnetic ‘islands’, and (2) electric fields parallel to the local magnetic field. The Fermi mechanism drives volume-filling energization that scales strongly with particle energy, whereas parallel electric fields are localized to micro-scale dissipation regions and scale weakly with energy, instead generating bulk heating.

We also show that a background axial or ‘guide’ field plays a vital important role in controlling the acceleration efficiency. In the absence of a guide field, reconnection dynamics are effectively two-dimensional, and particles become trapped in stagnant island cores where acceleration ceases. In a system where the guide field is much stronger than the reconnecting field, magnetic field contraction is inhibited and the Fermi mechanism is suppressed. The most efficient acceleration occurs with a moderate guide field (comparable to the reconnecting field) that drives turbulent transport, enabling particles to escape island cores and re-accelerate while not substantially suppressing the Fermi mechanism. We present a model for the macro-scale evolution of the guide field during a solar/stellar flare and demonstrate consistency with solar hard X-ray observations.