Sept 12: Bart Ripperda and Fabio Bacchini (KU Leuven)

"Numerical Comparison between Relativistic Particle Pushers for Astrophysics"

Plasma processes cover a large range of energy and length scales, from the global fluid scalesto the fundamental particle scales. While the large scales can be covered within the magnetohydrodynamics (MHD) approximation, the small scales cannot. The macroscopic evolution of a plasma often develops relatively slowly, even in relativistic regimes. The macroscopic scale is however tightly coupled to faster phenomena occurring at smaller scales. In astrophysics many of these phenomena occur in the setting of relativistic magnetized plasmas. Around compact objects like black holes or neutron stars relativistic effects have to be taken into account for both the global flow and the particles. However, even in the solar
corona or the Earth's magnetosphere particles can accelerate to mildly relativistic energies. At relativistic energies the particle equations of motion become nonlinear due to the occurrence of the Lorentz factor. There are several numerical methods to treat particle motion accurately. Here we aim to test a selection of available methods applied to known tests for which analytic solutions are available. The accuracy and performance of the particle pushers will be tested for various regimes, from Newtonian to highly relativistic energies in idealized setups relevant in astrophysics. Accuracy is assessed by determining how well conserved quantities are evolved. This study focuses on the particle pusher and therefore only static,
spatially uniform and non-uniform electromagnetic fields are considered. The pushers considered are commonly used in MHD codes to evolve test particles in a global (magnetized) fluid flow and in particle-in-cell (PIC) codes to evolve both particles and electromagnetic fields (cite codes). In both methods the electromagnetic fields have to be interpolated to the particle position typically. Interpolation errors are tested by feeding the
pusher with an analytic spatially varying field and comparing to the results with an interpolated field. We also show the extension of the tested schemes to a covariant form allowing to resolve particle dynamics in general relativistic plasma dynamics.

July 7: Bart Ripperda and Fabio Bacchini (KU Leuven)

"Particle acceleration in relativistic plasmas"

We analyze particle acceleration in explosive reconnection events in magnetically dominated proton-electron plasmas. Reconnection is driven by large-scale magnetic stresses in interacting current-carrying flux tubes. Our model relies on development of current-driven instabilities on macroscopic scales. These tilt-kink instabilities develop in an initially force-free equilibrium of repelling current channels. Using MHD methods we study a 3D model of repelling and interacting flux tubes in which we simultaneously evolve test particles, guided by electromagnetic fields obtained from MHD. We identify two stages of particle acceleration; Initially particles accelerate in the current channels, after which the flux ropes start tilting and kinking and particles accelerate due to reconnection processes in the plasma. The explosive stage of reconnection produces non-thermal energy distributions with slopes that depend on plasma resistivity and the initial particle velocity. We also discuss the influence of the length of the flux ropes on particle acceleration and energy distributions. This study extends previous 2.5D results to 3D setups, providing all ingredients needed to model realistic scenarios like solar flares, black hole flares and particle acceleration in pulsar wind nebulae: formation of strong resistive electric fields, explosive reconnection and non-thermal particle distributions. By assuming initial energy equipartition between electrons and protons, applying low resistivity in accordance with solar corona conditions and limiting the flux rope length to a fraction of a solar radius we obtain realistic energy distributions for solar flares with non-thermal power law tails and maximum electron energies up to 11 MeV and maximum proton energies up to 1 GeV.

June 9: David Eichler (Ben Gurion University)

"An Alternative Explanation for the Energy Dependent Boron to Carbon Ratio in Cosmic Rays"

May 19: Bei Wang (Princeton PICSciE & IPCC)

"Particle-In-Cell Optimization on Multi/Many-core Architectures"

We have witnessed a rapid evolution of computing architectures due to power constrains in the last decade. Understanding how to efficiently utilize these systems in the context of demanding numerical algorithms is an urgent task for many application scientists. In this talk, we describe approaches we use to develop a highly scalable particle-in-cell (PIC) code across one of the broadest sets of computer architectures, including multicore CPU, GPU and Intel Xeon Phi. In particular, we describe our “lessons learned” and “best practices” in optimizing PIC algorithm on Knights Landing (KNL), the 2nd generation Intel Xeon Phi processor.

May 12: Russell Kulsrud (Princeton) and Rashid Sunyaev (MPA & IAS)

"Diffusion of mass through tera gauss fields on the surface of neutron stars in HXRBs"

A large amount of mass falls on the polar region of neutron star in Xray binaries and the question is, is the mass completely frozen on the field lines or can it diffuse through them? In this talk we present a mechanism for the latter possibility. A strong MHD instability occurs in the top layers of the neutron star driven by the incoming mass. This instability has the same properties as the Schwarzschild instability in the solar convection zone. It gives rise to a turbulent cascade which mixes up the field lines so that lines originally far apart can come with a resistivity diffusion distance and transfer the masses between them. However, the lines of force themselves are not disrupted. This leads to an equilibrium which is marginal with respect to the instability just as happens in the Schwarzschild case.

April 28: Luca Comisso (Princeton)

"Relativistic Reconnection: from flat to curved spacetime"

In recent years, the classical Sweet-Parker and Petschek models have been extended in the special relativistic regime, both for MHD plasmas [1] and two-fluid electron-positron plasmas [2]. Nevertheless, there could be situations, like in the vicinity of black holes, where also general relativistic effects can become important. Here, we present a two-fluid description of the relativistic reconnection process for pair plasmas in the flat spacetime limit [2], and then we analyze the reconnection process in the MHD approximation for plasmas around rotating black holes [3]. A simple generalization of the Sweet-Parker model is used as a first approximation to the problem, and the reconnection rate, as well as other important properties of the reconnection layer, has been calculated taking into account the effect of spacetime curvature.

[1] Y. E. Lyubarsky, Mon. Not. R. Astron. Soc. 358, 113 (2005)
[2] L. Comisso and F.A. Asenjo, Phys. Rev. Lett. 113, 045001 (2014)
[3] F.A. Asenjo and L. Comisso, Phys. Rev. Lett. 118, 055101 (2017)

April 21: Jonathan Zrake (Columbia)

"Magnetic relaxation and turbulence in pulsar wind nebulae"

Pulsar wind nebulae (PWNe) are energized by the electromagnetic spin-down power of a rapidly rotating neutron star. Their emission is primarily synchrotron, produced by relativistic electrons radiating in a sub-equipartition magnetic field. The processes by which a pulsar wind, which is born in a strongly magnetized state, eventually shares its energy with electrons, have been a long-standing question in the theory of pulsars and their nebulae (sometimes referred to as the sigma-problem). I will discuss how dissipation in PWNe may be understood in terms of a process known as magnetic relaxation, and give an overview in general physics terms of recent advances in this topic. MHD simulations reveal the process is generally turbulent, and that magnetic field structures tend to organize themselves spatially, even when the field lacks net magnetic helicity. I will discuss how this process helps to explain the magnetization level of the Crab's synchrotron nebula.

April 14: Massimo Cappi (INAF/IASF-Bologna)

"The Athena X-ray Observatory: scientific objectives and mission study"

Athena is a large X-ray Observatory proposed to address the Science Theme “The Hot and Energetic Universe”, which has been selected by ESA in its Cosmic Vision program.
After reviewing its core science goals, the astrophysics and cosmic evolution of large-scale hot structures and black holes in the Universe, and (some of) its Observatory capabilities, I will present the mission telescope and instruments, to be implemented as a Large mission planned for launch in 2028.

March 17: Andrei Beloborodov (Columbia)

"Magnetic flares near accreting black holes"

A radiative mechanism is proposed for magnetic flares near luminous accreting black holes. It is based on recent first-principle simulations of magnetic reconnection, which show a hierarchical chain of fast-moving plasmoids. The reconnection occurs in a compact region (comparable to the black hole radius), and the chain experiences fast Compton cooling accompanied by electron-positron pair creation. The distribution of plasmoid speeds is shaped by radiative losses, and the self-regulated chain radiates its energy in hard X-rays. The mechanism is illustrated by Monte-Carlo simulations of the transfer of seed soft photons through the reconnection layer. The emerging radiation spectrum has a cutoff near 100 keV similar to the hard-state spectra of X-ray binaries and AGN. We discuss how the chain cooling differs from previous phenomenological emission models, and suggest that it can explain the hard X-ray activity of accreting black holes from first principles. Particles accelerated at the X-points of the chain produce an additional high-energy component, explaining "hybrid Comptonization" observed in Cyg X-1.

March 10: Alex Lazarian (University of Wisconsin - Madison)

"New observational techniques to trace magnetic fields and explore turbulence: first results"

I shall introduce two new techniques of magnetic field tracing. The first one uses Doppler-shifted emission lines and employs the gradients of velocity centroids in order to trace magnetic fields in the diffuse interstellar media as well as to trace regions of star formation associated with the gravitational collapse. I shall provide the theoretical justification of the use of the measure, its numerical testing as well as the comparison of the directions obtained with the velocity centroid gradients using GALFA HI data and those of magnetic field as traced by Planck. The second measure is the synchrotron intensity gradients that also trace magnetic field and, unlike synchrotron polarization, are insensitive to Faraday rotation. I shall also show its correspondence with the magnetic field tracing by Planck and discuss the synergy of using it with low frequency polarization studies. I shall discuss the promise of the new techniques both for the star formation and CMB foreground studies.

February 24: Kenta Hotokezaka (Center for Computational Astrophysics)

"Gravitational-wave Astronomy of compact binary mergers"

The discovery of gravitational waves from merging binary black holes (BBHs) by the two Advanced LIGO detectors has opened gravitational-wave astronomy. One of the biggest questions arose after the discovery is how do so massive BBHs form in close binaries. I discuss the progenitor scenarios of BBHs focusing on the low spins inferred from the three detected events. I show that, among known objects, Wolf-Rayet stars seem the only progenitors consistent with the low spins. I also discuss the possible connection between BBH mergers and long GRBs. I will also talk about electromagnetic counterparts of neutron star binary mergers. I will show that we will have chances to discover r-process macronovae/kilonovae and their radio remnants after gravitational-wave merger events.