2017

Dec 8: Wenbin Lu (University of Texas at Austin)

"The Radiation Mechanism of Fast Radio Bursts"

Recent observations show that fast radio bursts (FRBs) are energetic but probably non-catastrophic events from cosmological distances. The properties of their progenitors are largely unknown, despite many attempts to determine them using the event rate, duration and energetics. Understanding the radiation mechanism for FRBs should provide the missing insights regarding their progenitors, which will be described.

The high brightness temperatures (> 1e35 K) of FRBs mean that the emission process must be coherent. Two general types of coherent radiation mechanisms are considered --- maser and the antenna mechanism. We use the observed properties of the repeater FRB 121102 to constrain the plasma conditions for these two mechanisms. We have looked into a wide variety of maser mechanisms operating in vacuum or plasma and find that none of them can explain the high luminosity of FRBs without invoking unrealistic or fine-tuned plasma conditions. The most favorable mechanism is antenna curvature emission by charge bunches which is powered by magnetic reconnection near the surface of a magnetar (B > 1e14 G). We show that the plasma in the twisted magnetosphere of a magnetar may be clumpy due to two-stream instability. When magnetic reconnection occurs, the pre-existing density clumps may provide charge bunches for the antenna mechanism to operate.

*Wenbin will be visiting Peyton Dec 6-8. If you would like to meet with Wenbin, sign up here.

Dec 1: Sam Totorica (Stanford University)

"Particle acceleration in laser-driven magnetic reconnection"

Magnetic reconnection is a promising candidate mechanism for accelerating the nonthermal particles associated with explosive astrophysical phenomena. Laboratory experiments with high-power lasers can play an important role in the study of the detailed microphysics of reconnection and the dominant particle acceleration mechanisms. In this talk I will present the results of two- and three-dimensional particle-in-cell (PIC) simulations used to explore particle acceleration in conditions relevant for current and future laser-driven reconnection experiments. These simulations indicate that laser-driven plasmas offer a promising platform for studying particle acceleration from reconnection, with the potential to reach multi-plasmoid regimes of strong astrophysical interest.

Due to limitations such as noise from numerical collisions and the large number of simulation particles required to capture the development of nonthermal tails in the particle distribution, multiscale PIC simulations like those used to study reconnection are extremely challenging. In the second half of the talk I will discuss the novel simplex-in-cell algorithm that holds promise for overcoming these difficulties by interpreting the simulation particles as the vertices of a mesh that traces the evolution of the distribution function in phase space, rather than fixed-shape clouds of charge. Using test problems including the Weibel instability I will show how this new view retains fine-scale structure in the distribution function and can drastically reduce the number of simulation particles required to reach a given noise level.

* To meet with Sam, sign up here.

Nov 17: Maxim Barkov (Purdue University)

"Pulsar/Stellar wind collision in 3D and The origin of the X-ray-emitting object moving away from PSR B1259-63"

For the first time, we simulate in 3 dimensions the interaction of isotropic stellar and relativistic pulsar winds along one full orbit, on scales well beyond the binary size. We used the code PLUTO to carry out relativistic hydrodynamical simulations in 2 and 3 dimensions of the interaction between a slow dense wind and a mildly relativistic wind with Lorentz factor 2, along one full orbit in a region up to ~100 times the binary size. The simulations in 3 dimensions confirm previous results in 2 dimensions, showing: a strong shock induced by Coriolis forces that terminates the pulsar wind also in the opposite direction to the star; strong bending of the shocked-wind structure against the pulsar motion; and the generation of turbulence. The shocked flows are also subject to a faster development of instabilities in 3 dimensions, which enhances shocks, two-wind mixing, and large-scale disruption of the shocked structure. In addition to the Kelvin-Helmholtz instability, discussed in the past, we find that the Richtmyer-Meshkov and the Rayleigh-Taylor instabilities are very likely acting together in the shocked flow evolution.

A mysterious X-ray-emitting object has been detected moving away from the high-mass gamma-ray binary PSR B1259-63, which contains a non-accreting pulsar and a Be star whose winds collide forming a complex interaction structure. Given the strong eccentricity of this binary, the interaction structure should be strongly anisotropic, which together with the complex evolution of the shocked winds, could explain the origin of the observed moving X-ray feature. We propose here that a fast outflow made of a pulsar-stellar wind mixture is always present moving away from the binary in the apastron direction, with the injection of stellar wind occurring at orbital phases close to periastron passage. This outflow periodically loaded with stellar wind would move with a high speed, and likely host non-thermal activity due to shocks, on scales similar to those of the observed moving X-ray object. Such an outflow is thus a very good candidate to explain this X-ray feature. This, if confirmed, would imply pulsar-to-stellar wind thrust ratios of 0.1, and the presence of a jet-like structure on the larger scales, up to its termination in the interstellar medium.

Nov 10: Philipp Moesta (Berkeley)

"The most powerful transients in 3D"

Extreme (hyperenergetic/superluminous) core-collapse supernovae belong to the most energetic transients in the universe and are key in the supernova-GRB connection. I will discuss the unique challenges in both input physics and computational modeling for these systems involving all four fundamental forces and highlight recent breakthroughs in full 3D simulations. I will pay particular attention to how these simulations can be used to reveal the engines driving the explosion and conclude by discussing what remains to be done in order to maximize what we can learn from current and future time-domain transient surveys.

Nov 3: Matt Kunz (Princeton University)

“Kinetic Turbulence in Weakly Collisional, Space and Astrophysical Plasmas”

In this talk, I will provide an update of my group's efforts to elucidate, from first principles, the effects of cosmic magnetism and plasma microphysics on the macroscopic behavior of an important class of space and astrophysical plasmas: those that are so hot and diffuse that they cannot be rigorously described as fluids. These plasmas include the solar wind, low-luminosity black-hole accretion flows, and the intracluster medium of galaxy clusters. Currently, the lack of a rigorous theory for the plasma microphysics of these systems is a formidable obstacle to answering a wide variety of space and astrophysics questions. I will begin by showing recent results from hybrid-kinetic simulations of Alfvénic turbulence in the solar wind, which address (among other things) why its constituent particles are preferentially heated perpendicular to the magnetic field. This physics, along with the expansion of the solar wind, endow that plasma with an anisotropic pressure. Gyrokinetic theory and kinetic simulations that explore how this pressure anisotropy affects Alfvénic turbulence and particle heating in the solar wind (and in more general settings) will be presented. In particular, the role of microphysically regulated pressure anisotropy on angular-momentum transport, ion versus electron heating, the propagation of Alfvén waves, and the onset of magnetic reconnection in high-beta plasmas will be highlighted.

Oct 20: Brian Metzger (Columbia University)

"The Multi-Messenger Picture of a Neutron Star Merger"

On August 17 the LIGO/Virgo gravitational wave observatories detected the first binary neutron star merger event (GW170817), a discovery followed by the most ambitious electromagnetic (EM) follow-up campaign ever conducted. A gamma-ray burst (GRB) of short duration and very low luminosity was discovered by the Fermi and INTEGRAL satellites within 2 seconds of the merger. Within 11 hours, a bright but rapidly-fading thermal optical counterpart was discovered in the galaxy NGC 4993 at a distance of only 40 Mpc. The properties of the optical transient match remarkably well predictions for “kilonova” emission powered by the radioactive decay of heavy nuclei synthesized in the expanding merger ejecta by the r-process. The rapid spectral evolution of the kilonova emission to near-infrared wavelengths demonstrates that a portion of the ejecta contains heavy lanthanide nuclei. Two weeks after the merger, rising non-thermal X-ray and radio emission were detected from the position of the optical transient, consistent with delayed synchrotron afterglow radiation from an initially off-axis relativistic jet with the properties consistent with those of (on-axis) cosmological short GRB. I will describe a unified scenario for the range of EM counterparts from GW170817 and their implications for the astrophysical origin of the r-process and the properties of neutron stars. I will preview the upcoming era of multi-messenger astronomy, once Advanced LIGO/Virgo reach design sensitivity and a neutron star merger is detected every few weeks.

Oct 13: Shigeo Kimura (The Pennsylvania State University)

"Shear Acceleration in Active Galactic Nuclei"

Active galactic nuclei have accretion flows and/or relativistic jets. In the accretion flows, the magnetorotational instability generates turbulence, which induces magnetic reconnection that can produce cosmic rays (CRs). These CRs can be further accelerated by stochastic acceleration and/or shear acceleration. In the first half of this talk, I discuss the particle acceleration inside the accretion flows using test-particle simulations in turbulence generated by magnetorotational instability. The relativistic jets are also widely believed to produce a high-energy cosmic rays. In the last half, I talk about a radio galaxy model of ultrahigh-energy cosmic-ray (UHECR) production, where shear acceleration mechanism accelerates heavy nuclei up to 10^20 EeV. If the galactic cosmic rays are re-accelerated through this mechanism, our model can reproduce the spectrum and composition ratio of UHECRs.

Oct 6: Silvio Cerri (Princeton)

"Solar wind turbulence in the kinetic range: a hybrid-kinetic approach"

Nearly all astrophysical plasmas are in a turbulent state: understanding the properties of turbulent fluctuations and their dissipation is a fundamental step to understand how turbulence feeds back on the evolution of such systems.

In this context, space plasmas are probably the best laboratory for the study of collisionless plasma turbulence, as the Earth’s environment has become accessible to increasingly accurate direct measurements. In situ observations in the solar wind and in the terrestrial magnetosheath have indeed provided relevant constraints on the turbulent energy spectra, determining the typical values for their slopes and revealing the presence of breaks in the electromagnetic fluctuations cascade at kinetic scales.

While the energy is cascading toward smaller and smaller scales, the first break is encountered at the proton kinetic scales and separates the so-called "inertial range" spectrum, developing at the ("large") magnetohydrodynamic scales, from the kinetic spectrum that arises at scales smaller than the proton gyroradius (also referred to as the "dissipation" or "dispersion" range spectrum). Such a transition is clear evidence of a change in the physics underlying the cascade process, and its understanding in terms of kinetic processes is today a matter of a strong debate.

In this talk I present some most recent developments in the investigation of the properties of the subproton-scale cascade via high-resolution hybrid-kinetic (fully-kinetic ions and fluid electrons) simulations.

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.