Recent Changes

Friday, January 19

  1. page 2018 Archive edited Jan 26: Bart Ripperda and Fabio Bacchini (KU Leuven) "Generalized, energy-conserving numer…

    Jan 26: Bart Ripperda and Fabio Bacchini (KU Leuven)
    "Generalized, energy-conserving numerical integration of geodesics in general relativity"
    The numerical integration of particle trajectories in curved spacetimes is fundamental for obtaining realistic models of the particle dynamics around massive compact objects such as black holes and neutron stars. Generalized algorithms capable of handling generic metrics are required for studies of both standard spacetimes Schwarzschild and Kerr metrics) and non-standard spacetimes (e.g. Schwarzschild metric plus non-classical perturbations or multiple black hole metrics). The most commonly employed explicit numerical schemes (e.g. Runge-Kutta) are incapable of producing highly accurate results at critical points, e.g. in the regions close to the event horizon where gravity causes extreme curvature of the spacetime, at an acceptable computational cost. Here, we describe a generalized algorithm for the numerical integration of time-like (massive particles) and null (photons) geodesics in any given 3+1 split spacetime. We introduce a new, exactly energy-conserving implicit integration scheme based on the preservation of the underlying Hamiltonian, and we compare its properties with a standard fourth-order Runge-Kutta explicit scheme and an implicit midpoint scheme. We test the numerical performance of the three schemes against analytical solutions of test particle and photon orbits in Schwarzschild and Kerr spacetimes. We also prove the versatility of our framework in handling more exotic metrics such as Morris-Thorne wormholes and quantum-perturbed Schwarzschild black holes. The generalized approach is also discussed in the perspective of future extensions to more complex particle dynamics, e.g. the addition of the Lorentz force acting on charged particles.

    (view changes)
    9:05 am

Friday, December 1

  1. page 2017 Archive edited ... Recent observations show that fast radio bursts (FRBs) are energetic but probably non-catastro…
    ...
    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"
    (view changes)
    11:46 am
  2. page 2017 Archive edited ... Magnetic reconnection is a promising candidate mechanism for accelerating the nonthermal parti…
    ...
    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"
    (view changes)
    7:01 am

Thursday, November 30

  1. page 2017 Archive edited Dec 8: Wenbin Lu (University of Texas at Austin) "The Radiation Mechanism of Fast Radio Bu…

    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.

    Dec 1: Sam Totorica (Stanford University)
    "Particle acceleration in laser-driven magnetic reconnection"
    (view changes)
    4:41 pm

Friday, November 24

  1. page 2017 Archive edited Dec 1: Sam Totorica (Stanford University) "Particle acceleration in laser-driven magnetic …

    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.

    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"
    (view changes)
    5:37 pm

Wednesday, November 8

  1. page 2017 Archive edited Nov 17: Maxim Barkov (Purdue University) "Pulsar/Stellar wind collision in 3D and The orig…

    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"
    (view changes)
    6:58 pm

Saturday, November 4

  1. page 2017 Archive edited Nov 10: Philipp Moesta (Berkeley) "The most powerful transients in 3D" Extreme (hype…

    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”
    (view changes)
    3:45 pm

Sunday, October 29

  1. page 2017 Archive edited Nov 3: Matt Kunz (Princeton University) “Kinetic Turbulence in Weakly Collisional, Space and As…

    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"
    (view changes)
    5:57 pm

Monday, October 16

  1. page 2017 Archive edited Oct 20: Brian Metzger (Columbia University) "The Multi-Messenger Picture of a Neutron Star…

    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"
    (view changes)
    12:54 pm

Thursday, October 5

  1. page home edited ... The seminar covers a wide range of topics in plasma astrophysics and high energy astrophysics.…
    ...
    The seminar covers a wide range of topics in plasma astrophysics and high energy astrophysics.
    Time: Fridays 12:30-1:30 pm
    location:Location: Dome Room
    Department of Astrophysical Sciences - Princeton University
    4 Ivy Ln. - Peyton Hall
    (view changes)
    5:15 pm

More