496 research outputs found
Ion acceleration in non-relativistic astrophysical shocks
We explore the physics of shock evolution and particle acceleration in
non-relativistic collisionless shocks using multidimensional hybrid
simulations. We analyze a wide range of physical parameters relevant to the
acceleration of cosmic rays (CRs) in astrophysical non-relativistic shock
scenarios, such as in supernova remnant (SNR) shocks. We explore the evolution
of the shock structure and particle acceleration efficiency as a function of
Alfv\'enic Mach number and magnetic field inclination angle . We show
that there are fundamental differences between high and low Mach number shocks
in terms of the electromagnetic turbulence generated in the pre-shock zone and
downstream; dominant modes are resonant with the streaming CRs in the low Mach
number regime, while both resonant and non-resonant modes are present for high
Mach numbers. Energetic power law tails for ions in the downstream plasma can
account for up to 15% of the incoming upstream flow energy, distributed over
of the particles in a power law with slope in energy. The
energy conversion efficiency (for CRs) peaks at to
and , and decreases for higher Mach numbers, down to for
. Accelerated particles are produced by Diffusive Shock Acceleration
(DSA) and by Shock Drift Acceleration (SDA) mechanisms, with the SDA
contribution to the overall energy gain increasing with magnetic inclination.
We also present a direct comparison between hybrid and fully kinetic
particle-in-cell results at early times; the agreement between the two models
justifies the use of hybrid simulations for longer-term shock evolution. In SNR
shocks, particle acceleration will be significant for low Mach number
quasi-parallel flows (, ). This finding underscores the
need for effective magnetic amplification mechanism in SNR shocks
Long Term Evolution of Magnetic Turbulence in Relativistic Collisionless Shocks
We study the long term evolution of magnetic fields generated by an initially
unmagnetized collisionless relativistic shock. Our 2D particle-in-cell
numerical simulations show that downstream of such a Weibel-mediated shock,
particle distributions are approximately isotropic, relativistic Maxwellians,
and the magnetic turbulence is highly intermittent spatially, nonpropagating,
and decaying. Using linear kinetic theory, we find a simple analytic form for
these damping rates. Our theory predicts that overall magnetic energy decays
like with , which compares favorably with
simulations, but predicts overly rapid damping of short wavelength modes.
Magnetic trapping of particles within the magnetic structures may be the origin
of this discrepancy. We conclude that initially unmagnetized relativistic
shocks in electron-positron plasmas are unable to form persistent downstream
magnetic fields. These results put interesting constraints on synchrotron
models for the prompt and afterglow emission from GRBs.Comment: 4 pages, 3 figures, contributed talk at the workshop: High Energy
Phenomena in Relativistic Outflows (HEPRO), Dublin, 24-28 September 2007;
Downsampled version for arXiv. Full resolution version available at
http://astro.berkeley.edu/~pchang/proceedings.pd
Dissipative Pulsar Magnetosphere
Dissipative axisymmetric pulsar magnetosphere is calculated by a direct
numerical simulation of the Strong-Field Electrodynamics equations. The
magnetic separatrix disappears, it is replaced by a region of enhanced
dissipation. With a better numerical scheme, one should be able to calculate
the bolometric lightcurves for a given conductivity.Comment: 2 pages, 10 figures, minor changes for the journa
On the Cosmic Ray Driven Firehose Instability
The role of the non-resonant firehose instability in conditions relevant to
the precursors of supernova remnant shocks is considered. Using a second order
tensor expansion of the Vlasov-Fokker-Planck equation we illustrate the
necessary conditions for the firehose to operate. It is found that for very
fast shocks, the diffusion approximation predicts that the linear firehose
growth rate is marginally faster than its resonant counterpart. Preliminary
hybrid MHD-Vlasov-Fokker-Planck simulation results using young supernova
relevant parameters are presented.Comment: Contribution to the 6th International Symposium on High Energy
Gamma-Ray Astronomy (Gamma2016), Heidelberg, Germany. To be published in the
AIP Conference Proceeding
Impulsive acceleration of strongly magnetized relativistic flows
The definitive version can be found at: http://onlinelibrary.wiley.com/ Copyright Royal Astronomical SocietyThe strong variability of magnetic central engines of active galactic nuclei (AGNs) and gamma-ray bursts (GRBs) may result in highly intermittent strongly magnetized relativistic outflows. We find a new magnetic acceleration mechanism for such impulsive flows that can be much more effective than the acceleration of steady-state flows. This impulsive acceleration results in kinetic-energy-dominated flows that are conducive to efficient dissipation at internal magnetohydrodynamic shocks on astrophysically relevant distances from the central source. For a spherical flow, a discrete shell ejected from the source over a time t0 with Lorentz factor Γ∼ 1 and initial magnetization σ0=B20/4πρ0c2≫ 1 quickly reaches a typical Lorentz factor Γ∼σ1/30 and magnetization σ∼σ2/30 at the distance R0≈ct0. At this point, the magnetized shell of width Δ∼R0 in the laboratory frame loses causal contact with the source and continues to accelerate by spreading significantly in its own rest frame. The expansion is driven by the magnetic pressure gradient and leads to relativistic relative velocities between the front and back of the shell. While the expansion is roughly symmetric in the centre of the momentum frame, in the laboratory frame, most of the energy and momentum remains in a region (or shell) of width Δ∼R0 at the head of the flow. This acceleration proceeds as Γ∼ (σ0R/R0)1/3 and σ∼σ2/30 (R/R0)-1/3 until reaching a coasting radius Rc∼R0σ20, where the kinetic energy becomes dominant: Γ∼σ0 and σ∼ 1 at Rc. The shell then starts coasting and spreading (radially), its width growing as Δ∼R0(R/Rc), causing its magnetization to drop as σ∼Rc/R at R > Rc. Given the typical variability time-scales of AGNs and GRBs, the magnetic acceleration in these sources is a combination of the quasi-steady-state collimation acceleration close to the source and the impulsive (conical or locally quasi-spherical) acceleration farther out. The interaction with the external medium, which can significantly affect the dynamics, is briefly addressed in the discussion.Peer reviewe
Modeling high-energy pulsar lightcurves from first principles
Current models of gamma-ray lightcurves in pulsars suffer from large
uncertainties on the precise location of particle acceleration and radiation.
Here, we present an attempt to alleviate these difficulties by solving for the
electromagnetic structure of the oblique magnetosphere, particle acceleration,
and the emission of radiation self-consistently, using 3D spherical
particle-in-cell simulations. We find that the low-energy radiation is
synchro-curvature radiation from the polar-cap regions within the light
cylinder. In contrast, the high-energy emission is synchrotron radiation that
originates exclusively from the Y-point and the equatorial current sheet where
relativistic magnetic reconnection accelerates particles. In most cases,
synthetic high-energy lightcurves contain two peaks that form when the current
sheet sweeps across the observer's line of sight. We find clear evidence of
caustics in the emission pattern from the current sheet. High-obliquity
solutions can present up to two additional secondary peaks from energetic
particles in the wind region accelerated by the reconnection-induced flow near
the current sheet. The high-energy radiative efficiency depends sensitively on
the viewing angle, and decreases with increasing pulsar inclination. The
high-energy emission is concentrated in the equatorial regions where most of
the pulsar spindown is released and dissipated. These results have important
implications for the interpretation of gamma-ray pulsar data.Comment: 14 pages, 11 figures, Accepted for publication in MNRA
Self-inhibiting thermal conduction in high-beta, whistler-unstable plasma
A heat flux in a high- plasma with low collisionality triggers the
whistler instability. Quasilinear theory predicts saturation of the instability
in a marginal state characterized by a heat flux that is fully controlled by
electron scattering off magnetic perturbations. This marginal heat flux does
not depend on the temperature gradient and scales as . We confirm this
theoretical prediction by performing numerical particle-in-cell simulations of
the instability. We further calculate the saturation level of magnetic
perturbations and the electron scattering rate as functions of and the
temperature gradient to identify the saturation mechanism as quasilinear.
Suppression of the heat flux is caused by oblique whistlers with
magnetic-energy density distributed over a wide range of propagation angles.
This result can be applied to high- astrophysical plasmas, such as the
intracluster medium, where thermal conduction at sharp temperature gradients
along magnetic-field lines can be significantly suppressed. We provide a
convenient expression for the amount of suppression of the heat flux relative
to the classical Spitzer value as a function of the temperature gradient and
. For a turbulent plasma, the additional independent suppression by the
mirror instability is capable of producing large total suppression factors
(several tens in galaxy clusters) in regions with strong temperature gradients.Comment: accepted to JP
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