1,134 research outputs found
On the Spin-Down of Intermittent Pulsars
Magnetospheres of pulsars are thought to be filled with plasma, and
variations in plasma supply can affect both pulsar emission properties and
spin-down rates. A number of recently discovered "intermittent" pulsars switch
between two distinct states: an "on", radio-loud state, and an "off",
radio-quiet state. Spin-down rates in the two states differ by a large factor,
, which is not easily understood in the context of current
models. In this Letter we present self-consistent numerical solutions of "on"
and "off" states of intermittent pulsar magnetospheres. We model the "on" state
as a nearly ideal force-free magnetosphere with abundant magnetospheric plasma
supply. The lack of radio emission in the "off" state is associated with plasma
supply disruption that results in lower plasma density on the open field lines.
We model the "off" state using nearly vacuum conditions on the open field lines
and nearly ideal force-free conditions on the closed field lines, where plasma
can remain trapped even in the absence of pair production. The toroidal
advection of plasma in the closed zone in the "off" state causes spin-downs
that are a factor of higher than vacuum values, and we naturally
obtain a range of spin-down ratios between the "on" and "off" states, , which corresponds to a likely range of pulsar inclination angles of
. We consider the implications of our model to a number of
poorly understood but possibly related pulsar phenomena, including nulling,
timing noise, and rotating radio transients.Comment: 6 pages, 4 figures, submitted to ApJ Letter
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
A distance formula related to a family of projections orthogonal to their symmetries
Let u be a hermitian involution, and e an orthogonal projection, acting on
the same Hilbert space. We establish the exact formula, in terms of the norm of
eue, for the distance from e to the set of all orthogonal projections q from
the algebra generated by e,u, and such that quq=0.Comment: 6 pages, 1 figur
Simulations of Ion Acceleration at Non-relativistic Shocks. I. Acceleration Efficiency
We use 2D and 3D hybrid (kinetic ions - fluid electrons) simulations to
investigate particle acceleration and magnetic field amplification at
non-relativistic astrophysical shocks. We show that diffusive shock
acceleration operates for quasi-parallel configurations (i.e., when the
background magnetic field is almost aligned with the shock normal) and, for
large sonic and Alfv\'enic Mach numbers, produces universal power-law spectra
proportional to p^(-4), where p is the particle momentum. The maximum energy of
accelerated ions increases with time, and it is only limited by finite box size
and run time. Acceleration is mainly efficient for parallel and quasi-parallel
strong shocks, where 10-20% of the bulk kinetic energy can be converted to
energetic particles, and becomes ineffective for quasi-perpendicular shocks.
Also, the generation of magnetic turbulence correlates with efficient ion
acceleration, and vanishes for quasi-perpendicular configurations. At very
oblique shocks, ions can be accelerated via shock drift acceleration, but they
only gain a factor of a few in momentum, and their maximum energy does not
increase with time. These findings are consistent with the degree of
polarization and the morphology of the radio and X-ray synchrotron emission
observed, for instance, in the remnant of SN 1006. We also discuss the
transition from thermal to non-thermal particles in the ion spectrum
(supra-thermal region), and we identify two dynamical signatures peculiar of
efficient particle acceleration, namely the formation of an upstream precursor
and the alteration of standard shock jump conditions.Comment: 21 pages, 14 figures, Minor changes reflecting the version accepted
to Ap
Ab-initio pulsar magnetosphere: three-dimensional particle-in-cell simulations of axisymmetric pulsars
We perform first-principles relativistic particle-in-cell simulations of
aligned pulsar magnetosphere. We allow free escape of particles from the
surface of a neutron star and continuously populate the magnetosphere with
neutral pair plasma to imitate pair production. As pair plasma supply
increases, we observe the transition from a charge-separated electrosphere
solution with trapped plasma and no spin-down to a solution close to the ideal
force-free magnetosphere with electromagnetically-dominated pulsar wind. We
calculate the magnetospheric structure, current distribution and spin-down
power of the neutron star. We also discuss particle acceleration in the
equatorial current sheet.Comment: 6 pages, 5 figures, published in ApJ Letter
Laser Shaping and Optimization of the Laser-Plasma Interaction
The physics of energy transfer between the laser and the plasma in laser
wakefield accelerators is studied. We find that wake excitation by arbitrary
laser shapes can be parameterized using the total pulse energy and pulse
depletion length. A technique for determining laser profiles that produce the
required plasma excitation is developed. We show that by properly shaping the
longitudinal profile of the driving laser pulse, it is possible to maximize
both the transformer ratio and the wake amplitude, achieving optimal
laser-plasma coupling. The corresponding family of laser pulse shapes is
derived in the nonlinear regime of laser-plasma interaction. Such shapes
provide theoretical upper limit on the magnitude of the wakefield and
efficiency of the accelerating stage by allowing for uniform photon
deceleration inside the laser pulse. We also construct realistic optimal pulse
shapes that can be produced in finite-bandwidth laser systems and propose a
two-pulse wake amplification scheme using the optimal solution.Comment: 12 pages, 5 figures, contributed to the Advanced Accelerator Concepts
2000 worksho
Synthetic Spectra from PIC Simulations of Relativistic Collisionless Shocks
We extract synthetic photon spectra from first-principles particle-in-cell
simulations of relativistic shocks propagating in unmagnetized pair plasmas.
The two basic ingredients for the radiation, namely accelerated particles and
magnetic fields, are produced self-consistently as part of the shock evolution.
We use the method of Hededal & Nordlund (2005) and compute the photon spectrum
via Fourier transform of the electric far-field from a large number of
particles, sampled directly from the simulation. We find that the spectrum from
relativistic collisionless shocks is entirely consistent with synchrotron
radiation in the magnetic fields generated by Weibel instability. We can
recover the so-called "jitter'' regime only if we artificially reduce the
strength of the electromagnetic fields, such that the wiggler parameter K = qB
lambda/mc^2 becomes much smaller than unity ("B" and "lambda" are the strength
and scale of the magnetic turbulence, respectively). These findings may place
constraints on the origin of non-thermal emission in astrophysics, especially
for the interpretation of the hard (harder than synchrotron) low-frequency
spectrum of Gamma-Ray Bursts.Comment: 5 pages, 3 figures, submitted to ApJ Letter
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