78 research outputs found
Simulator method and apparatus for practicing the mating of an observer-controlled object with a target
A servo controlled target replica, and a surface bearing a computer generated line drawing of an object are individually viewed by separate television cameras allowing a two-dimensional composite of the target replica and the object to be displayed on a monitor simulating what an observer would see through a window in a spacecraft. The target replica is coded along one self coordinate axis in such a way that the distance of an elemental area on the target along the axis is capable of being remotely readout by a television camera. A third television camera responsive to the code reads out this information by which the Z-coordinate, relative to the observer, can be calculated, on-line with the scan, for the contents of each picture element of the scene televised by the target camera
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
A stability property of a force-free surface bounding a vacuum gap
A force-free surface (FFS) is a sharp boundary separating a void
from a region occupied by a charge-separated force-free plasma. It is proven
here under very general assumptions that there is on a simple
relation between the charge density on the plasma side and the derivative
of \delta=\E\cdot\B along \B on the vacuum side (with \E denoting the
electric field and \B the magnetic field). Combined with the condition
on , this relation implies that a FFS has a general
stability property, already conjectured by Michel (1979, ApJ 227, 579): turns out to attract charges placed on the vacuum side if they are of the
same sign as . In the particular case of a FFS existing in the
axisymmetric stationary magnetosphere of a "pulsar", the relation is given a
most convenient form by using magnetic coordinates, and is shown to imply an
interesting property of a gap. Also, a simple proof is given of the
impossibility of a vacuum gap forming in a field \B which is either uniform
or radial (monopolar)
Electrodynamic Structure of an Outer Gap Accelerator: Location of the Gap and the Gamma-ray Emission from the Crab Pulsar
We investigate a stationary pair production cascade in the outer
magnetosphere of a spinning neutron star. The charge depletion due to global
flows of charged particles, causes a large electric field along the magnetic
field lines. Migratory electrons and/or positrons are accelerated by this field
to radiate curvature gamma-rays, some of which collide with the X-rays to
materialize as pairs in the gap. The replenished charges partially screen the
electric field, which is self-consistently solved together with the
distribution functions of particles and gamma-rays. If no current is injected
at neither of the boundaries of the accelerator, the gap is located around the
conventional null surface, where the local Goldreich-Julian charge density
vanishes. However, we first find that the gap position shifts outwards (or
inwards) when particles are injected at the inner (or outer) boundary. Applying
the theory to the Crab pulsar, we demonstrate that the pulsed TeV flux does not
exceed the observational upper limit for moderate infrared photon density and
that the gap should be located near to or outside of the conventional null
surface so that the observed spectrum of pulsed GeV fluxes may be emitted via a
curvature process. Some implications of the existence of a solution for a super
Goldreich-Julian current are discussed.Comment: 17 pages, 12 figures, submitted to Ap
Large-Amplitude, Pair-Creating Oscillations in Pulsar and Black Hole Magnetospheres
A time-dependent model for pair creation in a pulsar magnetosphere is
developed. It is argued that the parallel electric field that develops in a
charge-starved region (a gap) of a pulsar magnetosphere oscillates with large
amplitude. Electrons and positrons are accelerated periodically and the
amplitude of the oscillations is assumed large enough to cause creation of
upgoing and downgoing pairs at different phases of the oscillation. With a
charge-starved initial condition, we find that the oscillations result in
bursts of pair creation in which the pair density rises exponentially with
time. The pair density saturates at , where is the parallel electric field in the
charge-starved initial state, and is the Lorentz factor for
effec tive pair creation. The frequency of oscillations following the pair
creation burst is given roughly by . A positive feedback keeps the system stable, such that the average pair
creation rate balances the loss rate due to pairs escaping the magnetosphere.Comment: 21 pages, 6 figures, ApJ submitte
The diocotron instability in a pulsar cylindrical electrosphere
The physics of the pulsar inner magnetosphere remains poorly constrained by
observations. Although about 2000 pulsars have been discovered to date, little
is known about their emission mechanism. Large vacuum gaps probably exist and a
non-neutral plasma made of electrons in some regions and of positrons in some
other regions fills space to form an electrosphere. The purpose of this work is
to study the stability properties of the differentially rotating equatorial
disk in the pulsar's electrosphere for which the magnetic field is assumed to
be dipolar. In contrast to previous studies, the magnetic field is not
restricted to be uniform. A pseudo-spectral Galerkin method using Tchebyshev
polynomials expansion is developed to compute the spectrum of the diocotron
instability in a non-neutral plasma column confined between two cylindrically
conducting walls. Moreover, the inner wall carries a given charge per unit
length in order to account for the presence of a charged neutron star at the
centre of the electrosphere. We show several eigenfunctions and eigenspectra
obtained for different initial density profiles and electromagnetic field
configurations useful for laboratory plasmas. The algorithm is very efficient
in computing the fastest growing modes. Applications to a cylindrical
electrosphere are also shown for several differential rotation profiles. It is
found that the growth rates of the diocotron instability are of the same order
of magnitude as the rotation rate.Comment: Accepted by A&
Investigations of the magnetospheric plasma distribution in the vicinity of a pulsar - I Basic formulation
The magnetospheric plasma distribution in the vicinity of a pulsar at various
inclination angles is investigated using a new relativistic, parallel 3D
Particle-In-Cell (PIC) code DYMPHNA3D. DYMPHNA3D uses a superposition of the
electromagnetic fields associated with a rotating magnetised conducting sphere
in a vacuum (the pulsar fields) and the electromagnetic fields due to the
presence of the magnetospheric plasma surrounding the pulsar (the plasma
fields), as the total fields. The plasma is moved self-consistently through the
magnetosphere using the PIC methodology. Our initial simulation results are
presented here. These show similar solutions to those obtained from previous
numerical simulations, which show the fundamental plasma distribution in the
vicinity of an aligned rotating neutron star to consist of two polar domes and
an equatorial torus of trapped non-neutral plasma of opposite sign. The aligned
case being the case in which the inclination angle between the magnetic dipole
moment and the rotation axis of the star is zero. Furthermore, our code allows
for off-axis simulations and we have found that this plasma distribution
collapses into a Quad-Lobe charge-separated non-neutral magnetospheric plasma
distribution in the case of an orthogonal rotator, i.e., the case in which the
magnetic dipole moment is oriented at right angles to the rotation axis of the
neutron star, with the plasma remaining trapped close to the stellar surface by
the force-free surfaces. We find that if initialised with a Goldreich-Julian
type distribution, the system is seen to collapse rapidly into these stable
Dome-Torus structures.Comment: 19 pages, 4 figures, accepted by Ap
The magnetron instability in a pulsar's cylindrical electrosphere
(abridged) The physics of the pulsar magnetosphere remains poorly constrained
by observations. Little is known about their emission mechanism. Large vacuum
gaps probably exist, and a non-neutral plasma partially fills the neutron star
surroundings to form an electrosphere. We showed that the differentially
rotating equatorial disk in the pulsar's electrosphere is diocotron unstable
and that it tends to stabilise when relativistic effects are included. However,
when approaching the light cylinder, particle inertia becomes significant and
the electric drift approximation is violated. In this paper, we study the most
general instability, i.e. by including particle inertia effects, as well as
relativistic motions. This general non-neutral plasma instability is called the
magnetron instability. We linearise the coupled relativistic cold-fluid and
Maxwell equations. The non-linear eigenvalue problem for the perturbed
azimuthal electric field component is solved numerically. The spectrum of the
magnetron instability in a non-neutral plasma column confined between two
cylindrically conducting walls is computed for several cylindrical
configurations. For a pulsar electrosphere, no outer wall exists. In this case,
we allow for electromagnetic wave emission propagating to infinity. When the
self-field induced by the plasma becomes significant, it can first increase the
growth rate of the magnetron instability. However, equilibrium solutions are
only possible when the self-electric field, measured by the parameter and tending to disrupt the plasma configuration, is bounded to an upper
limit, . For close to but smaller than this value
, the instability becomes weaker or can be suppressed as was the
case in the diocotron regime.Comment: Accepted by A&
The Axisymmetric Pulsar Magnetosphere
We present, for the first time, the structure of the axisymmetric force-free
magnetosphere of an aligned rotating magnetic dipole, in the case in which
there exists a sufficiently large charge density (whose origin we do not
question) to satisfy the ideal MHD condition, , everywhere.
The unique distribution of electric current along the open magnetic field lines
which is required for the solution to be continuous and smooth is obtained
numerically. With the geometry of the field lines thus determined we compute
the dynamics of the associated MHD wind. The main result is that the
relativistic outflow contained in the magnetosphere is not accelerated to the
extremely relativistic energies required for the flow to generate gamma rays.
We expect that our solution will be useful as the starting point for detailed
studies of pulsar magnetospheres under more general conditions, namely when
either the force-free and/or the ideal MHD condition are not
valid in the entire magnetosphere. Based on our solution, we consider that the
most likely positions of such an occurrence are the polar cap, the crossings of
the zero space charge surface by open field lines, and the return current
boundary, but not the light cylinder.Comment: 15 pages AAS Latex, 5 postscript figure
A Particle Simulation for the Axisymmetric Pulsar Magnetosphere: II. the case of dipole field
The main issue of the pulsar magnetosphere is how the rotation power is
converted into both particle beams which causes pulsed emissions, and a highly
relativistic wind of electron-positron plasmas which forms surrounding nebulae
shining in X-rays and TeV gamma-rays. As a sequel of the first paper (Wada &
Shibata 2007), we carried out a three dimensional particle simulation for the
axisymmetric global magnetosphere. We present the results of additional
calculations, which are higher resolution model and higher pair creation rate
cases, and a detailed analysis for the solution. We confined to demonstrate the
cases of low pair creation rate, i.e., the magnetic field is fixed dipole. The
radiation drag of the plasma is taken in a form with the curvature radius along
the dipole magnetic field. The electrostatic interactions are calculated by a
programmable special purpose computer, GRAPE-DR (Makino et al 2007). Once pair
creation is onset in the outer gaps, the both signed particles begin to drift
across the closed magnetic field due to radiation drag, and they create
outflow. Eventually, the steady magnetosphere has outer gaps, both signed
outflow of plasma and a region in which the electric field is dominant
extending from the equator. In the steady state, the magnetic field made by
magnetospheric current is comparable to the dipole magnetic field outside of
several light radii from the star. In much more pair creation rate model, the
effect of modification of the magnetic field will bring about modification of
the outflow of the plasma, requiring further study with higher pair creation
rate model in a subsequent paper.Comment: 18 pages, 12 figures, accepted for publication in MNRA
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