470 research outputs found
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
Collimation of Highly Variable Magnetohydrodynamic Disturbances around a Rotating Black Hole
We have studied non-stationary and non-axisymmetric perturbations of a
magnetohydrodynamic accretion onto a rotating (Kerr) black hole. Assuming that
the magnetic field dominates the plasma accretion, we find that the accretion
suffers a large radial acceleration resulting from the Lorentz force, and
becomes highly variable compared with the electromagnetic field there. In fact,
we further find an interesting perturbed structure of the plasma velocity with
a large peak in some narrow region located slightly inside of the
fast-magnetosonic surface. This is due to the concentrated propagation of the
fluid disturbances in the form of fast-magnetosonic waves along the separatrix
surface. If the fast-magnetosonic speed is smaller in the polar regions than in
the equatorial regions, the critical surface has a prolate shape for radial
poloidal field lines. In this case, only the waves that propagate towards the
equator can escape from the super-fast-magnetosonic region and collimate
polewards as they propagate outwards in the sub-fast-magnetosonic regions. We
further discuss the capabilities of such collimated waves in accelerating
particles due to cyclotron resonance in an electron-positron plasma.Comment: 15 pages, 6 postscript figures, LaTe
The origins of Causality Violations in Force Free Simulations of Black Hole Magnetospheres
Recent simulations of force-free, degenerate (ffde) black hole magnetospheres
indicate that the fast mode radiated from (or near) the event horizon can
modify the global potential difference in the poloidal direction orthogonal to
the magnetic field, V, in a black hole magnetosphere. There is a fundamental
contradiction in a wave that alters V coming from near the horizon. The
background fields in ffde satisfy the ``ingoing wave condition'' near the
horizon (that arises from the requirement that all matter is ingoing at the
event horizon), yet outgoing waves are radiated from this region in the
simulation. Studying the properties of the waves in the simulations are useful
tools to this end. It is shown that regularity of the stress-energy tensor in a
freely falling frame requires that the outgoing (as viewed globally) waves near
the event horizon are redshifted away and are ineffectual at changing V. It is
also concluded that waves in massless MHD (ffde) are extremely inaccurate
depictions of waves in a tenuous MHD plasma, near the event horizon, as a
consequence black hole gravity. Any analysis based on ffde near the event
horizon is seriously flawed.Comment: 9 pages to appear in ApJ Letter
Electrodynamics of Outer-Gap Accelerator: Formation of Soft Power-law Spectrum Between 100 MeV and 3 GeV
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 gamma-rays via curvature and inverse-Compton processes. Some of such
gamma-rays 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 energy distribution of particles and
gamma-rays at each point along the field lines. By solving the set of Maxwell
and Boltzmann equations, we demonstrate that an external injection of charged
particles at nearly Goldreich-Julian rate does not quench the gap but shifts
its position and that the particle energy distribution cannot be described by a
power-law. The injected particles are accelerated in the gap and escape from it
with large Lorentz factors. We show that such escaping particles migrating
outside of the gap contribute significantly to the gamma-ray luminosity for
young pulsars and that the soft gamma-ray spectrum between 100 MeV and 3 GeV
observed for the Vela pulsar can be explained by this component. We also
discuss that the luminosity of the gamma-rays emitted by the escaping particles
is naturally proportional to the square root of the spin-down luminosity.Comment: Accepted to Astroph. J, 25 pages, 13 figure
Gamma-ray Emission from an Outer-Gap Accelerator: Constraints on Magnetospheric Current, Magnetic Inclination, and Distance for PSR B1055-52
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. By solving the Vlasov
equations describing this pair production cascade, we demonstrate the existence
of a stationary gap in the outer magnetosphere of PSR B1055-52 for a wide range
of current densities flowing in the accelerator: From sub to super
Goldreich-Julian values. However, we find that the expected GeV spectrum
becomes very soft if the current density exceeds the Goldreich-Julian value. We
also demonstrate that the GeV spectrum softens with decreasing magnetic
inclination and with increasing distance to this pulsar. We thus conclude that
a sub-Goldreich-Julian current, a large magnetic inclination, and a small
distance (500 pc, say) are plausible to account for EGRET observations.
Furthermore, it is found that the TeV flux due to inverse Compton scatterings
of infrared photons whose spectrum is inferred from the Rayleigh-Jeans side of
the soft blackbody spectrum is much less than the observational upper limit.Comment: 16 pages, 13 figures, ApJ in pres
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