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