Luminosity Evolution of Gamma-ray Pulsars

We investigate the electrodynamic structure of a pulsar outer-magnetospheric particle accelerator and the resultant gamma-ray emission. By considering the condition for the accelerator to be self-sustained, we derive how the trans-magnetic-field thickness of the accelerator evolves with the pulsar age. It is found that the thickness is small but increases steadily if the neutron-star envelope is contaminated by sufficient light elements. For such a light element envelope, the gamma-ray luminosity of the accelerator is kept approximately constant as a function of age in the initial ten thousand years, forming the lower bound of the observed distribution of the gamma-ray luminosity of rotation-powered pulsars. If the envelope consists of only heavy elements, on the other hand, the thickness is greater but increases less rapidly than what a light element envelope has. For such a heavy element envelope, the gamma-ray luminosity decreases relatively rapidly, forming the upper bound of the observed distribution. The gamma-ray luminosity of a general pulsar resides between these two extreme cases, reflecting the envelope composition and the magnetic inclination angle with respect to the rotation axis. The cutoff energy of the primary curvature emission is regulated below several GeV even for young pulsars, because the gap thickness, and hence the acceleration electric field is suppressed by the polarization of the produced pairs.Comment: Accepted to ApJ; 10 pages, 6 figure

Luminosity Evolution of Rotation-powered Gamma-ray Pulsars

We investigate the electrodynamic structure of a pulsar outer-magnetospheric particle accelerator and the resultant gamma-ray emission. By considering the condition for the accelerator to be self-sustained, we derive how the trans-magnetic-field thickness of the accelerator evolves with the pulsar age. It is found that the thickness is small but increases steadily if the neutron-star envelope is contaminated by sufficient light elements. For such a light element envelope, the gamma-ray luminosity of the accelerator is kept approximately constant as a function of age in the initial ten thousand years, forming the lower bound of the observed distribution of the gamma-ray luminosity of rotation-powered pulsars. If the envelope consists of only heavy elements, on the other hand, the thickness is greater but increases less rapidly than what a light element envelope has. For such a heavy element envelope, the gamma-ray luminosity decreases relatively rapidly, forming the upper bound of the observed distribution. The gamma-ray luminosity of a general pulsar resides between these two extreme cases, reflecting the envelope composition and the magnetic inclination angle with respect to the rotation axis.Comment: 2012 Fermi Symposium proceedings -eConf C12102

High Energy Emission from Rotation-Powered Pulsars: Outer-gap vs. Slot-gap Models

We explore particle accelerator electrodynamics in the magnetosphere of a rapidly rotating neutron star (NS). We address the importance of a self-consistent treatment of pair production, solving the Poisson equation describing the acceleration electric field, the Boltzmann equations for produced electrons and positrons, and the radiative transfer equation simultaneously. It is demonstrated that the accelerator solution is obtained if we only specify the NS spin period, magnetic dipole moment, magnetic inclination angle with respect to the rotation axis, and the NS surface temperature, and that the solution corresponds to a quantitative extension of previous outer-gap models. We apply the scheme to the Crab pulsar and show that the predicted pulse profiles and phase-resolved spectrum are roughly consistent with observations. Applying the same scheme to the slot-gap model, we show that this alternative model predicts too small photon flux to reproduce observations, because the gap trans-field thickness is significantly restricted by its pair-free condition.Comment: 15 pages, 19 figures; submitted to the Open Astronomy Journa

Particle Accelerator in Pulsar Magnetospheres: Super Goldreich-Julian Current with Ion Emission from the Neutron Star Surface

We investigate the self-consistent electrodynamic structure of a particle accelerator in the Crab pulsar magnetosphere on the two-dimensional poloidal plane, solving the Poisson equation for the electrostatic potential together with the Boltzmann equations for electrons, positrons and gamma-rays. If the trans-field thickness of the gap is thin, the created current density becomes sub-Goldreich-Julian, giving the traditional outer-gap solution but with negligible gamma-ray luminosity. As the thickness increases, the created current increases to become super-Goldreich-Julian, giving a new gap solution with substantially screened acceleration electric field in the inner part. In this case, the gap extends towards the neutron star with a small-amplitude positive acceleration field, extracting ions from the stellar surface as a space-charge-limited flow. The acceleration field is highly unscreened in the outer magnetosphere, resulting in a gamma-ray spectral shape which is consistent with the observations.Comment: 36 pages, 14 figures, accepted to Astroph.

High-energy Emission from Pulsar Outer Magnetospheres: Two-dimensional Electrodynamics and Phase-averaged Spectra

We investigate particle accelerators in rotating neutron-star magnetospheres, by simultaneously solving the Poisson equation for the electrostatic potential together with the Boltzmann equations for electrons, positrons and photons on the poloidal plane. Applying the scheme to the three pulsars, Crab, Vela and PSR B1951+32, we demonstrate that the observed phase-averaged spectra are basically reproduced from infrared to very high energies. It is found that the Vela's spectrum in 10-50 GeV is sensitive to the three-dimensional magnetic field configuration near the light cylinder; thus, a careful argument is required to discriminate the inner-gap and outer-gap emissions using a gamma-ray telescope like GLAST. It is also found that PSR B1951+32 has a large inverse-Compton flux in TeV energies, which is to be detected by ground-based air Cerenkov telescopes as a pulsed emission.Comment: 8 pages, 5 figures; ApJ in press. The title is modified from the text to be distinguished from astro-ph/0307236 (a book chapter

Composition of active galactic nuclei jets: pair-plasma dominance in the 3C 345 and 3C 279 jets

We investigate whether the parsec-scale jets of quasars 3C 345 and 3C 279 are dominated by a normal (proton-electron) plasma or a pair (electron-positron) plasma. We first present a new method to compute the kinetic luminosity of a conical jet by using the core size observed at a single very long baseline interferometry frequency. The deduced kinetic luminosity gives electron densities of individual radio-emitting components as a function of the composition. We next constrain the electron density independently by using the theory of synchrotron self-absorption. Comparing the two densities, we can discriminate the composition. We then apply this procedure to the five components in the 3C 345 jet and find that they are pair-plasma dominated at 14 epochs out of the total 19 epochs at which the turnover frequencies are reported, provided that the bulk Lorentz factor is less than 15 throughout the jet. We also investigate the composition of the 3C 279 jet and demonstrate that its two components are likely pair-plasma dominated at all the four epochs, provided that their Doppler factors are less than 10, which are consistent with observations. The conclusions do not depend on the lower cutoff energy of radiating particles.Comment: 14 pages, 5 figures, 3 tables (tables 2 and 3 are in ps format

Does a strong particle accelerator arise very close to the light cylinder in a pulsar magnetosphere?

We examine if an efficient particle acceleration takes place by a magnetic-field-aligned electric field near the light cylinder in a rotating neutron star magnetosphere. Constructing the electric current density with the actual motion of collision-less plasmas, we express the rotationally induced, Goldreich-Julian charge density as a function of position. It is demonstrated that the 'light cylinder gap', which emits very high energy photons via curvature process by virtue of a strong magnetic-field-aligned electric field very close to the light cylinder, will not arise in an actual pulsar magnetosphere.Comment: 3 pages, 1 figure; Monthly Notices of Royal Astronomical Society Letters in pres

Energetic Gamma Radiation from Rapidly Rotating Black Holes

Supermassive black holes are believed to be the central power house of active galactic nuclei. Applying the pulsar outer-magnetospheric particle accelerator theory to black-hole magnetospheres, we demonstrate that an electric field is exerted along the magnetic field lines near the event horizon of a rotating black hole. In this particle accelerator (or a gap), electrons and positrons are created by photon-photon collisions and accelerated in the opposite directions by this electric field, efficiently emitting gamma-rays via curvature and inverse-Compton processes. It is shown that a gap arises around the null charge surface formed by the frame-dragging effect, provided that there is no current injection across the gap boundaries. The gap is dissipating a part of the hole's rotational energy, and the resultant gamma-ray luminosity increases with decreasing plasma accretion from the surroundings. Considering an extremely rotating supermassive black hole, we show that such a gap reproduces the significant very-high-energy (VHE) gamma-ray flux observed from the radio galaxy IC 310, provided that the accretion rate becomes much less than the Eddington rate particularly during its flare phase. It is found that the curvature process dominates the inverse-Compton process in the magnetosphere of IC~310, and that the observed power-law-like spectrum in VHE gamma-rays can be explained to some extent by a superposition of the curvature emissions with varying curvature radius. It is predicted that the VHE spectrum extends into higher energies with increasing VHE photon flux.Comment: 21 pages, 20 figures. Accepted for publication in ApJ, December 14, 201

Three-dimensional non-vacuum pulsar outer-gap model: Localized acceleration electric field in the higher altitudes

We investigate the particle accelerator that arises in a rotating neutron-star magnetosphere. Solving the Poisson equation for the electro-static potential, the Boltzmann equations for relativistic electrons and positrons, and the radiative transfer equation simultaneously, we demonstrate that the electric field is substantially screened along the magnetic field lines by the pairs that are created and separated within the accelerator. As a result, the magnetic-field-aligned electric field is localized in the higher altitudes near the light cylinder and efficiently accelerates the positrons created in the lower altitudes outwards but not the electrons inwards. The resulting photon flux becomes predominantly outwards, leading to typical double-peak light curves, which are commonly observed from many high-energy pulsars.Comment: 5 pages, 5 figures; Accepted for publication in ApJ Letter

Gamma-ray emission from Outer-Gap of pulsar magnetosphere

We develop a model for gamma-ray emission from the outer magnetosphere of pulsars (the outer-gap model). The charge depletion causes a large electric field which accelerates electrons and positrons. We solve the electric field with radiation and pair creation processes self-consistently, and calculate curvature spectrum and Inverse-Compton (IC) spectrum. We apply this theory to PSR B0833-45 (Vela) and B1706-44 for which their surface magnetic fields, observed thermal X-rays are similar to each other. We find that each observed cut-off energies of the gamma-rays are well explained. By inclusion of emission outside the gap, the spectrum is in better agreement with the observations than the spectrum arising only from the inside of the gap. The expected TeV fluxes are much smaller than that observed by CANGAROO group in the direction of B1706-44.Comment: 8pages, 3figures, to appear in "The Univese Viewed in Gamma-rays