Full simulation of the open field lines above pulsar's polar cap - Part I

We have programmed a full simulation of the open field lines above the polar cap of a magnetized pulsar, using time dependent Particle In Cell (PIC) numerical code (Birdsall and Langdon 1991). We consider the case of free ejection of electrons from the star surface, a Space Charge Limited Flow (SCLF) model. We report here the first results of the simulation. Electrons are accelerating along the open field lines to the flat space-time SCLF maximum Lorentz factor prediction, with oscillation pattern. Than we add the General Relativistic Frame Dragging correction that provide the particles the high Lorentz factor (10^6 - 10^7) needed to initiate pair production. The electrons accelerate according to the analytic expressions given in Muslimov and Tsygan 1992, and Muslimov and Harding 1997, with oscillation pattern. Electron-positron pair production is now being programmed, using the cross sections appears in the literature, and Monte-Carlo code. After completing this stage, we will automatically get: a) the positron return current (thus we could calculate the polar cap heating and the X-ray emission). b). The photons and electrons observed spectrum (photons and electrons that escape the magnetosphere after the cascade). c). The pulsar death line (pulsars with not enough pair production). d). The PFF height (pair formation front location). Those results will be report in a different paper.Comment: 7 pages, 6 figure

Pulsar Polar Cap Heating and Surface Thermal X-Ray Emission I. Curvature Radiation Pair Fronts

We investigate the effect of pulsar polar cap (PC) heating produced by positrons returning from the upper pair formation front. Our calculations are based on a self-consistent treatment of the pair dynamics and the effect of electric field screening by the returning positrons. We calculate the resultant X-ray luminosities, and discuss the dependence of the PC heating efficiencies on pulsar parameters, such as characteristic spin-down age, spin period, and surface magnetic field strength. In this study we concentrate on the regime where the pairs are produced in a magnetic field by curvature photons emitted by accelerating electrons. Our theoretical results are not in conflict with the available observational X-ray data and suggest that the effect of PC heating should significantly contribute to the thermal X-ray fluxes from middle-aged and old pulsars. The implications for current and future X-ray observations of pulsars are briefly outlined.Comment: 28 pages, 7 figures, accepted for publication in Ap

General Relativistic Effect of Gravitomagnetic Charge on Pulsar Magnetosphere and Particle Acceleration in a Polar Cap

We study magnetospheric structure surrounding rotating magnetized neutron star with nonvanishing NUT (Newman-Tamburino-Unti) parameter. For the simplicity of calculations Goldreich-Julian charge density is analyzed for the aligned neutron star with zero inclination between magnetic field, gravitomagnetic field and rotation axis. From the system of Maxwell equations in spacetime of slowly rotating NUT star, second-order differential equation for electrostatic potential is derived. Analytical solution of this equation indicates the general relativistic modification of an accelerating electric field and charge density along the open field lines by the gravitomagnetic charge. The implication of this effect to the magnetospheric energy loss problem is underlined. In the second part of the paper we derive the equations of motion of test particles in magnetosphere of slowly rotating NUT star. Then we analyze particle motion in the polar cap and show that NUT parameter can significantly change conditions for particle acceleration.Comment: 21 pages, 6 figures, accepted for publication in Ap

High-Altitude Particle Acceleration and Radiation in Pulsar Slot Gaps

We explore the pulsar slot gap electrodynamics up to very high altitudes, where for most relatively rapidly rotating pulsars both the standard small-angle approximation and the assumption that the magnetic field lines are ideal stream lines break down. We address the importance of the electrodynamic conditions at the slot gap boundaries and the occurrence of a steady-state drift of charged particles across the slot gap field lines at very high altitudes. These boundary conditions and the deviation of particle trajectories from stream lines determine the asymptotic behavior of the scalar potential at all radii from the polar cap to near the light cylinder. As a result, we demonstrate that the steady-state accelerating electric field must approach a small and constant value at high altitude above the polar cap. This parallel electric field is capable of maintaining electrons moving with high Lorentz factors (a few times 10^7) and emitting curvature gamma-ray photons up to nearly the light cylinder. By numerical simulations, we show that primary electrons accelerating from the polar cap surface to high altitude in the slot gap along the outer edge of the open field region will form caustic emission patterns on the trailing dipole field lines. Acceleration and emission in such an extended slot gap may form the physical basis of a model that can successfully reproduce some pulsar high-energy light curves.Comment: 26 pages, 2 figures, to appear in the Astrophysical Journal, May 10, 200

The effect of photo-electric absorption on space-charge limited flow in pulsars

Photo-electric absorption of blackbody photons is an important process which limits the acceleration of ions under the space-charge limited flow boundary condition at the polar caps of pulsars with positive corotational charge density. Photo-electric cross-sections in high magnetic fields have been found for the geometrical conditions of the problem, and ion transition rates calculated as functions of the surface temperatures on both the polar cap and the general neutron-star surface. The general surface temperature is the more important and, unless it is below 10^5 K, limits the acceleration electric field in the open magnetosphere to values far below those needed either for electron-positron pair creation or slot-gap X-ray sources. But such ion beams are unstable against growth of a quasi-longitudinal Langmuir mode at rates that can be observationally significant as a source of coherent radio emission.Comment: 7 pages; to be published in Monthly Notices of the Royal Astronomical Societ

Pair Multiplicities and Pulsar Death

Through a simple model of particle acceleration and pair creation above the polar caps of rotation-powered pulsars, we calculate the height of the pair-formation front (PFF) and the dominant photon emission mechanism for the pulsars in the Princeton catalog. We find that for most low- and moderate-field pulsars, the height of the pair formation front and the final Lorentz factor of the primary beam is set by nonresonant inverse Compton scattering (NRICS), in the Klein-Nishina limit. NRICS is capable of creating pairs over a wide range of pulsar parameters without invoking a magnetic field more complicated than a centered dipole, although we still require a reduced radius of curvature for most millisecond pulsars. For short-period pulsars, the dominant process is curvature radiation, while for extremely high-field pulsars, it is resonant inverse Compton scattering (RICS). The dividing point between NRICS dominance and curvature dominance is very temperature-dependent; large numbers of pulsars dominated by NRICS at a stellar temperature of $10^6$ K are dominated by curvature at $10^5$ K. We apply these results to pulsar death-line calculations and to the issue of particle injection into the Crab Nebula.Comment: 14 pages, 7 figures, to appear in Ap