A Particle Simulation for the Pulsar Magnetosphere: Relationship of Polar Cap, Slot Gap, and Outer Gap

To explain the pulsed emission of the rotation powered pulsars from radio to gamma-ray, the polar cap models, the slot gap models, and the outer gap models are proposed. The recent observations suggest that these models are likely to co-exist in the same magnetosphere. If so, their mutual relation is known to be troublesome (Harding 2009) due to the boundary conditions and the direction of the current which are properly assumed in each acceleration models. We performed a particle simulation for the global magnetospheric structure. Based on the simulation, we present a new picture of the global structure of the pulsar magnetosphere. It is found that a new dead zone is formed along the current neutral line which separates the oppositely directed current. We shall call this the current- neutral zone. We suggest that the polar cap accelerators and the slot gaps locate above the current-neutral zone, and the outer gap exist between the current neutral zone and the traditional dead zone. We also give an estimate of the super-rotation region.Comment: 10 pages, 5 figures, Accepted for publication in PAS

Polarization of the Crab Nebula with disordered magnetic components

In this paper, we present an expanding disc model to derive polarization properties of the Crab nebula. The distribution function of the plasma and the energy density of the magnetic field are prescribed as function of the distance from the pulsar by using the model by Kennel and Coroniti (1984) with $\sigma = 0.003$, where $\sigma$ is the ratio of Poynting flux to the kinetic energy flux in the bulk motion just before the termination shock. Unlike previous models, we introduce disordered magnetic field, which is parameterized by the fractional energy density of the disordered component. Flow dynamics is not solved. The mean field is toroidal. Averaged polarization degree over the disc is obtained as a function of inclination angle and fractional energy density of the disordered magnetic field. It is found for the Crab that the disordered component has about 60 percent of the magnetic field energy. This value is also supported by the facts that the disc appears not `lip-shape' but as `rings' in the intensity map as was observed, and that the highest polarization degree of $\sim 40$ percent is reproduced for rings, being consistent with the observation. We suggest that because the disordered field contributes rather pressure than tension, the pinch force may be over-estimated in previous relativistic magnetohydrodynamic simulations. Disruption of the toroidal magnetic field with alternating direction, which is proposed by Lyubarski (2003), may actually takes place. The relativistic flow speed, which is indicated by the front-back contrast, can be detected in asymmetry in distributions of the position angle and depolarization.Comment: 7 pages, 8 figures, accepted for publication in MNRA

On the Global Structure of Stellar Magnetospheres with Stellar Winds

The solutions for the magnetic field and stellar wind in axially symmetric stellar magnetospheres are obtained through perturbational method in three artificial extreme situations, i.e., the limits of weak magnetic field, strong magnetic field and weak electromagnetic coupling. The set of basic equations are derived from MHD equations in the two-fluid approximation of a plasma by assuming quasi-neutrality and small mass ratio of electrons to protons. It is emphasized in this treatment that, since the inertial term in the generalized Ohm\u27s law has generally a non-zero rotation, the \u27violation of flux-freezing\u27 arises even in a perfectly conducting plasma. This fact makes it possible for a stellar wind to blow across the magnetic lines of force. The global structure of a stellar magnetosphere is inferred from the results obtained in the three extreme situations. It is suggested that the magnetosphere have generally the closed magnetic lines of force and the stellar wind blows across them forming a current sheet which may result in a very elongated shape of the lines of force at about the equatorial plane

Formation of the Ring-like Structure in the SN 1987A Nebula due to the Magnetic Pressure of the Toroidal Field

Several weeks after the explosion of supernova (SN) 1987A, the UV flash of the SN illuminated a ring-like structure in the circumstellar material at about 0.65 ly from the SN. The interaction between the stellar winds from the SN progenitor is considered to be the candidate for the formation of the circumstellar structure. In the case that the stellar winds are spherically symmetric, the interaction should result in a shell-like structure. However, in this paper we show that the magnetic field in the winds causes an anisotropy which leads to the formation of a ring-like structure. When the fast wind of the blue supergiant phase of the progenitor sweeps up the surrounding slow wind of the red-supergiant phase, the magnetic field as well as the wind material are piled up in the interaction region. Since the magnetic energy increases in proportion to the square of the amplitude, the magnetic field exhibits its effect prominently at the interaction region; due to the magnetic pressure force the material at lower latitudes is compressed into a ring-like structure. It is suggested that this magnetic process can also explain the newly observed pair of rings of the SN 1987A nebula.Comment: 18 pages LaTeX, 3 PostScript figure

On the X-ray Image of The Crab Nebula: Comparison with Chandra Observations

An axisymmetric model for the Crab Nebula is constructed to examine the flow dynamics in the nebula. The model is based on that of Kennel and Coroniti (1984), although we assume that the kinetic-energy-dominant wind is confined in an equatorial region. The evolution of the distribution function of the electron-positron plasma flowing out in the nebula is calculated. Given viewing angles, we reproduce an image of the nebula and compare it with Chandra observation. The reproduced image is not a ring-like but rather 'lip-shaped'. It is found that the assumption of toroidal field does not reproduce the Chandra image. We must assume that there is disordered magnetic field with an amplitude as large as the mean toroidal field. In addition, the brightness contrast between the front and back sides of the ring cannot be reproduced if we assume that the magnetization parameter $\sigma$ is as small as $\sim 10^{-3}$. The brightness profile along the semi-major axis of the torus is also examined. The non-dissipative, ideal-MHD approximation in the nebula appears to break down. We speculate that if the magnetic energy is released by some process that produce turbulent field in the nebula flow and causes heating and acceleration, e.g. by magnetic reconnection, then the present difficulties may be resolved (i.e. we can reproduce a ring image, and a higher brightness contrast). Thus, the magnetization parameter $\sigma$ can be larger than previously expected.Comment: 8 pages, 4 figures. accepted for publication in MNRA