Fermi Gamma-ray Pulsar High-energy Characteristics Research

Pulsars are rapidly rotating neutron star with a strong magnetic field. Recently, more than a hundreds of pulsars (called as Fermi gamma-ray pulsars) have been detected to emit gamma-rays by the Fermi Gamma Ray Space Telescope. These observations provide useful information for us to study particle accelerations and radiations in pulsar’s magnetosphere. Based on these observed data, the high-energy properties of Fermi gamma-ray pulsars have been studied in this thesis, including three parts.Firstly I studied energy dependent light curves and phase-averaged spectra of high-energy gamma-rays from the Crab pulsar. The Crab pulsar is one of the brightest gamma-ray pulsars in the galaxy. Energy-dependent light curves and phase-resolved spectra of high-energy gamma-rays emission from the Crab pulsar have been detected recently by the Fermi Large Area Telescope (LAT). Within the framework of a two-pole, three-dimensional outer gap model, I calculated the energy-dependent light curves and phase-resolved spectra in the inertial observer’s frame. Calculating results show that (1) the observed gamma-ray properties from both Fermi-LAT and MAGIC can be reproduced well in this model;(2) the first peak of the light curves in the energy region less than～10GeV comes from the sum of emissions from both the north and south poles, and the second peak comes only from the emission from the south pole; however, the relative contribution of the two poles to the first peak changes with increasing gamma-ray energy, and the light curve in the energy region greater than～20GeV comes completely from the emission of the south pole; and (3) gamma-rays in the energy region greater than100MeV are produced through inverse Compton scattering from secondary pairs and the survival curvature photons, where the latter dominate over gamma-ray emission in the energy region greater than several GeV.Secondly, I investigated the emission patterns and light curves of gamma rays in pulsar magnetosphere with a current-induced magnetic field. It is generally believed that the high-energy pulsed emissions are produced in the pulsar magnetosphere, therefore how to approximate the pulsar magnetosphere is a very important sub-ject. A dipole magnetic field approximation is usually used to describe the pulsar magnetosphere. However, since radiating charges accelerated in the gaps of the pulsar magnetosphere will create some currents in the open zone, the current will induce a magnetic field, and then will result in a distorted magnetic field relative to the dipole field. Therefore, a more real magnetosphere should be approximated by the dipole field with the current-induced magnetic field perturbation. Based on the solution of a static dipole with the magnetic field induced by some currents (per-turbation field), I derived the solutions of a static as well as a retarded dipole field with the perturbation field in the Cartesian coordinates. Similar to the static dipole field, the retarded magnetic field can be expressed as the sum of the pure retarded dipolar magnetic field and the retarded perturbation field. I have used the solution of the retarded magnetic field to investigate the influence of the perturbation field on the emission patterns and light curves, and applied the perturbed solutions to calculate the gamma-ray light curves for the case of the Vela pulsar. I find that the perturbation field induced by the currents will change the emission patterns and then the light curves of gamma rays, especially for a larger perturbation field. Cal-culating results indicate that the perturbation field created by the outward-flowing (inward-flowing) electrons (positrons) can decrease the rotation effect on the mag-netosphere and makes emission pattern appear to be smoother relative to that of the pure retarded dipole, but the perturbation field created by the outward-flowing (inward-flowing) positrons (electrons) can make the emission pattern less smooth.Finally, I studied the phase-averaged spectra and luminosity of gamma-ray emissions from young isolated pulsars. For explaining averaged radiation properties of the young isolated pulsars observed by Fermi Space Telescope, I proposed a revised outer gap model. In the revised version of the outer gap, there are two possible cases for the outer gaps; the fractional size of the outer gap is estimated through the photon-photon pair process in the first case (Case Ⅰ) and is limited by the critical field lines in the second case (Case Ⅱ). The fractional size is described by that in Case Ⅰ if the fractional size at the null charge surface in Case I is smaller than that in Case Ⅱ, vice versa. Such an outer gap can extend from the inner boundary whose radial distance to the neutron star is less than that of the null charge surface to the light cylinder for a γ-ray pulsar with a given magnetic inclination. When the shape of the outer gap is determined, assuming that high-energy emission at an averaged radius of the field line in the center of the outer gap with a Gaussian distribution of the parallel electric field along the gap height represents typical emission, the phase-averaged γ-ray spectrum for a given pulsar can be estimated in the revised model with three model parameters. The model is applied to explain the phase-averaged spectra of Vela (Case Ⅰ) and Geminga (Case Ⅱ) pulsars. Also the model is used to fit the phase-averaged spectra of54young, isolated γ-ray pulsars, and then calculate γ-ray luminosities and compare them with the observed data by Fermi-LAT.