Project Description: |
We explore theoretically the emission of gamma rays from the magnetospheres of neutron stars using Monte Carlo simulations. The emission of gamma rays is initiated from the acceleration of charges - electrons, and positrons, along magnetic field lines with a parallel electric field. Two different theories describe the location and geometry of the acceleration and subsequent emission process. The polar cap model locates the acceleration zone right above the stellar surface at the magnetic poles, while the outer gap model describes the acceleration far out in the magnetosphere near the light cylinder where the speed of the co-rotating magnetic field lines approaches the speed of light. The geometries of the beams predicted by these two models are quite distinct. However, since there are only eight, possibly ten, known gamma-ray pulsars, the issue is not quite settled.
The gamma-ray telescopes, AGILE, launched in the spring of 2007, and Fermi Gamma-ray Space Telescope (FGST), launched in June of 2008 will discover many new gamma-ray pulsars. In this research project, we also study the radio emission from neutron stars. There are nearly two thousand radio pulsars known. While radio pulsars have been known since the mid-sixties, the mechanism of emission has not been understood. Therefore, our understanding of radio emission is purely from an empirical or phenomenological approach. Radio astronomers generally agree that the radio emission originates along magnetic field lines above the stellar surface. We model the radio emission with core beam center along the magnetic pole near the surface, maybe at similar altitudes above the surface where the gamma-ray emission occurs. In addition, typically the radio pulse profiles indicate the presence of one or two conal beams being emitted from a ring-shape region concentric with the magnetic axis at higher altitudes than the region of the core emission. Using the characteristics of radio pulsars with profiles containing five and three peaks, we attempt to improve the understanding of the characteristics of the core and cone beams and how the intensity of these beams depends on observed quantities like the pulsar period and period derivative. We have developed a computer code that simulates the characteristics of both radio and gamma-ray emission predicting the number of radio and gamma-ray pulsars observed by various radio surveys and gamma-ray instruments like EGRET, AGILE and FGST. Studies of the correlations of the radio and gamma-ray pulse profiles will provide a framework to differentiate between the competing pulsar models.
A second area of investigation in this project involves an aspect of the microphysics in the neutron star magnetosphere, where soft thermal X-rays from the stellar surface undergo inverse Compton scattering from relativistic electrons accelerated along magnetic field lines above the polar cap. We seek to develop a mathematical framework that is useful to astrophysicists to describe inverse Compton scattering in very strong magnetic fields. We will develop analytic expressions for the cross section in the highly supercritical fields associated with soft gamma-ray repeaters and anomalous X-ray pulsars, a class of pulsars known as magnetars. These expressions will be useful to magnetar modelers who are currently using Compton scattering cross sections that do not consider the effects of the strong magnetic field environment.
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