A Monte Carlo study of the statistical properties of gamma-ray pulsars in the gould belt

by Leung, Po

Abstract (Summary)
(Uncorrected OCR) Abstract of thesis entitled


submitted by Leung Po

for the Degree of Master of Philosophy at The University of Hong Kong

in August 2003

In this project, a study was made of the outer-gap size, I-ray luminosity and death lines of I-ray emission from pulsars with outer gaps. When the geometry of the dipole magnetic field is taken into account, the derived outer gap size is a function of period (P), magnetic field (B) ofthe neutron star and the average radial distance to the neutron star < r >, which depends on the magnetic inclination angle (0;). The fractional size of the outer gap, f(P, B, < r > (0;)), was used to describe the thickness of the outer gap and was determined by the electron-positron pair production. The outer gap will not exist if f(r, 0;) > 1. In estimating the fractional size of the outer gap, two possible X-ray fields

were considered: (i) X-rays produced by the neutron star cooling and polar cap heating, and (ii)X-rays produced by the bombardment of the relativistic particles from the outer gap on the stellar surface (the outer gap is assumed to be self-sustained). For all I-ray pulsars with self-sustained outer gaps simulated by using Monte Carlo methods, I-ray luminosity L, satisfies L, ex L~d; where the value of 6 depends on the sensitivity of the I-ray detector. For the Energetic Gamma Ray Experiment Telescope (EGRET), 6 is '" 0.38 whereas 6 is '" 0.46 for the Gamma Ray Large Area Space Telescope (GLAST). For I-ray pulsars with Lsd :::; L~dit, 6 is '" 1. L~dit = 1.5 X 1034 pl/3erg-1 is determined by f( < r >'" rL) = 1. These results are roughly consistent with the observed luminosity of I-ray pulsars. Further, the death lines of the pulsars with outer gaps in the two possible X-ray fields were given and a comparison made with the observed data.

In addition, I-ray emission properties of pulsars were investigated by using a new self-consistent outer-gap model. The outer gap can exist in pulsars with age up to '" 3 X 106 years, if their magnetic inclination angles are sufficiently large. The mature I-ray pulsars, whose ages are between 1 to 3 million years, are able to move up to high galactic latitude. Moreover, their I-ray luminosities are weaker and their spectra are much softer than those of young I-ray pulsars with ages < 106 years. A Monte Carlo technique was used to simulate the properties of I-ray pulsars in the Galaxy as well as in the Gould Belt. It was found that the I-ray pulsars detected by EGRET are predominantly mature pulsars with periods'" (0.1-0.8)s, for 1 b I> 5? A comparison was made of model spatial, energy-flux distributions and log N-log S for mature I-ray pulsars with those unidentified sources at medium latitude detected by EGRET. It was concluded that these unidentified sources at medium latitude belong to this class of mature

I-ray pulsars. Furthermore, the relation between I-ray luminosity L, and the spin-down power Lsd for this group of pulsars satisfies L, ex L~d instead of L, ex L~d for I-ray pulsars in the galactic plane, where (3 "-' 1 and 6 "-' 0.4 respecti vely.

Bibliographical Information:


School:The University of Hong Kong

School Location:China - Hong Kong SAR

Source Type:Master's Thesis

Keywords:gamma rays pulsars monte carlo method


Date of Publication:01/01/2003

© 2009 All Rights Reserved.