A Monte Carlo study of the statistical properties of gamma-ray pulsars in the gould belt
A MONTE CARLO STUDY OF THE STATISTICAL PROPERTIES OF GAMMA-RAY PULSARS IN THE GOULD BELT
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.
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