Dr. Gonthier, Dr. Harding
Supported by the Michigan Space Grant Consortium and Research Corporation
A pulsar is a rapidly rotating neutron star that emits electromagnetic radiation from its poles in various forms, such as gamma rays, x-rays, visible light, infrared, or radio waves. The typical neutron star has a mass of 1.4 solar masses, a radius on the order of 10 km, a density of about 6.65x1014 g/cm3, and an average gravity of 1.86x1014 cm/s2. Its period can be between 1 ms and 5 s, and is very accurate: accuracy challenging even the best atomic clocks.
As the pulsar rotates, the velocities of the particles traveling along the co-rotating magnetic field lines are limited to the speed of light (c = v = r*w. Therefore, there is a cutoff radius created where particles reach this speed, and thus forms the "light cylinder." In the polar cap model, the emission from the pulsar comes from its polar caps, which are regions near the magnetic poles where magnetic field lines do not reconnect to the pulsar. As the pulsar spins on its rotational axis, these "light cones" sweep across the Earth in the same manner as does light from a lighthouse.
While there have been more than 700 radio pulsars detected, only 8 gamma ray pulsars are known. This is mainly because while radio waves pass easily through the Earth's atmosphere, many gamma rays are absorbed, and thus are harder to detect with ground-based equipment. Whereas radio pulsars are detected with ground-based equipment with low flux thresholds, gamma-ray pulsars are detected from Earth's orbit with satellites having higher flux thresholds. The comparison between radio and gamma-ray pulsars is important because all but one of the gamma-ray pulsars are also detectable as radio pulsars. If the period of the gamma-ray pulsar is known from its radio emission, then it is much easier to search for the period in the gamma ray region.
Research this summer was conducted at NASA/Goddard Space Flight Center in Maryland. It consisted of continuing work on a developing code to predict the observability of gamma-ray pulsars in our galaxy. This code creates pulsars using a Monte Carlo technique and basing initial positions on probabilities given in Sturner and Dermer, 1996 [ApJ 461:872]. Various properties of the pulsars, such as period, period derivative and magnetic field, are calculated and these pulsars are evolved to the present time via a Runge-Kutta calculation with gravitational potentials. Gamma-ray and radio selection effects are then used to predict the observability of these simulated pulsars. Most of my work this summer has involved comparing calculated data with observed data and working on the Runge-Kutta section of code.
Results from this model will be compared with gamma-ray pulsars detected by the Compton Gamma-Ray Observatory (CGRO) and will also be used to predict the observability of gamma-ray pulsars detected by the Gamma-ray Large Area Telescope (GLAST) scheduled for launch in 2004.