Hope College Department of Physics and Engineering
Research Experiences for Undergraduates
Summer 2001
Project Summary

 

Project Title: Geometry of Radio Pulsars using Monte-Carlo Techniques
Student Name: Allison Bultemeier
Student’s home institution: Hope College
Research Advisor(s): Dr. Peter Gonthier
Source of Support: NSF-REU

The research that I was involved in this summer dealt with the continuation of Michelle Ouellette’s computer simulation that was started a few years ago. The program that she began attempts to model the properties and distribution of observed pulsars. A pulsar is a rapidly rotating neutron star that emits gamma rays, x-rays, light waves, and radio waves in the form of radiation from its poles. Using Monte Carlo techniques, pulsars are randomly generated by following sets of survey parameters given from many publications. In addition, many of the pulsar’s properties are calculated, such as its period, period derivative, and magnetic field, then the program rapidly evolves the pulsar to the present time.

I worked with a few different areas of the computer code, the first being simply trying to make the program run faster and more efficient. When the program first begins to run, it has to generate over a million events so that we can get a large enough number of events that are also radio pulsars. Running the entire program takes around twelve hours to complete, which is not practical to use every day. I made a part of the program that will create these events only once and store them in a file for later use. In doing this, the computer needs not create millions of events each time we need to run the program to get the properties of these pulsars, which essentially saves time each time the program is run.

The second part of the program that I worked with was adding the geometry associated with a neutron star into the program. In other years, a simple emission geometry had been used. We were hoping that by adding in the geometry, our simulated pulsars would more accurately represent the observed pulsars. In order to do this, we used the parameters and mathematical models given in the publication The Full Monte by Z. Arzoumanian.

A neutron star emits radiation at its magnetic poles in an effective cone shape, the same as a lighthouse emits light rays in pulses as the light rotates. As the star rotates, these cones of emission sweep past the earth causing the pulse affect. Emissions that are seen from the pulsar are in cone and core components. The cone components are the outside edge of the cone of radiation. There is also a core component, which is emission that comes from the center of this cone, and is also the most intense. The important angles when looking at the geometry are the angle associated with the direction of the viewer and the angle associated with the orientation of the magnetic field of the pulsar. My program randomly generated these angles and used them to eventually calculate the flux of each pulsar, an integrated luminosity determined in Arzoumanian’s publication.

In other years, it was not clear what was responsible for the radio luminosity, so it was obtained by dithering the normalized luminosity distribution. By adding in the geometry, which tells how the energy is distributed, and using Arzoumanian’s integrated luminosity that related the period and the period derivative, we hoped to find that this was responsible for the radio luminosity.

Including the geometry of radio pulsars did not radically change the resulting data. For the most part, variables such as the age, distance, flux, and period of the generated radio pulsars improved slightly. We were hoping that the geometry and new luminosity would help get rid of some older pulsars that were computer generated but were not observed, but that was not the case. As in earlier years, a field decay was implemented so our generated pulsars more closely represented the observed ones.

Looking ahead, this program will be used to predict how many radio and gamma-ray pulsars should be found by the telescope GLAST which is to be sent up in 2005. I added in the geometry of the radio pulsars; the same needs to be done with the gamma-ray pulsars.
 

Slide show of Allison Bultemeier's work