Martin knecht's pictureMartin knecht

Hope College

Dr. Gonthier

Supported by NSF-REU


This summer I had the opportunity to work at Goddard Space Flight Center in Greenbelt, Maryland through a REU program offered by Hope College in Holland, Michigan. I worked with John Shoemaker, an Undergrad from Hope College, and Dr. Peter Gonthier, a professor at Hope College. Dr. Gonthier has been collaborating with Alice Harding and Matthew Baring, who are NASA scientists currently working on a theoretical model of gamma ray pulsars with the Laboratory for High Energy Astrophysics (LHEA) at Goddard.

This opportunity gave me a chance to learn about a field of physics I have never been exposed to. I also got to work in a new environment. All my previous experiences have been in an experimental work setting rather than the theoretical setting of this summer. It was quite a change!

My project for the summer was to modify a computer model that produces a gamma ray spectrum of a pulsar by simulating various mechanisms within its magnetosphere. These mechanisms include cyclo-synchrotron radiation, electron-positron pair production, photon splitting, and eventually inverse-compton scattering. The simulated spectrua can then be compared with observations of gamma ray pulsars from the Compton Gamma Ray Observatory.

Cyclo-synchrotron radiation is caused by electrons or positrons spiraling along the pulsars field lines. These particles can originate from either the surface of the pulsar, or from photon pair production. The quantum treatment of this radiation is cyclotron radiation where particles orbit at a quantized radii or energy levels. These energy levels are called Landau states. When a charged particle decays from a higher to lower Landau state it produces a photon. A single particle will continue it's decay until it reaches the ground state energy level. At larger Landau states the radiation can be computed by a high energy approximation called synchrotron radiation which is used to save computation time in the program. In the computer model, cyclotron radiation was used for Landau states below twenty and synchrotron radiation was used otherwise.

After these photons are produced, pair production or photon splitting can occur. Pair production is a QED effect which allows a photon to split into a electron-positron pair, if it exceeds a certain kinematic threshold and is in the presence of a magnetic field. The produced pair can then go through cyclo-synchrotron radiation. Photon splitting is an exotic QED effect which can only occur in magnetic fields close to the quantum critical field (Bc = 4.413 x 1013G). In this high magnetic field a single photon is able to split into two lower energy photons. This splitting is dependent on the photons polarization with respect to the magnetic field. A perpendicular polarized photon can split into two parallel polarized photons, while parallel photons can not split.

The previous computer model did not keep track of the photons polarization emitted by electrons and positrons, so the full effect of the photon splitting cascade could not be seen. I assisted in implementing the photon polarization into the code so a full cascade could be seen. I did a lot of optimizing, debugging and testing, also.

This summer was an excellent opportunity for me. I was able to learn a great deal about pulsars, and the models and mechanisms describing them. I also learned about astrophysics in general by attending many talks on a variety of subject matters. I was able to work in a totally new environment and talk with many experts in various fields of physics and astronomy who were more than happy to answer any questions I that I had. I learned a great deal about programming and computer modeling including different methods of numerical integration, interpolation, memory handling, file manipulation, and optimization. I am excited by this, since I applied for this particular REU opportunity for programming experience like this.