Hope College Physics Department
Research Experiences for Undergraduates
Summer 2013
Project Summary


Project Title:

Compton Scattering Cross Sections in Strong Magnetic Fields: Advances for Neutron Star Applications

Student Name: Jesse Ickes
Student's Home Institution: Hope College
Research Advisor: Dr. Peter Gonthier
Source of Support:

This work is made possible by the generous support of the National Science Foundation (Grant No. AST-1009731), the NASA Astrophysics Theory and Fundamental Program (NNX09AQ71G and 12-ATP12-0169) and the Michigan Space Grant Consortium as well as internal funds from the Hope College Physics Department.

Various telescopes including RXTE, INTEGRAL, Suzaku, and Fermi have detected steady non-thermal X-ray emission in the range of 10-200 keV from strongly magnetic neutron stars known as magnetars. Magnetic inverse Compton scattering is believed to be the leading mechanism for the production of this intense X-ray radiation. We have simple, correct analytic expressions for the special case of resonant Compton upscattering at large Lorentz factors implementing spin-dependent widths. In the ultra-relativistic regime the photon is incident parallel to the magnetic field line in the electron rest frame resulting in significant simplification of the cross section. The conclusion of this effort is that spin effects of the intermediate state are significant very near the resonance.  Through multiple scatterings, this efficient mechanism can quickly cool electrons to mildly relativistic energies. The focus of our current work is to develop compact analytic expressions for the mildly-relativistic spin-averaged and spin-dependent cases where the angle of incidence must be included. With mildly-relativistic energies, the incoming photon can acquire substantial angles relative to the magnetic field lines, with the intermediate state being excited to arbitrary Landau levels. We are developing the spin-averaged and spin-dependent cross sections with photon polarization modes to further explore the role of the polarization effects in the high fields of magnetars. These efforts will provide applications to various neutron star problems, including computation of Eddington luminosities and polarization mode-switching rates in transient magnetar fireballs.

Publications and Presentations: