Modeling Comptonized Soft Gamma-Ray Flare Emission in Magnetars

William Marsh Rice University, Houston TX

Investigators

Abstract

Neutron stars that are magnetars are some of the most interesting compact objects in the Universe. There are perhaps as many as 30 presently identified with many of them exhibiting powerful outbursts of high energy X-rays. Magnetars present a unique forum for testing fundamental physics that is not presently accessible in terrestrial laboratories. Having great faith in the theory of quantum electrodynamics (QED), astrophysicists routinely model neutron star environments by using untested theoretical predictions for high magnetic field domains. Because magnetars provide the laboratory for performing these tests, understanding their character is central to using them as a proxy QED physics laboratory. This project makes significant steps in this direction by developing computer simulations to study the region above the surface of magnetars. This research project offers a significant element of graduate and undergraduate training with the involvement of students in research and teaching. One or two PhD students will be engaged as an integral part of the work with the results leading to doctoral theses. These students will be involved in undergraduate pedagogy by teaching at least one lecture per year on neutron stars in appropriate Rice courses. One or more undergraduates will be enlisted to work on smaller, self-contained portions of the research for their senior theses at Rice. These students are expected to present their research results to peers at Rice in research group seminars. Continuing an established relationship with the Houston Astronomical Society, the PI will also present aspects of this work and highlights of the exciting field of neutron stars to the general public. In more technical terms, this project develops a Monte Carlo simulation to describe radiative transfer in magnetar magnetospheres, mostly in closed field zones. The code will compute the interaction between X-rays and hot, relativistic electrons in Compton scattering events, tracking the energy exchange between them and accounting for changes in photon polarization. New physics formalism for the magnetic Compton cross section will be developed to account for the multiple cyclotron resonances that will dominate the opacity. Other processes will be modeled including photon splitting and magnetic pair creation, and their effect on attenuating soft gamma-rays above around 100 keV will be assessed. New physics developments of the magnetic two-photon pair creation cross section will be performed to facilitate the attenuation calculations. The creation of pairs will be incorporated and the generation of polarization signatures will be both a necessity for precision and also a highlight, increasing the diagnostic potential of the modeling. Specifically, capturing polarization information may provide the means to disentangle geometrical source information and the signatures of strong-field QED physics. Key physics analysis developments for positron-electron pair creation and Compton scattering will be a feature of the work and will provide new tools for astrophysicists to employ in other problems. As a broader scientific element, the polarization considerations of this program will aid in defining agendas and motivations for polarimetric telescopes in the X-ray and soft gamma-ray bands. Three outstanding questions pertaining to these sources that are at the forefront of scientific interest at the moment will be impacted significantly by this project. These are (i) whether or not magnetars are inherently different from normal pulsars, or is their emission driven by similar physics operating in different portions of neutron star parameter space, (ii) what causes the maximum energies of flares to be generally calibrated to around 100--300 keV; and (iii) where exactly is the site of energy injection in the magnetosphere that seeds non-thermal radiation in magnetars, i.e., how proximate is this locale to the stellar surface and/or the magnetic equator?