Collaborative Research: RUI: Theoretical Studies of the Atmospheres of Highly Magnetized Neutron Stars

Hope College, Holland MI

Investigators

Abstract

A magnetar is a neutron star, or extremely dense star, which possesses an extraordinarily large magnetic field. Magnetars, of which around 30 have been discovered, are some of the most intriguing compact objects in the Universe. A research program, which is a collaboration between William Marsh Rice University, Hope College and NASA's Goddard Space Flight Center, will lead to the development of state of the art theoretical models for the atmospheric emission of magnetars. The prime objective is to deliver a suite of observable signal predictions to enhance interpretation of data from X-ray telescopes, leading to a better understanding of neutron stars, magnetism and fundamental quantum physics. Computer simulations will be developed for the propagation of X-rays through magnetar atmospheres with arbitrary magnetic field orientations, including all locations on the neutron star surface, from magnetic pole to equator. The research includes significant graduate and undergraduate training. A PhD student at Rice University will be integral to the work, with the results leading to a doctoral thesis. Several undergraduates will work on self-contained portions of the research and present their discoveries to senior scientists and peers. The investigators will elucidate aspects of this work, and highlights of the exciting field of neutron stars, to the general public. Magnetars present a unique forum for testing fundamental physics that is not presently accessible in terrestrial laboratories. Better understanding of magnetars is central to using them as a proxy quantum electrodynamics (QED) physics laboratory. The researchers will help enable this by developing a comprehensive theoretical study of the emission from magnetar atmospheres, computing expectations for the polarization- dependent and angle-dependent emissivities at any position on the surface. The generation of polarization signatures will allow discrimination between the geometrical source information and the signatures of strong-field QED physics, like birefringent vacuum polarization. The work will feature upgrades of photospheric cyclotron absorption line physics and photon propagation, providing new tools for astrophysicists to employ in other neutron star problems. The results of this program will enhance potential science yields from polarimetric telescopes in the X-ray band. Four outstanding questions pertaining to magnetars will be significantly impacted by the research. These are (i) are magnetars inherently different from normal pulsars?, (ii) does the magnetic inclination angle of the rotator evolve with magnetar age, and can we measure it so as to more precisely calibrate the stellar field strength?, (iii) where, relative to the magnetic pole, is the site of heating in the surface that seeds quasi- thermal radiation in magnetars?, and (iv) can we use general relativity to determine the mass-to-radius ratio for magnetars, and thereby probe their equations of state? This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.