Collaborative Research: Pulsed X-ray Spectral and Polarization Signals from Magnetar Atmospheres

Magnetars are neutron stars with incredibly strong magnetic fields, the like of which are not possible to recreate in an Earth-bound laboratory. Currently only about 30 have been identified, and much about them remains unknown. A collaborative group of researchers at Rice University and Hope College will take significant strides in increasing the understanding of magnetars by developing a comprehensive simulation study of the emission from strongly-magnetized atmospheres straddling their surfaces. The project will provide training and support for the next generation of scientists, including a graduate student at Rice University and undergraduate students at Hope College, a primarily undergraduate institution. An on-line mini-course on compact object astrophysics for senior high school students will also be developed. This program will develop state-of-the-art models for the atmospheric emission of magnetars. An extant Monte Carlo simulation for polarized X-ray transport in fully ionized light element atmospheres will be upgraded to treat hydrostatic support by gas, radiation and magnetic pressure, and incorporate bremsstrahlung opacity and ion cyclotron absorption lines. It applies to arbitrary magnetic field orientations, addressing all locations on the neutron star surface, from the magnetic pole to the equator. Emissivities and polarization signals from hot active zones will be integrated, and as the light passes through the magnetosphere, modifications due to general relativity and the polarized birefringent vacuum will be tracked. The prime objective is to deliver a suite of signal predictions to enhance the interpretation of intensity and polarization data from X-ray telescopes. Tracking the polarization signatures enhances the precision of results, profoundly increasing the diagnostic potential of the modeling. Specifically, encapsulating polarization content will help enable the discrimination of geometrical atmosphere information from the signatures of strong-field QED physics, such as birefringent vacuum polarization, in their magnetospheres. Moreover, in combination with general relativistic lensing of light, this improved precision can afford potential probes of the mass-to-radius ratio of magnetars. 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.