SHEDDING NEW LIGHT ON MAGNETIC ACCRETION: A Comprehensive Study of the X-Ray Emission in Accreting Pulsars

Abstract

Neutron stars are evolutionary remnants of massive stars. They are extremely compact and challenge many of our current theories of the existence of matter at densities similar to that of the atomic nucleus. Highly magnetic neutron stars are known as pulsars. Many X-ray pulsars radiate due to the accumulation of material from a companion star in a binary system. This accretion process in the presence of an extreme magnetic field results in the formation of plasma funnels at the pulsar?s magnetic poles. The X-ray continuum is produced inside these extremely hot accretion columns. The magnetic fields of pulsars can be measured directly from the cyclotron resonant scattering feature sometimes observed in their X-ray spectra. To this day, the accreting pulsar continua have been described using standard phenomenological models that provide no physical insight. However, in the past few years, physically descriptive models have been under development. For the work presented in this dissertation, I analyzed data from the Japanese X-ray observatory, Suzaku. I first performed a detailed temporal and standard spectral analysis of the accreting pulsar XTE J1946+274. Building on that, I conducted a self-consistent study of the X-ray emission of a sample of nine accreting pulsars. The first step involved fitting a cutoff power-law model to the X-ray spectral of all sources. The second step involved testing the application of a newly implemented physical model to the same pulsar sample, and thereby obtaining physical descriptions of the accretion column structure and geometry. The physical parameters obtained include the radius, plasma (electron) temperature, and ratios of different Comptonization effects inside the accretion column. By comparing the physical and phenomenological spectral fit results, I provided the first observational proof that the plasma temperature inside the accretion column is related to the degree of curvature of the X-ray continuum. Lastly, I describe the remarkable self-consistency of the pulsar sample study by showing that the plasma temperature inside the accretion column, estimated from the thermal broadening effect of the cyclotron line, is consistent with the temperatures described by the physical model. To summarize, considerable progress has been made in recent years regarding the development of physical models describing the accretion process onto a highly magnetic neutron stars. I successfully applied a new model implementation and provided the first direct connection between physical parameters of the accretion process (magnetic field strength, plasma temperature, plasma density, mass accretion rate) and the X-ray continuum spectral shape.