Probing the Inner Regions Near Accreting Black Holes and Neutron Stars with Chandra

Abstract

Accretion disks are among the most important and well-studied objects in astrophysics. Disks play a critical role in both star formation and later stellar evolution, and are the site of planet formation. Compact objects accreting from a stellar companion may result in luminous and rapidly evolving UV or X-ray disks for white dwarf (WD) or stellar-mass black hole and neutron star (BH or NS) accretors, respectively. At the largest scales, accretion onto supermassive black holes is known to power active galactic nuclei (or, AGN). These are among the most powerful phenomena in the observable universe and their activity can have a profound effect on star formation and evolution of the host galaxy. Despite the astrophysical importance of the accretion process, as well as considerable observational and theoretical efforts, critical questions regarding specific physical mechanisms remain unanswered. There is broad theoretical consensus that angular momentum and mass transfer within the disk are likely mediated via magnetic processes; however, observational signatures of specific physical mechanisms are few and largely indirect, complicating efforts to characterize underlying processes. Likewise, magnetic processes may be central to the physics of the X-ray corona, potentially a cloud of hot in AGN and accreting NSs/stellar-mass BHs that imparts additional energy to disk photons via inverse Compton scattering. Our understanding of the geometry, energetics, and underlying physics of the X-ray Corona is limited and requires knowledge of its compactness - a difficult property to constrain observationally.

In this dissertation, I demonstrate how high-resolution X-ray spectroscopic studies of accreting black holes and neutron stars can address some of these issues. Specifically, I characterize absorption phenomena occurring above the disk's surface, such as disk winds and atmospheres, using astrophysical plasma models in order to probe the physical processes that underlie the accretion disk. We find that the highly ionized disk winds (outflowing at $0.1%-1%~c$) in BH candidate 4U 1630$-$472, for instance, are likely magnetic; the radial structure of these outflows may be indicative of magnetically driven accretion (Chapter II). Moreover, I present the discovery of gravitationally redshifted disk atmospheres in a sample of short-period neutron star systems. Like disk winds in BH systems, the location of these absorbers means that the atmosphere may require strong magnetic pressure support (Chapter III). Finally, I developed a new methodology for constraining the size of the central emitting regions of these systems, providing new angles on the physical nature of these emitters (Chapter IV).