Access Type

Open Access Dissertation

Date of Award

January 2024

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physics and Astronomy

First Advisor

Renee M. Ludlam

Abstract

A low-mass X-ray binary (LMXB) is a compact object, such as a neutron star (NS) or black hole (BH), which is accreting material from a stellar companion via a process called Roche-lobe overflow. An ultra-compact X-ray binary (UCXB) is a subset of these systems defined by a shorter orbital period (< 80 minutes, compared to the hours to days seen in LMXBs). This shorter orbital period implies that the companion is more compact than a main sequence type star, often being a white dwarf (WD). These systems are important to study, as the compact nature of both objects in a tight orbit produces low frequency gravitational waves that will affect the next generation of multi-messenger astronomy. By observing UCXBs we can help to understand every facet of these unique systems before such missions begin. Over the course of this dissertation, we study a NS-WD UCXB called 4U 0614+091 and a NS UCXB candidate called SLX 1735-269. The primary method used to study these systems is a process called reflection modeling. This process involves collecting X-ray spectra, and modeling various components; a thermal contribution from the NS, non-thermal contribution from an X-ray corona surrounding the compact object, and sometimes thermal contributions from the accretion disk itself when necessary. These make up the standard continuum for an LMXB, but an additional component is applied which accounts for photons from the corona illuminating the disk and being reprocessed by the elements therein. These elements differ between UCXBs and LMXBs, as UCXBs tend to have an overabundance of oxygen with respect to solar, while being nearly devoid of hydrogen, resulting in a unique feature in the low-energy band. We use this fact to attempt to verify the UCXB nature of SLX 1735-269 by comparing reflection models applied to the data. These models contain different chemical abundances, with some designed for standard LMXBs with a main sequence companion and another designed specifically for UCXBs with a WD companion. We find that it is more likely that the source is ultra-compact in nature, but our timing analysis yields no orbital period, so we can not verify this beyond the indirect evidence of abundances. We also use this method to analyzea series of observations of 4U0614+091, which is a confirmed UCXB. This source varies in flux quasi-periodically over the course of a few days. By capturing the source at various stages along this flux evolution and modeling the reflection, we find that the inner disk appears to move away from the NS at the lowest flux state. This is analogous to what is seen in BH LMXB systems, and explains an apparent anti-correlation in the flux of the illuminating corona and that of the reflected emission, as the accretion disk is physically further from the illuminating source. Finally, we focus on a long term analysis of the same source, taking 51 archival spectra and modeling them with a UCXB reflection abundances. We find the source in different states throughout the course of these observations, which allows us to study how certain reflection parameters change as the source varies overall. We find that our results are consistent with the X-ray corona moving closer to the NS during high flux states, where the bulk of the emission is at low energies. This unique subset of sources can help us understand accretion at various scales by drawing comparisons to more massive black hole counterparts.

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