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Access Type

WSU Access

Date of Award

January 2020

Degree Type


Degree Name



Physics and Astronomy

First Advisor

Edward M. Cackett


Neutron star low-mass X-ray binaries (NS LMXBs) are systems which consist of a NS and a low-mass companion star. They are naturally variable on a variety of timescales, and are even classified by their X-ray spectral variability on the timescales of hours to days. The most luminous NS LMXB sources are known as ‘Z’ sources, because they trace out a characteristic ‘Z’ shape when plotted on a color-color or hardness-intensity diagram. The physical mechanisms causing this variability are not well understood. To try to address this, we model spectra taken from different positions on a hardness-intensity diagram of two Z sources, GX 349+2 and Sco X-1. In doing so we utilize models of relativistic reflection off of the inner accretion disk, which manifests itself as a series of reflection features, the strongest of which is the Fe Kα line between 6 and 7 keV. We find that in both cases the inner disk radius is truncated and does not appear to change across the Z-track, which may be due to the NS boundary layer at high luminosities. At the shortest of timescales, the upper and lower kHz quasi-periodic oscillations (QPOs) are variations observed in the count rate of LMXB sources at frequencies between 300–1200 Hz. The energy-dependence of the time lags measured with the upper and lower kHz QPOs are different, which suggests that different physical mechanisms might produce them. Since time lags due to reverberation off of the accretion disk have been detected in some black hole systems, which also show signatures of reflection, it has been suggested that reverberation causes the time lags measured with the kHz QPOs. We model the lag-energy spectrum due to reverberation for the NS LMXB source 4U 1728−34, and show that reverberation is unable to produce the energy-dependence of the upper kHz QPOs. This implies that any physical models which attempt to explain the kHz QPOs must intrinsically produce the lags as well as their energy-dependence.

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