Access Type

Open Access Dissertation

Date of Award

January 2018

Degree Type


Degree Name



Physics and Astronomy

First Advisor

Edward Cackett


Kilohertz quasi-periodic oscillations or kHz QPOs are X-ray intensity variations observed

in neutron star low-mass X-ray binary (LMXB) systems. In such systems, matter is transferred

from a secondary low-mass star to a neutron star via the process of accretion. kHz

QPOs occur on the timescale of the inner accretion flow and may carry signatures of the

physics of strong gravity (c2 ~GM/R) and possibly clues to constraining the neutron star

equation of state (EOS). No model to date has been able to illuminate the origin of kHz

QPOs. Spectral-timing is a set of analysis techniques useful in deriving information about

the nature of physical processes occurring within the accretion flow on the timescale of the

kHz QPO. We present a comprehensive study of spectral-timing products of kHz QPOs from

systems where data is available in NASA’s Rossi X-ray Timing Explorer (RXTE) archive to

demonstrate the promise of these techniques and to gain insights regarding the origin of kHz

QPOs. Using data averaged over the entire RXTE archive, we show correlated time-lags

as a function of QPO frequency and energy, as well as energy-dependent covariance spectra

for the various LMXB systems where spectral-timing analysis is possible. The similarity in

trends in all sources suggest a common physical origin for kHz QPOs across the population.

The diferences in results between lower and upper kHz QPOs lend further support to the

evidence of the differing nature of the lower and upper kHz QPOs.