2018ApJ...860..167T

Kilohertz quasi-periodic oscillations or kHz QPOs occur on the orbital 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. Both the timing behavior of kHz QPOs and the time-averaged spectra of these systems have been studied extensively, yet no model completely describes all the properties of kHz QPOs. Here, we present a systematic study of spectral-timing products of kHz QPOs from low-mass X-ray binary systems through the use of archival Rossi X-ray Timing Explorer/Proportional Counter Array data. For the lower kHz QPOs in 14 objects and the upper kHz QPOs in six objects, we were able to obtain correlated time lags as a function of QPO frequency and energy, as well as energy-dependent covariance spectra and intrinsic coherence. For the lower kHz QPOs, we find a monotonic decrease in lags with increasing energy, rising covariance to ∼12 keV, and near unity coherence at all energies. For the upper kHz QPOs, we find near zero lags, rising covariance to ∼12 keV, and less well-constrained coherence at all energies. These results suggest that while kHz QPOs are likely produced by similar mechanisms across the population of LMXBs, the lower kHz QPOs are likely produced by a different mechanism than the upper kHz QPOs.