Mechanical model of a boundary layer for the parallel tracks of kilohertz quasi-periodic oscillations in accreting neutron stars

Abstract

Kilohertz-scale quasi-periodic oscillations (kHz QPOs) are a distinct feature of the variability of neutron star low-mass X-ray binaries. Among all the variability modes, they are especially interesting as a probe for the innermost parts of the accretion flow, including the accretion boundary layer (BL) on the surface of the neutron star. All the existing models of kHz QPOs explain only part of their rich phenomenology. Here, we show that some of their properties may be explained by a very simple model of the BL that is spun up by accreting rapidly rotating matter from the disk and spun down by the interaction with the neutron star. In particular, if the characteristic time scales for the mass and the angular momentum transfer from the BL to the star are of the same order of magnitude, our model naturally reproduces the so-called parallel tracks effect, when the QPO frequency is correlated with luminosity at time scales of hours but becomes uncorrelated at time scales of days. The closeness of the two time scales responsible for mass and angular momentum exchange between the BL and the star is an expected outcome of the radial structure of the BL.

arXiv comment8 pages, 7 figures, accepted for publication in Astronomy and Astrophysics

References in corpus (21)

• PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter
• A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation
• Millisecond Oscillations in X-Ray Binaries
• RXTE Observation of Cygnus X-1: II. Timing Analysis
• Non-linear X-ray variability in X-ray binaries and active galaxies
• Stringent constraints on neutron-star radii from multimessenger observations and nuclear theory
• X-Ray Variability Coherence: How to Compute it, What it Means, and How it Constrains Models of Cyg X-1 and GX 339-4
• Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
• Boundary layer, accretion disk and X-ray variability in luminous LMXBs
• Dependence of the Frequency of the Kilohertz Quasi-Periodic Oscillations on X-ray Count Rate and Colors in 4U 1608-52
• A Possible Explanation for the "Parallel Tracks" Phenomenon in Low-Mass X-Ray Binaries
• The distribution of kHz QPO frequencies in bright LMXBs
• Spectra of the spreading layers on the neutron star surface and constraints on the neutron star equation of state
• The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity
• Is there a link between the neutron-star spin and the frequency of the kilohertz quasi-periodic oscillations?
• Energy Release During Disk Accretion onto a Rapidly Rotating Neutron Star
• Magnetic activity in accretion disc boundary layers
• Angular Momentum Transport by Acoustic Modes Generated in the Boundary Layer I: Hydrodynamical Theory and Simulations
• On the spreading layer emission in luminous accreting neutron stars
• Kilohertz quasi-periodic oscillations from neutron star spreading layers
• Beat-Frequency Models of Kilohertz QPOs