The coherence of kHz quasi-periodic oscillations in the X-rays from accreting neutron stars

Abstract

We study in a systematic way the quality factor of the lower and upper kHz QPOs in a sample of low luminosity neutron star X-ray binaries, showing both QPOs varying over a wide frequency range. The sample includes 4U 1636-536, 4U 1608-522, 4U 1735-44, 4U 1728-34, 4U 1820-303 and 4U 0614+09. We find that all sources except 4U 0614+091 show evidence of a drop in the quality factor of their lower kHz QPOs at high frequency. For 4U 0614+091 only the rising part of the quality factor versus frequency curve has been sampled so far. At the same time, in all sources but 4U 1728-34, the quality factor of the upper kilo-Hz QPO increases all the way to the highest detectable frequencies. We show that the high-frequency behaviours of both the lower and upper kHz QPO quality factors are consistent with what is expected if the drop is produced by the approach of an active oscillating region to the innermost stable circular orbit: the existence of which is a key feature of General Relativity in the strong field regime. Within this interpretation, our results imply gravitational masses around 2 solar masses for the neutron stars in those systems.

Accepted for publication in MNRAS, 8 pages, 8 figures, figure 1 in color

Cited by in corpus (31)

• Neutron Star Observations: Prognosis for Equation of State Constraints
• The Masses and Spins of Neutron Stars and Stellar-Mass Black Holes
• Pulsars as Astrophysical Laboratories for Nuclear and Particle Physics
• Warps, bending and density waves excited by rotating magnetized stars: results of global 3D MHD simulations
• Accretion disks and coronae in the X-ray flashlight
• Quasiperiodic oscillations in a strong gravitational field around neutron stars testing braneworld models
• A Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with $\text{NuSTAR}$
• On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries
• RossiXTE monitoring of 4U 1636-53: I. Long-term evolution and kHz Quasi-Periodic Oscillations
• On mass-constraints implied by the relativistic precession model of twin-peak quasi-periodic oscillations in Circinus X-1
• Supporting evidence for the signature of the innermost stable circular orbit in Rossi X-ray data from 4U1636-536
• Discovery of the upper kilo-Hz QPO from the X-ray transient Aql X-1
• Timescale of twin-peak quasi-periodic oscillations and mass of accreting neutron stars
• Mass-angular-momentum relations implied by models of twin peak quasi-periodic oscillations
• On attributes of a Rotating Neutron star with a Hyperon core
• Upper-twin-peak quasiperiodic oscillation in x-ray binaries and the energy from tidal circularization of relativistic orbits
• High-frequency variability in neutron-star low-mass X-ray binaries
• Difficulties with the QPOs Resonance Model
• Relation between spectral changes and the presence of the lower kHz QPO in the neutron-star low-mass X-ray binary 4U 1636-53
• Relation between the properties of the kilohertz quasi-periodic oscillations and spectral parameters in 4U 1636$-$53
• Constraining models of twin peak quasi-periodic oscillations with realistic neutron star equations of state
• Simple analytic formula relating the mass and spin of accreting compact objects to their rapid X-ray variability
• Appearance of innermost stable circular orbits of accretion discs around rotating neutron stars
• Detection of a 1258 Hz high-amplitude kilohertz quasi-periodic oscillation in the ultra-compact X-ray binary 1A 1246-588
• On one-parametric formula relating the frequencies of twin-peak quasi-periodic oscillations
• Dynamics of charged particles and quasi-periodic oscillations in the vicinity of a distorted, deformed compact object embedded in a uniform magnetic field
• Kilohertz quasi-periodic oscillations in neutron-star X-ray binaries: Flattening of the lag spectrum with increasing luminosity
• Modeling the upper kHz QPOs of 4U 1728-34 with X-ray reverberation
• Evidences of an innermost stable bound orbit predicted by general relativity from the amplitude of the twin-peak quasi-periodic oscillations
• Accretion tori around rotating neutron stars II: Oscillations and precessions
• Lense-Thirring precession of neutron-star accretion flows: Relativistic versus classical precession