Drop of coherence of the lower kilo-Hz QPO in neutron stars: is there a link with the innermost stable circular orbit?

Abstract

Using all available archival data from the Rossi X-ray Timing Explorer (RXTE), we follow the frequency of the kilo-Hz QPOs in three low luminosity neutron star low mass X-ray binaries; namely 4U 1636-536, 4U 1608-522, and 4U1735-44. Following earlier work, we focus our analysis on the lower kilo-Hz QPO, for which we study the dependency of its quality factor (Q) amplitude as a function of frequency over a range covering from 500 Hz to 1000 Hz. As previously found for 4U 1636-536, we show that the quality factor of the lower kilo-Hz increases with frequency up to a maximum frequency around 800 Hz, beyond which an abrupt drop of its coherence is observed down to a limiting frequency where the QPO disappears completely. Simultaneously the amplitude of the QPOs is almost constant below the peak frequency and starts to decrease smoothly afterwards. The peak frequency is 850 Hz, 820 Hz, 740 Hz whereas the limiting frequency is 920 Hz, 900 Hz and 830 Hz for 4U 1636-536, 4U 1608-522 and 4U 1735-44 respectively. A ceiling of the lower QPO frequencies is also seen clearly in a frequency versus count rate diagram for all sources. This behavior is reproducible within an object and between objects. We suggest here that the drop of coherence of the lower QPO may be a geometry-related effect, which could be related to the last stable circular orbit.

4 pages, 3 color figures. Astronomical Notes, in press

References in corpus (3)

• Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar
• An abrupt drop in the coherence of the lower kilohertz QPO in 4U 1636-536
• On the high coherence of kilo-Hz Quasi-Periodic Oscillations

Cited by in corpus (28)

• The Masses and Spins of Neutron Stars and Stellar-Mass Black Holes
• Multi-resonance orbital model of high-frequency quasi-periodic oscillations: possible high-precision determination of black hole and neutron star spin
• X-ray time variability across the atoll source states of 4U 1636--53
• The coherence of kHz quasi-periodic oscillations in the X-rays from accreting neutron stars
• Neutron stars in Einstein-aether theory
• Quasiperiodic oscillations in a strong gravitational field around neutron stars testing braneworld models
• On mass-constraints implied by the relativistic precession model of twin-peak quasi-periodic oscillations in Circinus X-1
• Energy Dependent Time Delays of kHz Oscillations due to Thermal Comptonization
• Resonant radii of kHz quasi-periodic oscillations in Keplerian discs orbiting neutron stars
• Supporting evidence for the signature of the innermost stable circular orbit in Rossi X-ray data from 4U1636-536
• The kilohertz quasi-periodic oscillations during the Z and atoll phases of the unique transient XTE J1701--462
• On the distribution of frequency ratios of kHz QPOs
• 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
• The time derivative of the kilohertz quasi-periodic oscillations in 4U 1636-53
• Discovery of kilohertz quasi-periodic oscillations and state transitions in the low-mass X-ray binary 1E~1724-3045 (Terzan 2)
• Statistical properties of twin kHz QPO in neutron star LMXBs
• Relation between the properties of the kilohertz quasi-periodic oscillations and spectral parameters in 4U 1636$-$53
• Twin peak quasi-periodic oscillations and the kinematics of orbital motion in a curved space-time
• Simple analytic formula relating the mass and spin of accreting compact objects to their rapid X-ray variability
• On one-parametric formula relating the frequencies of twin-peak quasi-periodic oscillations
• Kilohertz quasi-periodic oscillations in neutron-star X-ray binaries: Flattening of the lag spectrum with increasing luminosity
• A model for upper kHz QPO coherence of accreting neutron star
• Morphological analysis on the coherence of kHz QPOs
• Constraining the mass and moment of inertia of neutron stars from quasi-periodic oscillations in X-ray binaries
• Statistical Analysis for the Q-factor of twin kHz QPOs
• Oscillation properties of relativistic tori in the vicinity of a distorted deformed compact object
• Restrictions to neutron star properties based on twin-peak quasi-periodic oscillations