Horizontal Branch Oscillations Research Articles

Hard apex transition in quasi-periodic oscillators - Closing of the accretion gap

We propose that the 'hard apex' transition in the X-ray two-color diagrams for low-mass X-ray binaries exhibiting quasi-periodic oscillation is associated with closure of a gap between the accretion disk and the star. At low accretion rates, gas crosses this gap intermittently. However, when the mass accretion rate increases, the disk thickens and its inner edge touches the star, thus forming a boundary layer through which the gas flows steadily. This explanation is viable provided that the equation of state of nuclear matter is not significantly harder than the Bethe-Johnson I prescription. Accretion gap scenarios are possibly distinguishable from models which invoke a small magnetosphere around the neutron star, in that they preclude large stellar magnetic fields and associate the high-frequency (horizontal-branch) oscillations with different sites.

Stability of slim accretion discs – Effects of central mass and viscosity

Slim accretion discs have a total luminosity of the order |$L/L_{E}=\dot{m}\sim 1$|⁠, where LE is the Eddington luminosity and |$\dot{m}=\dot{M}/\dot{M}_{c}$|⁠, where Me is a critical accretion rate, related to the Eddington one. The local stability properties of such discs are examined, in the three-dimensional parameter space spanned by the (α, μ, m) axes, where α and μ are two viscosity parameters, and m = M/M⊙ the central mass. We suggest that various types of observed quasi-periodic behaviour may be connected with slim disc instabilities. If this turns out to be correct, the so-called normal and horizontal branch oscillations could be due to unstable thermal and acoustic modes, respectively. It is subsequently shown that some of the observed short-term (quasi-periodic) variability in active galactic nuclei may also originate from short-wavelength acoustic modes in the innermost region of the disc. Consequently, observational characteristics, in connection with stability theory, may yield estimates of basic accretion parameters. In the case of the Seyfert galaxy NGC6814, this process seems to favour |$(\alpha ,\mu ,m,\dot{m})\simeq (0.5,0,{10}^{6},{10}^{-2})$|⁠. We finally conclude that this line of work may provide additional evidence for both accretion discs and black holes, in various compact sources.

Binary X‐Ray Sourcesa

The present state of understanding of binary X-ray sources is surveyed. The observed properties of the luminous quasi-periodic oscillation sources are discussed, and a recently developed, unified model of low-mass X-ray binary behavior is described. The most up-to-date models of X-ray spectral states, horizontal branch oscillations, and normal branch oscillations are examined.

Models of QPOs in luminous low-mass X-ray binaries

Recent observational results indicate that quasi-periodic intensity oscillations (QPOs) in luminous X-ray sources are of two or three distinct types. These types are defined by the spectral state of the source (whether on the horizontal, normal, or flaring branches of an X-ray color-color diagram) and by the properties of the QPOs themselves. The sources exhibiting these QPOs are thought to be low-mass binary'systems in which a neutron star is accreting matter from its companion via an accretion disk. Models proposed so far can be classified according to whether the magnetic field of the neutron star is or is not important, and whether the quasi-periodic intensity oscillations are produced by luminosity oscillations or beaming. The strong red noise that accompanies horizontal branch oscillations is expected in many models. The weakness of the red noise accompanying normal branch oscillations poses more or less of a difficulty for all models so far proposed. The hard lags observed in horizontal branch oscillations may be to Comptonization by a central corona. If so, these oscillations are probably caused by oscillations in the luminosity of the X-ray star. The existence of a central corona would also help to explain why periodic oscillations at the spin frequency of the neutron star are so weak in these sources. Obscuration by rotating or oscillating disk matter has several attractive features as an explanation of horizontal branch oscillations but is difficult to reconcile with the Comptonization interpretation of the hard lags and places severe restrictions on the angles of inclination of QPO systems and on the dynamics of the inner disk. At present, the beat-frequency model appears to be the most attractive model for horizontal branch oscillations. It provides a quantitative explanation of the frequencies, frequency-intensity correlations, and other properties of such oscillations. Calculations of the amplitudes of periodic oscillations at the spin frequency of the neutron star give amplitudes of a fraction of 1%, consistent with current observational limits. Detection of such oscillations remains the most important test of the model. An accretion disk instability may be the most promising explanation of normal branch oscillations.