Origin of 'normal-branch' quasiperiodic oscillations in low-mass X-ray binary systems

Abstract

QUASIPERIODIC intensity oscillations with frequencies in the range 6–10 Hz have recently been discovered in luminous low-mass X-ray binary systems^1,2. These so-called 'normal-branch' oscillations (NBOs) seem to be a nearly universal feature of those systems with luminosities near the Eddington critical luminosity L_E (refs 3,4), at which the outward force of radiation balances gravity. Here we report the results of numerical simulations of the approximately radial accretion flow expected near the neutron-star component, which show that the flow becomes overstable (that is, the flow oscillates, with increasing magnitude) when the luminosity rises to within a few per cent of L_E. If the radial inflow begins ∼300 km from the neutron star (as expected), the frequency of the oscillations is ∼5–10 Hz, comparable to the observed frequencies of NBOs. The time variation in optical depth is substantial, whereas the variation in luminosity is relatively small. The predicted amplitudes and phases of the resulting intensity oscillations agree with observations⁵ of the NBOs. We suggest that the 10–20 Hz oscillations in intensity observed in Scorpius X-l when it is on the flaring branch^3,4 are caused by photohydrodynamic modes that are excited by the oscillations in the radial flow and grow when the luminosity exceeds L_E.