ENERGY-DEPENDENCE OF NORMAL BRANCH QUASI-PERIODIC INTENSITY OSCILLATIONS IN LOW-MASS X-RAY BINARIES

The properties of the approximately 6 Hz quasi-periodic X-ray intensity oscillations observed in the low-mass X-ray binary Cyg X-2 when it is on the normal spectral branch are shown to be consistent with a model in which photons from a central source with a fixed spectrum are Comptonized by an oscillating radial inflow. As the electron scattering optical depth of the flow varies, the spectrum of the escaping X-rays appears to rotate about a pivot energy E(p) that depends mainly on the electron temperature in the flow. The temperature derived from the observed energy dependence of the Cyg X-2 normal branch oscillations is approximately 1 keV, in good agreement with the estimated Compton temperature of its X-ray spectrum. The mean optical depth tau of the Comptonizing flow is inferred to be about 10, while the change in tau over an oscillation is estimated to be about 1; both values are in good agreement with radiation hydrodcode simulations of the radial flow. The effect of induced scattering is investigated and used to place an approximate lower bound on the volume of the Comptonizing region. We conjecture that the 6 Hz normal branch oscillations observed in other Z-class sources are also produced largely by oscillations in the degree of Comptonization of a central source.