Intensity and source state dependence of the quasi-periodic oscillations in Scorpius X-1

Abstract

Quasi-periodic oscillations (QPO) at 6 Hz were discovered from EXOSAT observations of Sco X-1 in its quiescent state by Middleditch and Priedhorsky. The authors report here on the properties of the QPO seen in earlier EXOSAT observations of Sco X-1. During these observations QPO were also detected when Sco X-1 was in an active state, in ≡20 minute dips between flares and prior to a transition to an extended quiescent state; in these cases QPO frequency varied between 10 and 20 Hz and was strongly correlated with source intensity. The transition between an active and a quiescent state was marked by variations in QPO frequency between 6 and 16 Hz which occurred on time scales of several minutes, with no intensity correlation. The authors present an oscillating thick disc obscuration model to explain the observed QPO and X-ray spectral behavior from Sco X-1.