No QPO time lags from Sco X-1 as seen with EXOSAT: a comparison with Cyg X-2

Abstract

We have measured the phase-delay and rms amplitude spectra of Cyg X-2 and Sco X-1. Using EXOSAT data from the normal branch of Cyg X-2 we confirm earlier (Ginga) results, showing that at an energy near 6 keV there is both a minimum in the quasi-periodic oscillations QPO rms amplitude spectrum and a 150^deg phase jump in the QPO phase-delay spectrum. Surprisingly, using EXOSAT and Ginga data, we find no evidence for a phase jump of this kind in the phase-delay spectrum of Sco X-1 on either the normal or flaring branch. Upper limits (90% confidence) of 42^deg can be set on any phase jump in the energy range 2-10 keV on the normal branch, and 88^deg on the flaring branch. The QPO rms amplitude spectrum of Sco X-1 increases steeply with energy on both the normal and flaring branches. These results suggest that the X-ray spectrum pivots about an energy of < _~ 2 keV or > _~ 10 keV or that normal branch QPO of Sco X-1 does not have a pivoting spectrum. We discuss the implications of these results in terms of the radiation-hydrodynamic model for normal branch QPO.