No QPO time lags from Sco X-1 as seen with EXOSAT: a comparison with Cyg X-2
Stefan W. Dieters 1,2,
Brian A. Vaughan 1,3,
Erik Kuulkers 1,
Frederick K. Lamb 4 and
Michiel van der Klis 1
Received 6 April 1999 / Accepted 10 November 1999
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 150o 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 42o can be set on any phase jump in the energy range 2-10 keV on the normal branch, and 88o 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.
Key words: methods: data analysis stars: binaries: close stars: individual: Sco X-1 stars: individual: Cyg X-2 stars: neutron X-rays: stars
Present address: SD-50 NASA Marshall Space Flight Center, Huntsville, Alabama, AL 35812, USA, (firstname.lastname@example.org)
Present address: Space Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, and Astronomical Institute, Utrecht University, P.O. Box 80000, 3507 TA Utrecht, The Netherlands
Send offprint requests to: Stefan Dieters
© European Southern Observatory (ESO) 2000
Online publication: December 8, 1999