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Astron. Astrophys. 353, 203-210 (2000)

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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

1 Astronomical Institute "Anton Pannekoek", University of Amsterdam and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands.
2 Max-Planck-Institut für Astrophysik, Postfach 15 23, 85740 Garching bei Müchen, Germany.
3 Division of Physics, Mathematics, and Astronomy, Caltech, Pasadena, CA 91125, USA
4 Departments of Physics and Astronomy, University of Illinois at Urbana-Champaign, 1110 W. Green Street, Urbana, IL 61801, USA

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 [FORMULA]2 keV or [FORMULA]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, (stefan.dieters@msfc.nasa.gov)

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