Astron. Astrophys. 343, 197-201 (1999)

The phase-delay spectrum of GX 5-1 on its normal branch

B.A. Vaughan ^1,2, M. van der Klis ², W.H.G. Lewin ³, J. van Paradijs ^2,4, K. Mitsuda ⁵ and T. Dotani <sup>5

1</sup> Space Radiation Laboratory, California Institute of Technology, MC 220-47, Pasadena CA 91125, USA
² Astronomical Institute "Anton Pannekoek", University of Amsterdam and Center for High Energy Astrophysics, Kruislaan 403,
1098 SJ Amsterdam, The Netherlands
³ Massachusetts Institute of Technology, 37-627, Cambridge, MA 02139, USA
⁴ Physics Department, University of Alabama in Huntsville, Huntsville AL 35899, USA
⁵ Institute of Space and Astronautical Sciences, 3-1-1 Yoshinodai, Sagamihara-shi, Kanagawa-ken, 229 Japan

Received 2 October 1997 / Accepted 6 April 1998

Abstract

We have measured the phase differences between X-ray intensity variations in different energy bands in data taken with Ginga of the Z source GX 5-1 in its normal branch. We apply a optimal extraction technique to measure the phase-delay spectrum of the normal-branch quasi-periodic oscillations (QPO) and find a jump at 3.5 keV. X-ray intensity variations at the QPO frequency in energy channels below 3.5 keV are in phase with one another. The same is true of energy channels above 6 keV. However, intensity variations in energy channels above 6 keV lag those in channels below 3.5 keV by . We see no minimum in the fractional rms amplitude of the quasi-periodic oscillations at the jump energy, in contrast to what is seen in the Z source Cyg X-2, but because of limited statistics we cannot rule out such a minimum. We discuss our findings in the context of a radiation-hydrodynamics model for the normal-branch quasi-periodic oscillations.

Key words: accretion, accretion disks stars: individual: GX 5-1 stars: neutron X-rays: stars

Send offprint requests to: M. van der Klis

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Contents

• 1. Introduction
• 2. Observations
• 3. Analysis techniques
  • 3.1. Phase and time delay measurements
  • 3.2. Deadtime and cross talk
• 4. Results
• 5. Discussion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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