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

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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 5

1 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


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 [FORMULA] 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 [FORMULA][FORMULA]. 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

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999