1994A&A...289..795K -
Astronomy and Astrophysics, volume 289, 795-821 (1994/9-3)
Spectral and correlated timing behaviour of GX 5-1.
KUULKERS E., VAN DER KLIS M., OOSTERBROEK T., ASAI K., DOTANI T., VAN PARADIJS J. and LEWIN W.H.G.
Abstract (from CDS):
We have analyzed the X-ray spectral and fast timing behaviour of the Z-source GX5-1 using X-ray colour-colour diagrams (CDs), hardness-intensity diagrams (HIDs) and power spectra. We used all EXOSAT ME data on GX5-1, a total of seven observations during 1983-1985. We detect, for the first time in GX5-1, secular motion of the well known "Z" pattern through the CDs and HIDs. This behaviour resembles CygX-2, but in GX5-1 has a smaller amplitude. We find that the power spectra do not change when the "Z" moves, and are only dependent on the position inside the "Z". We also report, for the first time in GX5-1, indications of flaring-branch (FB) like behaviour, both during one of our EXOSAT observations, and during observations done with Ginga. During this FB stage, dips occur in the light curve. This was previously also seen in CygX-2 and GX340+0 when they were in the FB. We suggest that the secular variations and the presence of dips in the FB may be due to a higher inclination in these Z-sources than in the Z-sources GX17+2, ScoX-1 and GX349+2. Other observational differences between the Z-sources seem to confirm the existence of two groups with different inclinations. It is shown that our data of the quasi-periodic oscillations in the horizontal branch (HB) can not, using the beat-frequency model, constrain the predicted pulse period of the neutron star in GX5-1. As the secular changes of the position of the "Z" pattern in the CD and HID are relatively small we investigated in detail the systematic changes in the EXOSAT ME detectors using the only bright and steady X-ray "standard star", the Crab Nebula. We also redetermined the collimator responses of the eight argon detectors. As the observed count rate of GX5-1 is high, large dead time effects influence our data. We have examined the effects on the Poisson levels in the power spectra; we find corrections to the usually employed relations between observed count rate and Poisson level leading to improved estimates of the source variability characteristics. Finally, we determined an intrinsic CD of GX5-1 (i.e. a soft photon flux-ratio versus hard photon flux-ratio diagram) in a model independent way, by applying a first order correction for the detector efficiency. It is shown, using data of the Crab Nebula, that this method works reasonably well for the determination of intrinsic colours.
Abstract Copyright:
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Journal keyword(s):
accretion, accretion disks - instrumentation: detectors - binaries: close - stars: individual: GX 5-01 - stars: neutron: Crab Nebula - X-rays: stars
Errata:
erratum vol.295 p.842 (1995)
Simbad objects:
10
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