Discovery of KiloHertz Quasi-Periodic Oscillations in the Z source Cygnus X-2

Rudy Wijnands; Jeroen Homan; Michiel van der Klis; Erik Kuulkers; Jan van Paradijs; Walter H. G. Lewin; Frederick K. Lamb; Dimitrios Psaltis; Brian Vaughan

doi:10.1086/311138

arXiv · astro-ph/9711219

Discovery of KiloHertz Quasi-Periodic Oscillations in the Z source Cygnus X-2

Rudy Wijnands·Jeroen Homan·Michiel van der Klis·Erik Kuulkers·Jan van Paradijs·Walter H. G. Lewin·Frederick K. Lamb·Dimitrios Psaltis·Brian Vaughan

View original source ↗Read source document ↗Export metadata ↓

Abstract

During observations with RXTE we discovered two simultaneous kHz QPOs near 500 Hz and 860 Hz in the Z source Cyg X-2. In the X-ray color-color diagram and hardness-intensity diagram (HID) a clear Z track was traced out. A single kHz QPO peak was detected at the left end of the horizontal branch (HB) of the Z track, with a frequency of 731+/-20 Hz and an amplitude of 4.7+0.8-0.6 % rms in the energy band 5.0-60 keV. Further to the right on the HB, at somewhat higher count rates, an additional peak at 532+/-43 Hz was detected with an rms amplitude of 3.0+1.0-0.7 %. When the source moved down the HB, thus when the inferred mass accretion rate increased, the frequency of the higher-frequency QPO increased to 839+/-13 Hz, and its amplitude decreased to 3.5+0.4-0.3 % rms. The higher-frequency QPO was also detected on the upper normal branch (NB) with an rms amplitude of 1.8+0.6-0.4 % and a frequency of 1007+/-15 Hz; its peak width did not show a clear correlation with inferred mass accretion rate. The lower-frequency QPO was most of the time undetectable, with typical upper limits of 2 % rms, no conclusion on how this QPO behaved with mass accretion rate can be drawn. If the peak separation between the QPOs is the neutron star spin frequency (as required in some beat-frequency models) then the neutron star spin period is 2.9+/-0.2 ms (346+/-29 Hz).

Explore related subjects

astro-ph

Keep this discovery

Explore connections, maps & timelines

BibTeX RIS

Rudy Wijnands, Jeroen Homan, Michiel van der Klis, Erik Kuulkers, Jan van Paradijs, Walter H. G. Lewin, Frederick K. Lamb, Dimitrios Psaltis, Brian Vaughan. 1997-11-19. Discovery of KiloHertz Quasi-Periodic Oscillations in the Z source Cygnus X-2. https://doi.org/10.1086/311138

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Frame covariance of CMBR lensing in the exact 1+3 covariant radiation hierarchy

The exact $1+3$ covariant Boltzmann hierarchy for the cosmic microwave background contains acceleration, shear and vorticity couplings whose coefficients grow linearly in $\ell$. We show that all are the multipole form of the aberration operator in the photon Liouville equation. Under a change of threading, this operator shifts by the along-ray generator of a Lorentz boost. The acceleration coupling of $\ell$ to $\ell\pm1$ and the shear coupling of $\ell$ to $\ell\pm2$ are therefore frame-covariant descriptions of the same operator, related by an algebraic, pointwise relabelling of direction. With emitter and observer frames fixed, the observed sky is independent of the intermediate threading at every $\ell$. We derive this in $1+3$ covariant and gauge-invariant variables without constructing second-order gauge-invariant variables. The $O(ε^2\ell)$ content is gravitational lensing, as known in metric variables. Linearisation about a Friedmann--Lemaître--Robertson--Walker background removes these couplings, requiring lensing to be reinstated from the geodesic equation. Keeping the threading arbitrary makes frame covariance explicit and places observer aberration in the same operator. In the Newtonian threading we compute both halves of the deflection: the equivalence-principle contribution enters through aberration, while the spatial-curvature contribution arises from free streaming on perturbed spatial sections and reduces to the same operator. The lensing efficiency follows from the direction derivative. The angular-power-spectrum contribution is quartic in the potential: for a deflection statistically independent of the source, the two fourth-order terms carrying $\ell^2$ cancel where the spectrum is smooth on the deflection scale, and lensing redistributes power without adding ensemble variance.

astro-ph↗

arXiv2026

Cosmic Conundrums with Quantum Corrections

Darh energy was discovered over 25 years ago and we do not have an explanation of it. Dark matter comprises 95% of matter in the universe and we still don't know what it is. The Webb telescope has been finding fully formed galaxies with massive black holes millions of times the mass of the sun in the early universe and we don't have any explanation. A quantum density limitation will be used to solve these and other outstanding problems.

astro-ph↗

arXiv2026

On binary pulsars and the force of gravity

The energy-momentum budget of the astrophysical systems can be studied by the exact local conservation equation derived by Landau and Lifshitz. We show that a similar equation is valid for the Einstein-Cartan gravity. We reanalyze a binary pulsar system using the Landau-Lifshitz conservation equation and show that the orbital period change rate can be completely understood as a curvature backreaction process. Taking into account the detailed theoretical and observational research of relativistic binary pulsar systems, especially the system of Hulse and Taylor, we conclude that general relativity and astrophysical observations rule out the existence of gravitational radiation. We comment upon the LIGO GW events and their alternative explanation, as well as the recent pulsar timing arrays data.

astro-ph↗