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Observations of Quasi-Periodic Oscillations in Cyg X-2

Published online by Cambridge University Press:  04 August 2017

Günther Hasinger
Günther Hasinger*
Affiliation:
Max-Planck-Institut für extraterrestrische Physik, 8046 Garching bei München Federal Republic of Germany

Abstract

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EXOSAT observations of Cyg X-2 reveal two types of QPO's (quasi-periodic oscillations) which are associated with different spectral behaviour. QPO's in the range 18–55 Hz with a hard spectrum are found in all observations where the source is on the ‘horizontal branch’ in an hardness-versus-intensity diagram. On the ‘vertical branch’, however, QPO's with a soft spectrum are found at a stable frequency of 5.6 Hz. A multitude of QPO frequency-intensity relations, found in different ‘horizontal branch’ observations, coalesce to an almost unique relation when the QPO frequency is correlated with spectral hardness. A cross-correlation analysis of the rapid variability reveals a time-lag of hard photons by 1.5–3.8 ms; the lag being smaller when the QPO frequency is larger. All results are consistent with a model where QPO's represent the Kepler frequency at the edge of the neutron star magnetosphere. In particular the data do not require the assumption of a beat frequency. The derived surface magnetic field strength is of the order of 1010G.

Type
IV. Neutron Stellar Evolution
Information
Symposium - International Astronomical Union , Volume 125: The Origin and Evolution of Neutron Stars , 1987 , pp. 333 - 345
Copyright
Copyright © Reidel 1987 

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