Journal Article

Damped harmonic oscillator interpretation of the soft-state power spectra of Cyg X-1

Ranjeev Misra and Andrzej A. Zdziarski

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 387, issue 2, pages 915-920
Published in print June 2008 | ISSN: 0035-8711
Published online May 2008 | e-ISSN: 1365-2966 | DOI:
Damped harmonic oscillator interpretation of the soft-state power spectra of Cyg X-1

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We develop a model of an accretion disc in which the variability induced at a given radius is governed by a damped harmonic oscillator at the corresponding epicyclic frequency. That variability induces both linear and non-linear responses in the locally emitted radiation. The total observed variability of a source is the sum of these contributions over the disc radius weighted by the energy dissipation rate at each radius. It is shown that this simple model, which effectively has only three parameters including the normalization, can explain the range of the power spectra observed from Cyg X-1 in the soft state. Although a degeneracy between the black hole mass and the strength of the damping does not allow a unique determination of the mass, we can still constrain it to ≲(16–20) M. We also show that our model preserves the observed linear rms–flux relationship even in the presence of the non-linear flux response.

Keywords: accretion, accretion discs; black hole physics; X-rays: binaries; X-rays: individual: Cyg X-1; X-rays: stars

Journal Article.  4353 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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