EDP Sciences
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Volume 414, Number 3, February II 2004
Page(s) 1091 - 1104
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20031684

A&A 414, 1091-1104 (2004)
DOI: 10.1051/0004-6361:20031684

Long term variability of Cyg X-1

II. The rms-flux relation
T. Gleissner1, J. Wilms1, K. Pottschmidt2, 3, P. Uttley4, M. A. Nowak5 and R. Staubert1

1  Institut für Astronomie und Astrophysik - Abt. Astronomie, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
2  Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85748 Garching, Germany
3  INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland
4  School of Physics and Astronomy, University of Southampton, Southampton S017 1BJ, UK
5  MIT-CXC, NE80-6077, 77 Massachusetts Ave., Cambridge, MA 02139, USA

(Received 19 August 2003 / Accepted 30 October 2003 )

We study the long term evolution of the relationship between the root mean square (rms) variability and flux (the "rms-flux relation") for the black hole Cygnus X-1 as monitored from 1996 to 2003 with the Rossi X-ray Timing Explorer (RXTE). We confirm earlier results by Uttley & McHardy (2001) of a linear relationship between rms and flux in the hard state on time scales  > 5 s reflecting in its slope the fractional rms variability. We demonstrate the perpetuation of the linear rms-flux relation in the soft and the intermediate state. The existence of a non-zero intercept in the linear rms-flux relation argues for two lightcurve components, for example, one variable and one non-variable component, or a possible constant rms component. The relationship between these two hypothesized components can be described by a fundamental dependence of slope and intercept at time scales  $\la$10 ksec with long term averages of the flux.

Key words: black hole physics -- stars: individual: Cyg X-1 -- X-rays: binaries -- X-rays: general

Offprint request: T. Gleissner, gleiss@astro.uni-tuebingen.de

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