Long term variability of Cyg X-1

II. The rms-flux relation

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

Corresponding author: T. Gleissner, gleiss@astro.uni-tuebingen.de

Received: 19 August 2003
Accepted: 30 October 2003

Abstract

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 [CITE] 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 10 ksec with long term averages of the flux.