Long term variability of Cygnus X-1^⋆

VI. Energy-resolved X-ray variability 1999–2011

¹ Dr. Karl-Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics (ECAP), Friedrich Alexander Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: victoria.grinberg@fau.de
² Massachusetts Institute of Technology, Kavli Institute for Astrophysics, Cambridge MA 02139, USA
³ CRESST, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore MD 21250, USA
⁴ NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt MD 20771, USA
⁵ Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, 53121 Bonn, Germany
⁶ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁷ Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley CA 94720, USA
⁸ Laboratoire AIM, UMR 7158, CEA/DSM – CNRS – Université Paris Diderot, IRFU/SAp, 91191 Gif-sur-Yvette, France
⁹ Lawrence Livermore National Laboratory, 7000 East Ave., Livermore CA 94550, USA
¹⁰ Center for Astrophysics and Space Sciences, University of California San Diego, La Jolla, 9500 Gilman Drive, CA 92093, USA
¹¹ Alexander von Humboldt fellow  
¹² Ludwig-Maximilians University, Excellence Cluster “Universe”, Boltzmannstr. 2, 85748 Garching, Germany

Received: 1 November 2013
Accepted: 16 February 2014

Abstract

We present the most extensive analysis of Fourier-based X-ray timing properties of the black hole binary Cygnus X-1 to date, based on 12 years of bi-weekly monitoring with RXTE from 1999 to 2011. Our aim is a comprehensive study of timing behavior across all spectral states, including the elusive transitions and extreme hard and soft states. We discuss the dependence of the timing properties on spectral shape and photon energy, and study correlations between Fourier-frequency dependent coherence and time lags with features in the power spectra. Our main results follow. (a) The fractional rms in the 0.125–256 Hz range in different spectral states shows complex behavior that depends on the energy range considered. It reaches its maximum not in the hard state, but in the soft state in the Comptonized tail above 10 keV. (b) The shape of power spectra in hard and intermediate states and the normalization in the soft state are strongly energy-dependent in the 2.1–15 keV range. This emphasizes the need for an energy-dependent treatment of power spectra and a careful consideration of energy- and mass-scaling when comparing the variability of different source types, e.g., black hole binaries and AGN. PSDs during extremely hard and extremely soft states can be easily confused for energies above ~5 keV in the 0.125–256 Hz range. (c) The coherence between energy bands drops during transitions from the intermediate into the soft state but recovers in the soft state. (d) The time lag spectra in soft and intermediate states show distinct features at frequencies related to the frequencies of the main variability components seen in the power spectra and show the same shift to higher frequencies as the source softens. Our results constitute a template for other sources and for physical models for the origin of the X-ray variability. In particular, we discuss how the timing properties of Cyg X-1 can be used to assess the evolution of variability with spectral shape in other black hole binaries. Our results suggest that none of the available theoretical models can explain the full complexity of X-ray timing behavior of Cyg X-1, although several ansatzes with different physical assumptions are promising.