Institut für Astronomie und Astrophysik

Abteilung Astronomie

Preprint E/98

RXTE Observation of Cygnus X-1: II. Timing Analysis

Michael A. Nowak(1), Brian A. Vaughan(2), Jörn Wilms(3), James B. Dove(1,4,5), Mitchell C. Begelman(1,5)

(1) JILA, University of Colorado, Campus Box 440, Boulder, CO 80309-0440, USA
(2) Space Radiation Laboratory, California Institute of Technology, MC 220-47, Pasadena, CA 91125, USA
(3) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(4) also, Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
(5) also, Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA

1999, Astrophysical Journal, 510, 874-891

Abstract. We present timing analysis for a Rossi X-ray Timing Explorer observation of Cygnus X-1 in its hard/low state. This was the first RXTE observation of Cyg X-1 taken after it transited back to this state from its soft/high state. RXTE's large effective area, superior timing capabilities, and ability to obtain long, uninterrupted observations have allowed us to obtain measurements of the power spectral density (PSD), coherence function, and Fourier time lags to a decade lower in frequency and half a decade higher in frequency than typically was achieved with previous instruments. Notable aspects of our observations include a weak 0.005 Hz feature in the PSD coincident with a coherence recovery; a `hardening' of the high-frequency PSD with increasing energy; a broad frequency range measurement of the coherence function, revealing rollovers from unity coherence at both low and high frequency; and an accurate determination of the Fourier time lags over two and a half decades in frequency. As has been noted in previous similar observations, the time delay is approximately proportional to f^{-0.7}, and at a fixed Fourier frequency the time delay of the hard X-rays compared to the softest energy channel tends to increase logarithmically with energy. Curiously, the 0.01 - 0.2 Hz coherence between the highest and lowest energy bands is actually slightly greater than the coherence between the second highest and lowest energy bands. We carefully describe all of the analysis techniques used in this paper, and we make comparisons of the data to general theoretical expectations. In a companion paper, we make specific comparisons to a Compton corona model that we have successfully used to describe the energy spectral data from this observation.

Key words: accretion - black hole physics - X-rays: binaries

Paper (143k gzip'ed Postscript including figures)

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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de) Last updated 28-Jul-1998