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Astron. Astrophys. 363, 1013-1018 (2000)


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High frequencies in the power spectrum of Cyg X-1 in the hard and soft spectral states

M. Revnivtsev 1,2, M. Gilfanov 2,1 and E. Churazov 2,1

1 Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117810 Moscow, Russia
2 Max-Planck-Institute für Astrophysik, Karl-Schwarzschild-Strasse 1, 85740 Garching bei München, Germany

Received 3 July 2000 / Accepted 19 September 2000

Abstract

We analyzed a large number of RXTE/PCA observations of Cyg X-1 in the hard and soft spectral states with total exposure time of [FORMULA] and [FORMULA] ksec respectively and time resolution better than [FORMULA]s in order to investigate its variability down to a few milliseconds time scales. The model of modifications of the power density spectra due to the dead time effect was tested using RXTE observations of an extremely bright source - Sco X-1. The results of these tests demonstrated, that although some problems still remain, for sources like Cyg X-1 (which is an order of magnitude less bright than Sco X-1) present knowledge of the instrument is sufficient to reliably analyze the power spectra in the kHz frequency range.

In both spectral states of Cyg X-1 we detected statistically significant variability up to [FORMULA]150-300 Hz with a fractional rms amplitude above 100 Hz at the level of [FORMULA]. The power spectrum in the hard state shows a steepening at frequency of [FORMULA] Hz with the power law slope changing from [FORMULA] to [FORMULA]. In the soft state the slope of power spectrum changes from [FORMULA] to [FORMULA] at the frequency of [FORMULA] Hz without any evidence of further steepening up to [FORMULA] Hz. These break frequencies represent the highest characteristic frequencies detected in the power density spectrum of Cyg X-1 so far.

Key words: accretion, accretion disks – instabilities – stars: binaries: general – stars: neutron – X-rays: general – X-rays: stars

Send offprint requests to: M. Revnivtsev (revnivtsev@hea.iki.rssi.ru)

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000

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