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

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 <sup>2,1

1</sup> 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 and ksec respectively and time resolution better than 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 150-300 Hz with a fractional rms amplitude above 100 Hz at the level of . The power spectrum in the hard state shows a steepening at frequency of Hz with the power law slope changing from to . In the soft state the slope of power spectrum changes from to at the frequency of Hz without any evidence of further steepening up to 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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