OBSERVATIONS OF THE X-RAY SOURCE CYGNUS-X-1 FROM THE ROENTGEN OBSERVATORY ABOARD THE ORBITING MIR-KVANT MODULE

Between 1988 and 1994, the temporal and spectral behavior of the X-ray source Cygnus X-1, a black-hole candidate, was studied by the instrum ents of the Roentgen Observatory in the energy range 2 - 200 keV. Most of the time, the source was in a hard spectral state with a spectrum characteristic of Comptonization. ?he daily mean flux from the source in the energy range 45 - 140 keV varied approximately twofold, from si milar to 0.07 to similar to 0.15 photons s(-1) cm(-2). Considerable ch aotic fluctuations of the flux with an amplitude of the deviation from the mean as large as 15% were also observed on a time scale of a day. No systematic relation between the spectral hardness of Cygnus X-1 an d its luminosity was found, In February 1994, the hardness over the en tire X-ray range was significantly reduced, and a soft component with a blackbody temperature of similar to 0.5 keV appeared in the spectrum . The HEXE instrument detected a low-frequency noise in the power-dens ity spectrum of the source. In the frequency range 10(-2) - 10 Hz, the noise power-density spectrum is well fitted by a plateau in the low-f requency domain and by a power-law fall-off in the high-frequency doma in. The frequency corresponding to a break in the spectrum and the amp litude of the plateau showed statistically significant changes during the observations. One of the power spectra exhibited a broad (FWHM of the order of the central frequency) feature which may be interpreted a s being a peak of quasi-periodic oscillations. Measurements of the sou rce's mts in various energy bands suggest that the overall spectrum an d the spectrum of the variable component were identical. A statistical ly significant (3.5 sigma) correlation was also found between the lumi nosity of Cy,onus X-1 and its rms.