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Astron. Astrophys. 352, 182-188 (1999)

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Reflection and noise in Cygnus X-1

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

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

Received 24 June 1999 / Accepted 31 August 1999


We analyzed RXTE/PCA observations of Cyg X-1 from 1996-1998. We found a tight correlation between the characteristic noise frequencies (e.g. the break frequency [FORMULA]) and the spectral parameters in the low spectral state. The amplitude of reflection increases and the spectrum of primary radiation steepens as the noise frequency increases ([FORMULA] changes by a factor of [FORMULA] in our sample). This can be understood assuming that increase of the noise frequency is associated with the shift of the inner boundary of the optically thick accretion disk towards the compact object. The related increase of the solid angle, subtended by the disk, and of the influx of the soft photons to the Comptonization region lead to an increase of the amount of reflection and steepening of the Comptonized spectrum. The correlation between the slope of primary radiation and the amplitude of reflection extends to the soft spectral state.

A similar correlation between reflection and slope was found for the frequency resolved spectra in the 0.01-15 Hz frequency range. Such a correlation could appear if the longer time scale variations are associated with emission originating closer to the optically thick disk and, therefore, having steeper Comptonized spectra with larger reflection.

Key words: accretion, accretion disks – black hole physics – stars: binaries: general – stars: individual: Cyg X-1 – X-rays: general – X-rays: stars

Send offprint requests to: gilfanov@mpa-garching.mpg.de

© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999