Compton downscattering in Cyg X - 3 3 3 THE EFFECT OF COMPTON SCATTERING ON TIMING PROPERTIES

We consider the effect of Compton scattering by a Thomson-thick, low-temperature, plasma cloud surrounding the compact object in the binary system Cyg X-3 on its X-ray energy and power spectra. The presence of such a cloud was earlier inferred from the energy-independent orbital modulation of the X-ray flux and the lack of high frequencies in its power spectra. Here, we consider the effect of Compton downscattering by the cloud on the X-ray spectra, concentrating on the hard spectral state. The process reduces the energy of the high-energy break/cutoff in the spectra, which allows to determine the Thomson optical depth. This, in turn, allows determining the size of the plasma from the observed cutoff in the power spectrum, which we find to be ∼ (2–3)× 109 cm. At this size, the cloud will be in thermal equilibrium in the photon field of the X-ray source only, which yields the plasma temperature of ≃ 3 keV. At this temperature, the Thomson optical depth implied by the X-ray spectrum is ∼ 6. The physical origin of the cloud is likely to be collision of the very strong stellar wind of the companion Wolf-Rayet star with a small accretion disc formed by the wind accretion.