2004ApJ...615L.121L

We calculate the temperature and ionization balance in an outflow from an accreting black hole under illumination by hard radiation from the central object. Electron scattering of the Fe Kα photons within the highly ionized expanding flow leads to a decrease of their energy (redshift), which is of first order in v/c, when v is the outflow velocity and v is much less than the speed of light c. This photon redshift is an intrinsic property of any outflow for which divergence is positive. We also find that the equivalent widths of red-skewed Fe Kα that originated in the wind is on the order of keV. We conclude that redshifted lines are intrinsic properties of the powerful outflows that are observed in many compact objects. Downscattering of the primary line photons generated in the outflow (a more natural and probable mechanism than the general relativistic effects in the innermost part of the accretion flow) leads to the formation of red-skewed lines.