On the role of the UV and X-ray radiation in driving a disk wind in X-ray binaries

摘要

X-ray heating of the photosphere of an accretion disk is a possible mechanismto produce strong, broad UV emission lines in low mass X-ray binaries (LMXBs).However, detailed photoionization calculations show that this mechanism failsto produce sufficient emission measure. We present the results ofhydrodynamical calculations of the disk photosphere irradiated by strongX-rays. We attempt to determine whether LMXBs can harbor significant UV-drivendisk winds despite the effects of X-ray ionization. Such winds would be alikely candidate for the site of emission of UV lines and may better explainthe observations than the X-ray heated disk photosphere. We find that the localdisk radiation cannot launch a wind from the disk because of strong ionizingradiation from the central object. Unphysically high X-ray opacities would berequired to shield the UV emitting disk and allow the line force to drive adisk wind. However the same X-ray radiation that inhibits line driving heatsthe disk and can produce a hot bipolar wind or corona above the disk. To assessthe impact of X-ray heating upon driving of a disk wind by the line force inany system with an accretion disk we derive analytic formulae. In particular,we compare results of line-driven disk wind models for accretion disks in LMXBsand active galactic nuclei. The latter show spectral features associated with astrong and fast disk wind. The key parameter determining the role of the lineforce is not merely the presence of the luminous UV zone in the disk and thepresence of the X-rays, but also the distance of this UV zone from the center.