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

X-ray heating of the photosphere of an accretion disk is a possible mechanism to produce strong, broad UV emission lines in low-mass X-ray binaries (LMXBs). However, detailed photoionization calculations show that this mechanism fails to produce sufficient emission measure at suitable ionization and optical depth. We present the results of hydrodynamical calculations of the disk photosphere irradiated by strong X-rays. We attempt to determine whether LMXBs can harbor significant UV-driven disk winds despite the effects of X-ray ionization. Such winds would be a likely candidate for the site of emission of UV lines and may better explain the observations than the X-ray-heated disk photosphere. We find that the local disk radiation cannot launch a wind from the disk because of strong ionizing radiation from the central object. Unphysically high X-ray opacities would be required to shield the UV-emitting disk and allow the line force to drive a disk wind. However, the same X-ray radiation that inhibits line driving heats the disk and can produce a hot bipolar wind or corona above the disk. Our calculations are generally consistent with past work on the dynamics of coronae and winds from accretion disks in LMXBs. Additionally, our results are consistent with the UV observations of LMXB that show no obvious spectral features associated with strong and fast disk winds. To assess the impact of X-ray heating on driving of a disk wind by the line force in any system with an accretion disk, we derive analytic formulae. In particular, we compare results of line-driven disk wind models for accretion disks in LMXBs and active galactic nuclei. The latter show spectral features associated with a strong and fast disk wind, the wind that has been successfully modeled by Proga, Stone, & Kallman. The key parameter determining the role of the line force is not merely the presence of the luminous UV zone in the disk and the presence of the X-rays but also the distance of this UV zone from the center. The closer the UV zone to the center, the stronger the line force and subsequently the denser the disk wind launched by the line force. The density of the disk wind critically determines whether the wind will stay in a lower ionization state in the presence of the X-ray radiation and be further accelerated by the line force to supersonic velocities.