We model the visual light curve of CAL 87 based on the assumption that an accreting steadily burning white dwarf irradiates the accretion disk and the secondary star, as suggested by van den Heuvel et al. (1992). We use constraints on the geometry derived from the known orbital period. As sources of visual light we include the secondary star and an accretion disk with an optically thick, cold, clumpy spray at its riml presumably caused by an accretion stream of high mass flow rate impinging on the disk at the hot spot. This spray moving around the disk can account for the asymmetry in the light curve and the depth of the secondary minimum. It also might be the cause of the observed low X-ray luminosity if the white dwarf is permanently hidden by this disk.

We extended the disk corona model (Meyer & Meyer-Hofmeister 1994;Meyer, Liu, & Meyer-Hofmeister 2000a) to the inner region of galactic nuclei by including different temperatures in ions and electrons as well as Compton cooling. We found that the mass evaporation rate and hence the fraction of accretion energy released in the corona depend strongly on the rate of incoming mass flow from outer edge of the disk, a larger rate leading to more Compton cooling, less efficient evaporation and a weaker corona. We also found a strong dependence on the viscosity, higher viscosity leading to an enhanced mass flow in the corona and therefore more evaporation of gas from the disk below. If we take accretion rates in units of the Eddington rate our results become independent on the mass of the central black hole. The model predicts weaker contributions to the hard X-rays for objects with higher accretion rate like narrow-line Seyfert 1 galaxies (NLS1s), in agreement with observations. For luminous active galactic nuclei (AGN) strong Compton cooling in the innermost corona is so efficient that a large amount of additional heating is required to maintain the corona above the thin disk.

The condensation of matter from a corona to a cool, optically thick inner disk is investigated for black hole X-ray transient systems in the low/ hard state. A description of a simple model for the exchange of energy and mass between corona and disk originating from thermal conduction is presented, taking into account the effect of Compton cooling of the corona by photons from the underlying disk. It is found that a weak, condensation-fed inner disk can be present in the low/hard state of black hole transient systems for a range of luminosities that depends on the magnitude of the viscosity parameter. For $ 0.1Y0.4, an inner disk can exist for luminosities in the range $(0.001Y0.02) L Edd . The model is applied to the X-ray observations of the black hole candidate sources GX 339À4 and SWIFT J1753.5À0127 in their low/ hard state. It is found that Compton cooling is important in the condensation process, leading to the maintenance of cool inner disks in both systems. As the results of the evaporation/condensation model are independent of the black hole mass, it is suggested that such inner cool disks may contribute to the optical and ultraviolet emission of low-luminosity active galactic nuclei.

We apply the disk-corona evaporation model (Meyer & Meyer-Hofmeister) originally derived for dwarf novae to black hole systems. This model describes the transition of a thin cool outer disk to a hot coronal flow. The mass accretion rate determines the location of this transition. For a number of well-studied black hole binaries, we take the mass flow rates derived from a fit of the advection-dominated accretion flow (ADAF) model to the observed spectra (for a review, see Narayan, Mahadevan, & Quataert) and determine where the transition of accretion via a cool disk to a coronal flow/ADAF would be located for these rates. We compare this with the observed location of the inner disk edge, as estimated from the maximum velocity of the Halpha emission line. We find that the transition caused by evaporation agrees with this determination in stellar disks. We also show that the ADAF and the "thin outer disk + corona" are compatible in terms of the physics in the transition region.