SELF-CONSISTENT THERMAL ACCRETION DISK CORONA MODELS FOR COMPACT OBJECTS .1. PROPERTIES OF THE CORONA AND THE SPECTRUM OF ESCAPING RADIATION

We present the properties of accretion disk corona (ADC) models in whi ch the radiation field, the temperature, and the total opacity of the corona are determined self-consistently. We use a nonlinear Monte Carl o code to perform the calculations. As an example, we discuss models i n which the corona is situated above and below a cold accretion disk w ith a plane-parallel (slab) geometry, similar to the model of Haardt & Maraschi. By Comptonizing the soft radiation emitted by the accretion disk, the corona is responsible for producing the high-energy compone nt of the escaping radiation. Our models include the reprocessing of r adiation in the accretion disk. Here the photons either are Compton-re flected or photoabsorbed, giving rise to fluorescent line emission and thermal emission. The self-consistent coronal temperature is determin ed by balancing heating (due to viscous energy dissipation) with Compt on cooling, determined using the fully relativistic, angle-dependent c ross sections. The total opacity is found by balancing pair production s with annihilations. We find that, for a disk temperature kT(BB) less than or similar to 200 eV, these coronae are unable to have a self-co nsistent temperature higher than similar to 140 keV if the total optic al depth is greater than or similar to 0.2, regardless of the compactn ess parameter of the corona and the seed opacity. This limitation corr esponds to the angle-averaged spectrum of escaping radiation having a photon index greater than or similar to 1.8 within the 5-30 keV band. Finally, all models that have reprocessing features also predict a lar ge thermal excess at lower energies. These constraints make explaining the X-ray spectra of persistent black hole candidates with ADC models very problematic.