SPECTRUM OF A RELATIVISTIC ACCRETION DISK IN ACTIVE GALACTIC NUCLEI

We have calculated the spectrum of a relativistic accretion disk aroun d a non-rotating black hole seen from a distant observer. Radiation-tr ansfer equations were solved to obtain a spectral deformation due to t he Compton scattering within the disk, and the ray-tracing method was adopted to calculate the trajectories of photons emerging from the sur face of the disk. When a photon travels from deep inside the disk towa rds the surface, it becomes Compton up-scattered, producing a soft X-r ay excess in the X-ray spectrum, as is observed in active galactic nuc lei. We find that this tendency is enhanced by the general relativisti c Doppler and gravitational lensing effects, whereas the emissivity of a relativistic disk around a non-rotating black hole is systematicall y lower than that of a Newtonian disk for the same mass-accretion rate . As a result, these two opposite effects are counteracted when the in clination angle is large, i approximately 80-degrees; both relativisti c and Newtonian disks produce similar spectra. Even when the inclinati on angle is small, i approximately 0-degree, on the other hand, simila r spectra can be produced, if the mass-flow rate in the relativistic m odel is taken to be by a factor of approximately 1.7 higher than that of the Newtonian model.