Massive thin accretion discs -I. Calculated spectra

We present detailed calculations of the structure and the spectrum of massive, geometrically thin, 'bare' accretion discs. The calculations are for an a-disc, with various assumptions about the viscosity and full relativistic corrections. The radiative transfer equations are solved using the Eddington approximation for an atmosphere with a vertical temperature gradient. All significant sources of opacity, for T> 10⁴ K, are included, and all models are found to be optically thick throughout. The requirement of a geometrically thin disc forces a limit on the accretion rate, of L<0.3L_edd. Several previous disc calculations violate this limit and their results are questionable. All discs considered in this work are found to be radiation pressure dominated throughout the region where self-gravity dominates. Spectral changes due to electron scattering (modified blackbody and comptonization) are not significant in most models. The surface temperature is close to the effective temperature, even for regions where electron scattering effects are significant, due to the vertical temperature gradient, in contradiction to earlier findings. The upper limit on the accretion rate indicates that thin discs, with no corona, may not have enough soft X-rays to explain the observations of bright quasars. Relativistic effects modify the spectrum, considerably, at large viewing angles. We show several examples for this and calculate the angular dependence of the ionizing radiation and photons flux. This may have important implications on the modelling of AGN emission lines.