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We calculate optically thick but geometrically thin (and slim) accretion diskmodels and perform a ray-tracing of photons (in the Kerr geometry) to calculatethe observed disk spectra. Previously, it was a common practice to ray-tracephotons assuming that they are emitted from the Kerr geometry equatorial plane,z = 0. We show that the spectra calculated with this assumption differ fromthese calculated under the assumption that photons are emitted from the actualsurface of the disc, z = H(r). This implies that a knowledge of the location ofthe thin disks effective photosphere is relevant for calculating the spectra.In this paper we investigate, in terms of a simple toy model, a possibleinfluence of the (unknown, and therefore ad hoc assumed) vertical dissipationprofiles on the vertical structure of the disk and thus on the location of theeffective photosphere, and on the observed spectra. For disks with moderate andhigh mass accretion rates (\dot m>0.01\dot m_C) we find that the photospherelocation in the inner, radiation pressure dominated, disk region (where most ofthe radiation comes from) does not depend on the dissipation profile andtherefore emerging disk spectra are insensitive to the choice of thedissipation function. For lower accretion rates the photosphere locationdepends on the assumed vertical dissipation profile down to the disk inneredge, but the dependence is very weak and thus of minor importance. We concludethat the spectra of optically thick accretion disks around black holes shouldbe calculated with the ray-tracing from the effective photosphere and that,fortunately, the choice of a particular vertical dissipation profile does notsubstantially influence the calculated spectrum.

Vertical dissipation profiles and the photosphere location in thin and slim accretion disks