  Recent general relativistic magneto-hydrodynamic (MHD) simulations of
accretion onto black holes have shown that, contrary to the basic assumptions
of the Novikov-Thorne model, there can be substantial magnetic stress
throughout the plunging region. Additional dissipation and radiation can
therefore be expected. We use data from a particularly well-resolved simulation
of accretion onto a non-spinning black hole to compute both the radiative
efficiency of such a flow and its spectrum if all emitted light is radiated
with a thermal spectrum whose temperature matches the local effective
temperature. This disk is geometrically thin enough (H/r ~= 0.06) that little
heat is retained in the flow. In terms of light reaching infinity (i.e., after
allowance for all relativistic effects and for photon capture by the black
hole), we find that the radiative efficiency is at least ~=6-10% greater than
predicted by the Novikov-Thorne model (complete radiation of all heat might
yield another ~6%). We also find that the spectrum more closely resembles the
Novikov-Thorne prediction for a/M ~= 0.2--0.3 than for the correct value,
a/M=0. As a result, if the spin of a non-spinning black hole is inferred by
model-fitting to a Novikov-Thorne model with known black hole mass, distance,
and inclination, the inferred a/M is too large by ~= 0.2--0.3.

High Energy Astrophysical Phenomena;General Relativity and Quantum Cosmology