Radiative Efficiency and Thermal Spectrum of Accretion Onto Schwarzschild Black Holes

Recent general relativistic magnetohydrodynamic (MHD) simulations of accretion onto black holes (BHs) have shown that, contrary to the basic assumptions of the Novikov–Thorne (NT) 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 BH 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 BH), we find that the radiative efficiency is at least ≃ 6%–10% greater than predicted by the NT model (complete radiation of all heat might yield another ≃ 6%). We also find that the spectrum more closely resembles the NT 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 BH is inferred by model fitting to an NT model with known BH mass, distance, and inclination, the inferred a / M is too large by ≃ 0.2–0.3.