Geometrically Thin Sub-Eddington Active Galactic Nuclei Accretion Disks Show a Suppressed Lyman Edge

Abstract

Abstract The observed UV continua of active galactic nuclei (AGN) generally lack a strong intrinsic H I Lyman edge predicted by classical optically thick accretion disk atmosphere models. We revisit this long-standing problem using our previous sub-Eddington ( L / L Edd ∼ 0.03) geometrically thin 3D accretion disk simulation evolved with gray angle-dependent radiation magnetohydrodynamics (MHD) around a 10 8 M ⊙ black hole. This disk is magnetic-pressure-dominated and has surface densities more than 2 orders of magnitude below the corresponding radiation-pressure-supported α -disk prediction. By restarting the simulation with multiple frequency groups, we compute the emergent continuum directly from the frequency-dependent radiation fluxes and find no sharp H I Lyman edge at 13.6 eV. This suppressed Lyman edge is mainly caused by the low densities in a magnetic-pressure-supported disk where the Lyman opacity jump is far weaker than in standard disk models. These results indicate that magnetic pressure support may provide a possible solution to reconciling optically thick AGN disks with the smooth observed continuum emission around the Lyman edge. However, in this simulation, a substantial fraction of the emission near 13.6 eV is expected to originate at smaller radii closer to the central black hole, which is outside our simulation domain. More detailed predictions of the spectrum will require an extension to the inner disk, non-LTE radiation transfer, higher frequency resolution, and a survey across a broader range of accretion rates.