Geometrically Thin Sub-Eddington AGN Accretion Disks show a suppressed Lyman Edge

Abstract

The observed ultraviolet continua of active galactic nuclei (AGN) generally lack a strong intrinsic HI Lyman edge predicted by classical optically thick accretion disk atmosphere models. We revisit this long-standing problem using our previous sub-Eddington ($L/L_{\rm Edd}\sim 0.03$) geometrically thin 3D accretion disk simulation evolved with gray angle-dependent radiation magnetohydrodynamics (MHD) around a $10^8M_{\odot}$ black hole. This disk is magnetic pressure dominated, and has surface densities more than two 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 HI 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.