Iron Kα Emission from the Low‐Luminosity Active Galaxies M81 and NGC 4579

Abstract

We report on the XMM-Newton spectroscopy of the low-luminosity active galaxies (LLAGNs) M81 and NGC 4579, both of which have known black hole masses and well-sampled spectral energy distributions (SEDs). The iron Kα line profiles from both LLAGNs can be described in terms of two components: a narrow line at 6.4 keV, and a moderately broad line (FWHM ~ 2 × 104 km s -1) arising from highly ionized, He-like or H-like species (E ~ 6.8 keV). We interpret the broad lines arising from an accretion disk, the inner edge of which is restricted to large radii (rin ~ 100rg). However, the Eddington ratio, L/LEdd, of these sources is 3-4 orders of magnitude lower than that required to photoionize a cold disk to He-like iron. We suggest that the lines can be explained as collisionally ionized X-ray lines arising from the transition region between a hot (radiatively inefficient) flow in the inner regions and a cold disk outside r ~ 100rg. The accretion flow geometry probed by our XMM-Newton observations is consistent with the truncated disk models proposed to explain the SED of LLAGNs.