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~6.8keV). We interpret the broad lines arising from an accretion disk, the inner edge of which is restricted to large radii (r_in~100r_g). However, the Eddington ratio, L/L_Edd, 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~100r_g. The accretion flow geometry probed by our XMM-Newton observations is consistent with the truncated disk models proposed to explain the SED of LLAGNs.