High-resolution X-ray spectroscopy of a low-luminosity active galactic nucleus: The structure and dynamics of M81

We present Chandra HETGS observations of the low-luminosity active galactic nucleus (LLAGN) of M81. The HETGS is unique in providing high-resolution spectroscopy of the central 1″ of M81, including the iron K bandpass. The continuum is a power law of photon index Γ = 1.8, similar to that seen in highly luminous AGNs. Highly ionized emission lines, characteristic of gas at temperatures off = 10⁶-10⁸ K, are detected. Many of these thermal lines are velocity broadened, with a FWHM of approximately 1500 km s~ '. A separate thermal component is associated with a 2557 km s^_1 redshifted Fe XXXIemission line, characteristic of gas at temperatures T = 10^7.4-10⁸ K. Neutral Fe, Ar, and Si Kα fluorescence lines indicate the presence of cold, dense material. The Si Kα fluorescence line is velocity broadened, with a FWHM of 1200 km s^-1. If the fluorescence lines are produced by reflection from cold, Compton thick material, then the line strengths are not compatible with solar abundances, instead favoring enhanced Ar and Si abundances with respect to the Fe abundance. The Fe Kα line is narrow, with no evidence of a thin disk extending inside 55r_g (where r_g = GM/c² is the gravitational radius for a black hole of mass M). We show that a simple spectral model used to represent the expectations from a radiatively inefficient accretion flow (RIAF ) describes the X-ray data well, while in a companion paper we will show that jet models with parameters similar to fits of hard state X-ray binaries describe both the X-ray and broadband (radio/optical) spectra. The HETGS spectra we present here offer an unprecedented view of the inner workings of a low-luminosity accretion flow, and thus can quantitatively constrain theoretical accretion flow models of LLAGNs such as M81*.