X-ray Images of Hot Accretion Flows

We consider the X-ray emission due to bremsstrahlung processes from sphericallysymmetric low radiative-efficiency hot accretion flows around supermassive and galactic black holes. We calculate surface brightness profiles and Michelson visibility functions for a range of density profiles, ρ ∼ r−3/2+p, with 0 < p < 1, to allow for the presence of outflows. We find that although the 1 keV emitting region in these flows can always extend up to 10 Schwarzschild radii (RS), their surface brightness profiles and visibility functions are strongly affected by the specific density profile. The advection-dominated solutions with no outflows (p = 0) lead to centrally peaked profiles with characteristic sizes of only a few tens of RS. Solutions with strong outflows (p ∼ 1) lead to flat intensity profiles with significantly larger characteristic sizes of up to 10RS. This implies that low luminosity galactic nuclei, such as M87, may appear as extended X-ray sources when observed with current X-ray imaging instruments. We show that X-ray brightness profiles and their associated visibility functions may be powerful probes for determining the relevant mode of accretion and, in turn, the properties of hot accretion flows. We discuss the implications of our results for observations with the Chandra X-ray Observatory and the proposed X-ray interferometer MAXIM. Subject headings: accretion, accretion flows – black hole physics – radiation mechanisms: thermal bremsstrahlung – galaxies: nuclei – X-rays: galaxies