Prospects for AGN Studies with AXIS: AGN Fueling—Resolving Hot Gas inside Bondi Radius of SMBHs

Abstract

Hot gas around a supermassive black hole (SMBH) should be captured within the gravitational “sphere of in?uence”, characterized by the Bondi radius. Deep Chandra observations have spatially resolved the Bondi radii of ?ve nearby SMBHs that are believed to be accreting in hot accretion mode. Contrary to earlier hot accretion models that predicted a steep temperature increase within the Bondi radius, none of the resolved temperature pro?les exhibit such an increase. The temperature inside the Bondi radius appears to be complex, indicative of a multi-temperature phase of hot gas with a cooler component at about 0.2–0.3 keV. The density pro?les within the Bondi regions are shallow, suggesting the presence of strong out?ows. These ?ndings might be explained by recent realistic numerical simulations that suggest that large-scale accretion inside the Bondi radius can be chaotic, with cooler gas raining down in some directions and hotter gas out?owing in others. With an angular resolution similar to Chandra and a signi?cantly larger collecting area, AXIS will collect enough photons to map the emerging accretion ?ow within and around the “sphere of in?uence” of a large sample of active galactic nuclei (AGNs). AXIS will reveal transitions in the in?ow that ultimately fuels the AGN, as well as out?ows that provide feedback to the environment. This White Paper is part of a series commissioned for the AXIS Probe Concept Mission; additional AXIS White Papers can be found at the AXIS website.