The Megasecond Chandra X-Ray Visionary Project Observation of NGC 3115: Witnessing the Flow of Hot Gas within the Bondi Radius

Ka-Wah Wong, Minnesota State University Mankato
Jimmy A. Irwin, University of Alabama - Tuscaloosa
Roman V. Shcherbakov, University of Maryland at College Park
Mihoko Yukita, Johns Hopkins University
Evan T. Million, University of Alabama - Tuscaloosa
Joel N. Bregman, University of Michigan - Ann Arbor


Observational confirmation of hot accretion model predictions has been hindered by the challenge to resolve spatially the Bondi radii of black holes with X-ray telescopes. Here, we use the Megasecond Chandra X-ray Visionary Project observation of the NGC 3115 supermassive black hole to place the first direct observational constraints on the spatially and spectroscopically resolved structures of the X-ray emitting gas inside the Bondi radius of a black hole. We measured temperature and density profiles of the hot gas from a fraction out to tens of the Bondi radius (RB = 24-48 = 112-224 pc). The projected temperature jumps significantly from ~0.3 keV beyond 5'' to ~0.7 keV within ~4''-5'', but then abruptly drops back to ~0.3 keV within ~3''. This is contrary to the expectation that the temperature should rise toward the center for a radiatively inefficient accretion flow. A hotter thermal component of ~1 keV inside 3'' (~150 pc) is revealed using a two-component thermal model, with the cooler ~0.3 keV thermal component dominating the spectra. We argue that the softer emission comes from diffuse gas physically located within ~150 pc of the black hole. The density profile is broadly consistent with ρ∝r –1 within the Bondi radius for either the single temperature or the two-temperature model. The X-ray data alone with physical reasoning argue against the absence of a black hole, supporting that we are witnessing the onset of the gravitational influence of the supermassive black hole.