The Megasecond Chandra X-ray Visionary Project Observation of NGC 3115: Nature of Hot Gas within the Bondi Radius
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. The closest billion solar mass black hole in NGC 3115 provides such an opportunity. We present results from our Megasecond Chandra X-ray Visionary Project (XVP) observation of NGC 3115 to study the most detailed spatially and spectroscopically resolved structures of the X-ray-emitting gas inside the Bondi radius a black hole. Temperature and density profiles of the hot gas were measured from a fraction out to tens of the Bondi radius (R_B = 2.4-4.8 arcsec = 112-224 pc). In contrary to the expectation that the temperature should be rising toward the center for the radiatively inefficient accretion flow in NGC 3115, we found that there is significant softer emission (~0.3 keV) within a scale of 3 arcsec (~150 pc) compared to a simple hot accretion model. A hotter thermal component of ~1 keV inside 3 arcsec, which is closer to the predictions of hot accretion models, is revealed when we modeled the projected spectra using a two component thermal model, with the cooler ~0.3 keV thermal component dominating the spectra. We discuss possible origins of the softer thermal component and implications of the results.