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Register a new userResolving the Bondi Accretion Flow toward the Supermassive Black Hole of NGC 3115 with Chandra
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Ka-Wah Wong
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Gas undergoing Bondi accretion onto a supermassive black hole (SMBH) becomes hotter toward smaller radii. We searched for this signature with a Chandra observation of the hot gas in NGC 3115, which optical observations show has a very massive SMBH. Our analysis suggests that we are resolving, for the first time, the accretion flow within the Bondi radius of an SMBH. We show that the temperature is rising toward the galaxy center as expected in all accretion models in which the black hole is gravitationally capturing the ambient gas. There is no hard central point source that could cause such an apparent rise in temperature. The data support that the Bondi radius is at about 4 arcsec-5 arcsec (188-235 pc), suggesting an SMBH of 2 x 10^9 M_sun that is consistent with the upper end of the optical results. The density profile within the Bondi radius has a power-law index of 1.03^{+0.23}_{-0.21} which is consistent with gas in transition from the ambient medium and the accretion flow. The accretion rate at the Bondi radius is determined to be {dot M}_B = 2.2 x 10^{-2} M_sun yr^{-1}. Thus, the accretion luminosity with 10% radiative efficiency at the Bondi radius (10^{44} erg s^{-1}) is about six orders of magnitude higher than the upper limit of the X-ray luminosity of the nucleus.
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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 (XVP) 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 (R_B = 2.4-4.8 arcsec = 112-224 pc). The projected temperature jumps significantly from ~0.3 keV beyond 5 arcsec to ~0.7 keV within ~4-5 arcsec, but then abruptly drops back to ~0.3 keV within ~3 arcsec. 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 arcsec (~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 $sim 150$~pc from the black hole. The density profile is broadly consistent with rho ~ 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.
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