Galactic Nuclei

HST photometry and ground-based kinematics of nearby galaxies revealed that most galaxies have to harbor a central massive black hole (SMBH) [44]. However, only few galactic nuclei show up as AGN in the optical, radio, or X-ray wave bands. This can be either explained by an extremely low accretion rate or by an accretion process, which is radiatively inefficient.

The closest example of such an inactive SMBH is the black hole in the center of the Milky Way, Sgr A*, with a quiescent X-ray luminosity in the 0.5-7 keV band of ~2x 1033 erg s-1, which sometimes shows flares (see, e.g., [2] and Chap. 19.3). Also M 31 harbors a SMBH in the center, but no bright X-ray source coincides with the M 31 center. Chandra images show a 2.5a evidence for a faint (~1036 erg s-1) discrete source consistent with the position of M 31* [25], indicating again a radiatively inefficient accretion flow. These and further deep X-ray observations of SMBHs in nearby galaxies will place severe constraints on the radiative processes in such a flow.

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