Are There ADAFs in Nearby Giant Elliptical Galaxies?
  Recently it has been suggested that the hard x-ray power-law tails exhibited in the ASCA spectra of several nearby giant elliptical galaxies are caused by low-radiative efficiency accretion flows (ADAFs) onto a massive black hole in the centers of the galaxies. The estimated fluxes from these central ADAFs as derived from the ASCA x-ray spectral analyses indicate that they may be visible as x-ray point sources. We analyze archival ROSAT HRI images of three such galaxies, NGCs 1399, 4636, and 4696, to determine whether point x-ray sources consistent with ASCA-derived fluxes are present in the centers of the galaxies. We find that the upper limit for the flux of a central point x-ray source in each of these galaxies as determined by the ROSAT HRI data is only marginally consistent with the predicted flux inferred from the ASCA spectra. We suggest that although a central point source, such as a ADAF associated with a massive black hole, cannot be completely ruled out, they can only account for a fraction of the flux of the observed hard x-ray power law components. 
Martin E. Sulkanen (1 and 2), Joel N. Bregman (2) ((1) Space Sciences   Laboratory, NASA Marshall Space Flight Center, (2) D...