  We report the detection of hard X-ray emission components in the spectra of
six nearby, giant elliptical galaxies observed with the ASCA satellite. The
systems studied, which exhibit strong dynamical evidence for supermassive black
holes in their nuclei, are M87, NGC 1399 and NGC 4696 (the dominant galaxies of
the Virgo, Fornax and Centaurus clusters, respectively) and NGC 4472, 4636 and
4649 (three further giant ellipticals in the Virgo cluster). The ASCA data for
all six sources provide clear evidence for hard emission components, which can
be parameterized by power-law models with photon indices in the range \Gamma =
0.6-1.5 (mean value 1.2) and intrinsic 1-10 keV luminosities of 2 \times
10^{40} - 2 \times 10^{42} erg/s. Our results imply the identification of a new
class of accreting X-ray source, with X-ray spectra significantly harder than
those of binary X-ray sources, Seyfert nuclei or low luminosity AGN, and
bolometric luminosities relatively dominated by their X-ray emission. We
discuss various possible origins for the hard X-ray emission and argue that it
is most likely to be due to accretion onto the central supermassive black
holes, via low-radiative efficiency accretion coupled with strong outflows. In
the case of M87, our detected power-law flux is in good agreement with a
previously-reported measurement from ROSAT High Resolution Imager observations,
which were able to resolve the jet from the nuclear X-ray emission components.
We confirm previous results showing that the use of multiphase models in the
analysis of the ASCA data leads to determinations of approximately solar
emission-weighted metallicities for the X-ray gas in the galaxies. We also
present results on the individual element abundances in NGC 4636.
