  We present simultaneous XMM-Newton and NuSTAR observations spanning 3-78 keV
of the nearest radio galaxy, Centaurus A (Cen A). The accretion geometry around
the central engine in Cen A is still debated, and we investigate possible
configurations using detailed X-ray spectral modeling. NuSTAR imaged the
central region of Cen A with sub-arcminute resolution at X-ray energies above
10 keV for the first time, but finds no evidence for an extended source or
other off-nuclear point-sources. The XMM-Newton and NuSTAR spectra agree well
and can be described with an absorbed power-law with a photon index {\Gamma} =
1.815 +/- 0.005 and a fluorescent Fe K{\alpha} line in good agreement with
literature values. The spectrum does not require a high-energy exponential
rollover, with a constraint of E_fold > 1 MeV. A thermal Comptonization
continuum describes the data well, with parameters that agree with values
measured by INTEGRAL, in particular an electron temperature kTe between
~100-300 keV, seed photon input temperatures between 5-50 eV. We do not find
evidence for reflection or a broad iron line and put stringent upper limits of
R < 0.01 on the reflection fraction and accretion disk illumination. We use
archival Chandra data to estimate the contribution from diffuse emission,
extra-nuclear point-sources, and the outer X-ray jet to the observed NuSTAR and
XMM-Newton X-ray spectra and find the contribution to be negligible. We discuss
different scenarios for the physical origin of the observed hard X-ray
spectrum, and conclude that the inner disk is replaced by an
advection-dominated accretion flow or that the X-rays are dominated by
synchrotron self-Compton emission from the inner regions of the radio jet or a
combination thereof.
