  The detection of very hot plasma in the quiescent corona is important for
diagnosing heating mechanisms. The presence and the amount of such hot plasma
is currently debated. The SphinX instrument on-board CORONAS-PHOTON mission is
sensitive to X-ray emission well above 1 keV and provides the opportunity to
detect the hot plasma component. We analyzed the X-ray spectra of the solar
corona collected by the SphinX spectrometer in May 2009 (when two active
regions were present). We modelled the spectrum extracted from the whole Sun
over a time window of 17 days in the 1.34-7 keV energy band by adopting the
latest release of the APED database. The SphinX broadband spectrum cannot be
modelled by a single isothermal component of optically thin plasma and two
components are necessary. In particular, the high statistics and the accurate
calibration of the spectrometer allowed us to detect a very hot component at ~7
million K with an emission measure of ~2.7 x 10^44 cm^-3. The X-ray emission
from the hot plasma dominates the solar X-ray spectrum above 4 keV. We checked
that this hot component is invariably present both at high and low emission
regimes, i.e. even excluding resolvable microflares. We also present and
discuss a possible non-thermal origin (compatible with a weak contribution from
thick-target bremsstrahlung) for this hard emission component. Our results
support the nanoflare scenario and might confirm that a minor flaring activity
is ever-present in the quiescent corona, as also inferred for the coronae of
other stars.
