  We report on the discovery of a hard X-ray source with ASCA from a molecular
cloud in the vicinity of the SNR RX J1713.7-3946. The energy spectrum (1--10
keV) shows a flat continuum which is described by a power-law with photon index
1.0 +-0.4. We argue that this unusually flat spectrum can be best interpreted
in terms of characteristic bremsstrahlung emission from the
ionization-loss-flattened distribution of either sub-relativistic protons or
mildly-relativistic electrons. The strong shock of the SNR RX J1713.7-3946,
which presumably interacts with the molecular cloud, as evidenced by
observations of CO-lines, seems to be a natural site of acceleration of such
sub- or mildly-relativistic nonthermal particles. However, the observed X-ray
luminosity of 1.7 10^35 erg/s (for 6 kpc distance) requires that a huge kinetic
energy of about 10^50 erg be released in the form of nonthermal particles to
illuminate the cloud. The shock-acceleration at RX J1713.7-3946 can barely
satisfy this energetic requirement, unless (i) the source is located much
closer than 6 kpc and/or (ii) the mechanical energy of the explosion
essentially exceeds 10^51 erg. Another possibility would be that an essential
part of the "lost" energy is somehow converted to plasma waves, which return
this energy to nonthermal particles through their turbulent reacceleration on
plasma waves. Irrespective of mechanisms responsible for production of
high-energy particles, the flat X-ray emission seems to be a signature of a new
striking energetic phenomenon in molecular clouds.
