Flat Spectrum X-Ray Emission from the Direction of a Molecular Cloud Associated with SNR RX J1713.7–3946

We report on the discovery of a hard X-ray source with ASCA from a molecularcloud in the vicinity of the SNR RX J1713.7-3946. The energy spectrum (1--10keV) shows a flat continuum which is described by a power-law with photon index1.0 +-0.4. We argue that this unusually flat spectrum can be best interpretedin terms of characteristic bremsstrahlung emission from theionization-loss-flattened distribution of either sub-relativistic protons ormildly-relativistic electrons. The strong shock of the SNR RX J1713.7-3946,which presumably interacts with the molecular cloud, as evidenced byobservations of CO-lines, seems to be a natural site of acceleration of suchsub- or mildly-relativistic nonthermal particles. However, the observed X-rayluminosity of 1.7 10^35 erg/s (for 6 kpc distance) requires that a huge kineticenergy of about 10^50 erg be released in the form of nonthermal particles toilluminate the cloud. The shock-acceleration at RX J1713.7-3946 can barelysatisfy this energetic requirement, unless (i) the source is located muchcloser than 6 kpc and/or (ii) the mechanical energy of the explosionessentially exceeds 10^51 erg. Another possibility would be that an essentialpart of the "lost" energy is somehow converted to plasma waves, which returnthis energy to nonthermal particles through their turbulent reacceleration onplasma waves. Irrespective of mechanisms responsible for production ofhigh-energy particles, the flat X-ray emission seems to be a signature of a newstriking energetic phenomenon in molecular clouds.Comment: 6 pages, 2 figures, Accepted for publication in PAS