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

Abstract

Abstract We report on the discovery of a diffuse X-ray source with ASCA, presumably associated with a molecular cloud in the vicinity of the supernova remnant RX J1713.7$-$3946. The energy spectrum (1–10 keV) of the hard X-ray source shows a flat continuum, which is described by a power-law with a photon index of $ \Gamma = 1.0^{+0.4}_{-0.3}$. We argue that this unusually flat spectrum can be best interpreted in terms of characteristic bremsstrahlung emission from the loss-flattened distribution of either sub-relativistic protons or mildly relativistic electrons. The strong shock of RX J1713.7$-$3946, which is likely to interact with the molecular cloud, as evidenced by CO-line observations, seems to be a natural site of acceleration of such nonthermal particles. The observed luminosity of $ L_\mathrm{X} = 1.7 \times 10^{35} \,\mathrm{erg} \,\mathrm{s}^{-1}$ (for a distance of 6 kpc) seems to require a huge kinetic energy of about $ 10^{50} \,\mathrm{erg}$ 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 the preferred distance of 6 kpc and/or (ii) the mechanical energy of the supernova explosion essentially exceeds $ 10^{51} \,\mathrm{erg}$. Another possibility would be that an essential part of the lost energy due to the ionization and heating of gas, is somehow converted to plasma waves, which return this energy to nonthermal particles through their turbulent reacceleration on the plasma waves.