  Current observations of supernova remnant (SNR) RX J1713.7-3946 favor the
leptonic scenario for the TeV emission, where the radio to X-ray emission is
produced via the synchrotron process and the $\gamma$-ray emission is produced
via the inverse Comptonization of soft background photons, and the electron
distribution can be inferred from the observed $\gamma$-ray spectrum with a
spectral inversion method. It is shown that the observed correlation between
the X-ray and $\gamma$-ray brightness of SNR RX J1713.7-3946 can be readily
explained with the assumption that the energy density of energetic electrons is
proportional to that of the magnetic field in such a scenario. A 2D
magnetohydrodynamic simulation is then carried out to model the overall
emission spectrum. It is found that the total energy of electrons above $\sim
1$ GeV is equal to that of the magnetic field. This is the first piece of
observational evidence for energy equipartition between energetic electrons and
magnetic field in the downstream of strong collision-less astrophysical shocks
of SNRs.
