Yet another plasma diagnostic with He-like triplet: Probing energetic electrons behind SNR shocks with ASTRO-H SXS

We present a new X-ray line diagnostic to probe energetic electrons behind SNR shocks. SNR shocks are believed to be acceleration sites of the Galactic cosmic rays up to knee energy. In the early stage of acceleration, particles must have sufficient energies to cross the shock to enter diffusive shock acceleration. This requires supra-thermal energies for electrons, however, we currently do not know how electrons depart from the thermal pool to attain such energies. Hence observational constraints on the amount and energy distribution of supra-thermal electrons are awaited. Here we propose a new X-ray line diagnostic using the He-like triplet to detect and characterise the energy distribution of supra-thermal electrons. We simulate X-ray spectra of SNR plasma interacting with energetic electrons and find that the forbidden line of the He-like triplet is enhanced via inner-shell ionisation process of Li-like ions. Such an effect can be precisely measured by high-resolution spectroscopy with the coming ASTRO-H SXS instrument.