Schedule Feb 10, 2006
CR radiation spectra in SNR shocks near molecular clouds and other ISM environments: The case of SNR RX J1713.7-3946
M.A. Malkov, P.H. Diamond, R.Z. Sagdeev

The most favorable situation for proving that the main, hadronic CR component is accelerated in SNRs is when molecular clouds (MC) surround the SN shock. We suggest a new mechanism of spectrum formation in this situation. Using an analytic model of nonlinear diffusive shock acceleration, we calculate the spectra of protons and estimate the resulting $\gamma$-ray emission occurring when the SNR shock approaches a MC. We show that the spectrum develops a break in the TeV range and that its GeV component is suppressed. Possible applications of the new spectra to the recent CANGAROO and HESS observations of the SNR RX J1713.7-3946 are discussed.
We also suggest a physical mechanism whereby the acceleration time of CRs by shock waves can be significantly reduced with a simultaneousdevelopment of a break (knee) on the spectrum. This creates the possibility of particle acceleration beyond the galactic CR knee energy at $10^15$ eV. The acceleration results from a nonlinearmodification of the flow ahead of the shock supported by particles already accelerated to the knee momentum. The particles gain energy bybouncing off converging magnetic irregularities frozen into the flow in the shock precursor and not so much by re-crossing the shock itself. The acceleration rate is thus determined by the gradient of the flow velocity and turns out to be formally independent of the particle mean free path. The velocity gradient is, in turn, set bythe knee-particles. The acceleration rate of particles above the knee does not decrease with energy, unlike in the linear acceleration regime. The knee forms because particles above it are effectively confined to the shock while they are within limited domains in the momentum space, while other particles fall into \"loss-islands\",similar to the \"loss-cone\" of magnetic traps. This also maintains the steep velocity gradient and high acceleration rate.

Author entry (protected)