On the gamma-ray spectra radiated by protons accelerated in SNR shocks near molecular cloud.pdf

a r X i v : a s t r o - p h / 0 5 0 3 4 0 3 v 1 1 7 M a r 2 0 0 5 On the gamma-ray spectra radiated by protons accelerated in SNR shocks near molecular clouds: The case of SNR RX J1713.7-3946 M.A. Malkov 1 , P.H. Diamond 1 and R.Z. Sagdeev 2 Abstract Cosmic rays (CRs) are thought to be accelerated in SNRs. The most favorable sit- uation for proving that the main, hadronic CR component is accelerated there is when CRs interact with dense gases, such as molecular clouds (MC) which surround the SN shock. Here, a new mechanism of spectrum formation in partially ionized gases near SNRs is proposed. Using an analytic model of nonlinear diffusive shock acceleration, we calculate the spectra of protons and estimate the resulting γ-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. These modifications to the standard theory occur because of the proximity of the partially ionized MC-gas and because of the physics of particle and Alfven wave propagation inside the gas. Possi- ble applications of the new spectra to the recent CANGAROO and HESS observations of the SNR RX J1713.7-3946 are discussed. Subject headings: acceleration of particles—cosmic rays —shock waves—supernova remnants—turbulence 1. Introduction Cosmic rays (CRs) travel to us through the chaotic magnetic field of the Galaxy. Therefore, the only way to connect them to their accelerator is through the on-site radiation. Accelerated electrons have already been detected in supernova remnant (SNR) shocks (Koyama et al. 1995). However, electrons comprise only 1-2% of the CR intensity above 4-5 GeV. Therefore, distin- guishing between leptonic and hadronic origin of the observed TeV emission from SNRs is a key to the proof of the supernova origin of CRs. Simultaneous monitoring of X- and γ? energy bands is an indispensable tool for that purpose. The same TeV electrons radiate in X- and γ-rays via syn- chrot