Intrinsic Energy Cut-off in Diffusive Shock Acceleration: Possible Reason for Non-detection of TeV-protons in SNRs

Details Intrinsic Energy Cut-off in Diffusive Shock Acceleration: Possible Reason for Non-detection of TeV-protons in SNRs Intrinsic Energy Cut-off in Diffusive Shock Acceleration: Possible Reason for Non-detection of TeV-protons in SNRs

Dec 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aas...197.8011m&link_type=abstract

American Astronomical Society, 197th AAS Meeting, #80.11; Bulletin of the American Astronomical Society, Vol. 33, p.709

Astronomy and Astrophysics

Astronomy

Scientific paper

The theory of shock acceleration predicts the maximum particle energy to be limited only by the acceleration time and the shock size. This led to optimistic estimates for the galactic cosmic ray energy achievable in the SNR shocks. The estimates imply that the accelerated particles, while making no strong impact on the shock structure (test particle approach) are still scattered by strong self-generated Alfven waves (turbulent boost) needed to accelerate particles quickly. These two assumptions are, however, in conflict when applied to SNRs of the age required for cosmic ray acceleration to the ``knee'' energy. We study the combined effect of acceleration nonlinearity (shock modification by acclerated particles) and the turbulent boost of acceleration on the maximum energy achievable by this mechanism in a given time. We show that the refraction to shorter wave lengths in the nonlinearly modified flow causes enhanced losses of particles in the momentum range pmax/R < p < pmax , where R > 1 is the nonlinear pre-compression of the flow and pmax is the absolute maximum momentum that could be reached in an unimpeded (linear, but turbulently boosted) acceleration process. The particle spectrum behaves as p{-σ }e-√ {p//line{p}} at p > p*= pmax/R as opposed to the conventional power-law p{-σ } at p < p* . Since R itself is proportional to /line{p} that characterizes the energy content of accelerated particlles (since σ < 4 and p* < /line{p} < pmax ) the observationally important spectral break p* should grow slower than √ {pmax} . Moreover, due to the lack of particles at pmax (and thus waves in resonance with them) caused by the spectral break at p*}=p{max/R , the pmax(t) itself should advance much slower than the estimates based on the Bohm diffusion would predict. These nonlinear mechanisms of momentum limitation may result in significant reduction of both the absolute maximum momentum pmax and the observationally more important spectral break p* .

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