Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012apj...749..178b&link_type=abstract
The Astrophysical Journal, Volume 749, Issue 2, article id. 178 (2012).
Astronomy and Astrophysics
Astronomy
Acceleration Of Particles, Cosmic Rays, Ism: Bubbles, Scattering
Scientific paper
We present a simple continuous-time random-walk model for the transport of energetic particles accelerated by a collection of supernova explosions in a galactic superbubble, developed to simulate and highlight signatures of anomalous transport on the particles' evolution and their spectra in a multi-shock context. We assume standard diffusive shock acceleration (DSA) theory for each shock encounter. The superbubble (an OB stars association) is idealized as a heterogeneous region of particle sources and sinks bounded by a random surface. The model is based on two coupled stochastic differential equations and is applied for protons and alpha particles. Using characteristic values for a typical bubble, our simulations suggest that acceleration and transport in the bubble may be sub-diffusive. In addition, a spectral break in the particles' evolution and spectra is evident located at ≈1015 eV for protons and ≈3 × 1015 eV for alphas. Our simulations are consistent with a bubble's mean magnetic field strength of ≈1 μG and a shock separation distance ~0.1 × the characteristic radius of the bubble. The simulations imply that the diffusion coefficient (for the elementary shock acceleration process) is <~ 1027 cm2 s–1 at 1 GeV/c. While the sub-diffusive transport is readily attributed to the stochastic nature of the acceleration time according to DSA theory, the spectral break appears to be an artifact of transport in a finite medium. These simulations point to a new and intriguing phenomenon associated with the statistical nature of collective acceleration of high-energy cosmic rays in galactic superbubbles.
Barghouty A. F.
Schnee D. A.
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