11,689 research outputs found
A Novel Approach to Non linear Shock Acceleration
First order Fermi acceleration at astrophysical shocks is often invoked as a
mechanism for the generation of non-thermal particles. This mechanism is
especially simple in the approximation that the accelerated particles behave
like test particles, not affecting the shocked fluid. Many complications enter
the calculations when the accelerated particles have a backreaction on the
fluid, in which case we may enter the non linear regime of shock acceleration.
In this paper we summarize the main features of a semi-analytical approach to
the study of the non linearity in shock acceleration, and compare some of the
results with previous attempts and with the output of numerical simulations.Comment: To appear in the proceedings of the TAUP conference, September 8-12,
2001 - Gran Sasso Laboratory, Ital
Nonlinear Particle Acceleration in Relativistic Shocks
Monte Carlo techniques are used to model nonlinear particle acceleration in
parallel collisionless shocks of various speeds, including mildly relativistic
ones. When the acceleration is efficient, the backreaction of accelerated
particles modifies the shock structure and causes the compression ratio, r, to
increase above test-particle values. Modified shocks with Lorentz factors less
than about 3 can have compression ratios considerably greater than 3 and the
momentum distribution of energetic particles no longer follows a power law
relation. These results may be important for the interpretation of gamma-ray
bursts if mildly relativistic internal and/or afterglow shocks play an
important role accelerating particles that produce the observed radiation. For
shock Lorentz factors greater than about 10, r approaches 3 and the so-called
`universal' test-particle result of N(E) proportional to E^{-2.3} is obtained
for sufficiently energetic particles. In all cases, the absolute normalization
of the particle distribution follows directly from our model assumptions and is
explicitly determined.Comment: Updated version, Astroparticle Physics, in press, 29 pages, 13
figure
Non linear particle acceleration at non-relativistic shock waves in the presence of self-generated turbulence
Particle acceleration at astrophysical shocks may be very efficient if
magnetic scattering is self-generated by the same particles. This nonlinear
process adds to the nonlinear modification of the shock due to the dynamical
reaction of the accelerated particles on the shock. Building on a previous
general solution of the problem of particle acceleration with arbitrary
diffusion coefficients (Amato & Blasi, 2005), we present here the first
semi-analytical calculation of particle acceleration with both effects taken
into account at the same time: charged particles are accelerated in the
background of Alfven waves that they generate due to the streaming instability,
and modify the dynamics of the plasma in the shock vicinity.Comment: submitted to MNRA
Prediction, Retrodiction, and The Amount of Information Stored in the Present
We introduce an ambidextrous view of stochastic dynamical systems, comparing
their forward-time and reverse-time representations and then integrating them
into a single time-symmetric representation. The perspective is useful
theoretically, computationally, and conceptually. Mathematically, we prove that
the excess entropy--a familiar measure of organization in complex systems--is
the mutual information not only between the past and future, but also between
the predictive and retrodictive causal states. Practically, we exploit the
connection between prediction and retrodiction to directly calculate the excess
entropy. Conceptually, these lead one to discover new system invariants for
stochastic dynamical systems: crypticity (information accessibility) and causal
irreversibility. Ultimately, we introduce a time-symmetric representation that
unifies all these quantities, compressing the two directional representations
into one. The resulting compression offers a new conception of the amount of
information stored in the present.Comment: 17 pages, 7 figures, 1 table;
http://users.cse.ucdavis.edu/~cmg/compmech/pubs/pratisp.ht
- …