320 research outputs found
First order Fermi acceleration driven by magnetic reconnection
A box model is used to study first order Fermi acceleration driven by
magnetic reconnection. It is shown, at least in this simple model, that the
spectral index of the accelerated particles is related to the total compression
in the same way as in diffusive shock acceleration and is not, as has been
suggested, a universal spectrum. The acceleration time-scale is
estimated and some comments made about the applicability of the process.Comment: Accepted for MNRA
Analytical Study of Diffusive Relativistic Shock Acceleration
Particle acceleration in relativistic shocks is studied analytically in the
test-particle, small-angle scattering limit, for an arbitrary velocity-angle
diffusion function D. Accurate analytic expressions for the spectral index s
are derived using few (2-6) low-order moments of the shock-frame angular
distribution. For isotropic diffusion, previous results are reproduced and
justified. For anisotropic diffusion, s is shown to be sensitive to D,
particularly downstream and at certain angles, and a wide range of s values is
attainable. The analysis, confirmed numerically, can be used to test
collisionless shock models and to observationally constrain D. For example,
strongly forward- or backward-enhanced diffusion downstream is ruled out by GRB
afterglow observations.Comment: 4 pages, 2 figures, PRL accepted, minor change
Escaping the accelerator; how, when and in what numbers do cosmic rays get out of supernova remnants?
The escape of charged particles accelerated by diffusive shock acceleration
from supernova remnants is shown to be a more complex process than normally
appreciated. Using a box model it is shown that the high-energy end of the
spectrum can exhibit spectral breaks even with no formal escape as a result of
geometrical dilution and changing time-scales. It is pointed out that the bulk
of the cosmic ray particles at lower energies must be produced and released in
the late stages of the remnant's evolution whereas the high energy particles
are produced early on; this may explain recent observations of slight
compositional variations with energy. Escape resulting from ion-neutral
friction in dense and partially ionized media is discussed briefly and some
comments made on the use of so-called "free escape boundary conditions".
Finally estimates are made of the total production spectrum integrated over the
life of the remnant.Comment: To appear in MNRA
Self-Similar Collisionless Shocks
Observations of gamma-ray burst afterglows suggest that the correlation
length of magnetic field fluctuations downstream of relativistic non-magnetized
collisionless shocks grows with distance from the shock to scales much larger
than the plasma skin depth. We argue that this indicates that the plasma
properties are described by a self-similar solution, and derive constraints on
the scaling properties of the solution. For example, we find that the scaling
of the characteristic magnetic field amplitude with distance from the shock is
B \propto D^{s_B} with -1<s_B<=0, that the spectrum of accelerated particles is
dn/dE \propto E^{-2/(s_B+1)}, and that the scaling of the magnetic correlation
function is \propto x^{2s_B} (for x>>D). We show that the
plasma may be approximated as a combination of two self-similar components: a
kinetic component of energetic particles and an MHD-like component representing
"thermal" particles. We argue that the latter may be considered as infinitely
conducting, in which case s_B=0 and the scalings are completely determined
(e.g. dn/dE \propto E^{-2} and B \propto D^0). Similar claims apply to non-
relativistic shocks such as in supernova remnants, if the upstream magnetic
field can be neglected. Self-similarity has important implications for any
model of particle acceleration and/or field generation. For example, we show
that the diffusion function in the angle \mu of momentum p in diffusive shock
acceleration models must satisfy D_{\mu\mu}(p,D) = D^{-1}D'_{\mu\mu}(p/D), and
that a previously suggested model for the generation of large scale magnetic
fields through a hierarchical merger of current-filaments should be
generalized. A numerical experiment testing our analysis is outlined
(Abridged).Comment: 16 pages, 1 figure, accepted for publication in Ap
Pair Creation at Shocks: Application to the High Energy Emission of Compact objects
We investigate the effect of pair creation on a shock structure. Actually,
particles accelerated by a shock can be sufficiently energetic to boost, via
Inverse Compton (IC) process for example, surrounding soft photons above the
rest mass electron energy and thus to trigger the pair creation process. The
increase of the associated pair pressure is thus able to disrupt the plasma
flow and possibly, for too high pressure, to smooth it completely. Reversely,
significant changes of the flow velocity profile may modify the distribution
function of the accelerated particles, modifying consequently the pair creation
rate. Stationary states are then obtained by solving self-consistently for the
particle distribution function and the flow velocity profile. We discuss our
results and the application of these processes to the high energy emission and
variability of compact objects.Comment: 14 pages, 7 figures (uses newpasp.sty included). To appear in Proc.
of "X-ray astronomy 2000",(Palermo Sep. 2000), Eds. R. Giacconi, L. Stella,
S. Serio, ASP Conf. Series, in pres
A two-zone model for the emission from RX J1713.7-3946
We study the acceleration and radiation of charged particles in the shock
waves of supernova remnants using a recent version of the "box model".
According to this, particles are accelerated in an energy-dependent region
around the shock by the first order Fermi mechanism and lose energy through
radiation. The particle distribution function is obtained from a spatially
averaged kinetic equation that treats the energy losses self-consistently.
There exists also a second population that consists of those particles that
escape behind the shock where they also radiate. The energy distribution of
this population is calculated in a similar manner. The application of the model
to the supernova remnant RX J1713.7-3946, which was recently confirmed as a TeV
source by H.E.S.S., shows that the X-ray emission can be attributed to electron
synchrotron radiation while in gamma-rays there are contributions from both
electrons and protons, with protons playing the dominant role. Additionally,
there are strong indications that particles diffuse in turbulence that has a
Kolmogorov spectrum.Comment: 6 pages, 2 figures, accepted by Astronomy and Astrophysic
Diffuse Galactic Gamma Rays from Shock-Accelerated Cosmic Rays
A shock-accelerated particle flux \propto p^-s, where p is the particle
momentum, follows from simple theoretical considerations of cosmic-ray
acceleration at nonrelativistic shocks followed by rigidity-dependent escape
into the Galactic halo. A flux of shock-accelerated cosmic-ray protons with s ~
2.8 provides an adequate fit to the Fermi-LAT gamma-ray emission spectra of
high-latitude and molecular cloud gas when uncertainties in nuclear production
models are considered. A break in the spectrum of cosmic-ray protons claimed by
Neronov, Semikoz, & Taylor (PRL, 108, 051105, 2012) when fitting the gamma-ray
spectra of high-latitude molecular clouds is a consequence of using a
cosmic-ray proton flux described by a power law in kinetic energy.Comment: Version to correspond to published letter in PRL; corrected Fig.
The contribution of supernova remnants to the galactic cosmic ray spectrum
The supernova paradigm for the origin of galactic cosmic rays has been deeply
affected by the development of the non-linear theory of particle acceleration
at shock waves. Here we discuss the implications of applying such theory to the
calculation of the spectrum of cosmic rays at Earth as accelerated in supernova
remnants and propagating in the Galaxy. The spectrum is calculated taking into
account the dynamical reaction of the accelerated particles on the shock, the
generation of magnetic turbulence which enhances the scattering near the shock,
and the dynamical reaction of the amplified field on the plasma. Most
important, the spectrum of cosmic rays at Earth is calculated taking into
account the flux of particles escaping from upstream during the Sedov-Taylor
phase and the adiabatically decompressed particles confined in the expanding
shell and escaping at later times. We show how the spectrum obtained in this
way is well described by a power law in momentum with spectral index close to
-4, despite the concave shape of the instantaneous spectra of accelerated
particles. On the other hand we also show how the shape of the spectrum is
sensible to details of the acceleration process and environment which are and
will probably remain very poorly known.Comment: 19 pages, 8 figures, published version (references updated
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
The effect of the hot oxygen corona on the interaction of the solar wind with Venus
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95142/1/grl3589.pd
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