577 research outputs found
Critical self-organization of astrophysical shocks
There are two distinct regimes of the first order Fermi acceleration at
shocks. The first is a linear (test particle) regime in which most of the shock
energy goes into thermal and bulk motion of the plasma. The second is an
efficient regime when it goes into accelerated particles. Although the
transition region between them is narrow, we identify the factors that drive
the system to a {\it self-organized critical state} between those two. Using an
analytic solution, we determine this critical state and calculate the spectra
and maximum energy of accelerated particles.Comment: To appear in ApJL, Sec.3 extensively rewritten, 4 pages, Latex,
emulateapj.sty, eps
Cosmic ray acceleration parameters from multi-wavelength observations. The case of SN 1006
The properties of the Galactic supernova remnant SN 1006 are theoretically
reanalysed. Nonlinear kinetic theory is used to determine the acceleration
efficiency of cosmic rays (CRs) in the supernova remnant SN 1006. The known
range of astronomical parameters and the existing measurements of nonthermal
emission are examined in order to define the values of the relevant physical
parameters which determine the CR acceleration efficiency. It is shown that the
parameter values -- proton injection rate, electron to proton ratio and
downstream magnetic field strength -- are determined with the appropriate
accuracy. In particular also the observed azimuthal variations in the gamma-ray
morphology agree with the theoretical expectation. These parameter values,
together with the reduction of the gamma-ray flux relative to a spherically
symmetric acceleration geometry, allow a good fit to the existing data,
including the recently detected TeV emission by H.E.S.S. SN 1006 represents the
first example where a high efficiency of nuclear CR production, required for
the Galactic CR sources, is consistently established.Comment: 10 pages, 6 figures, accepted for publication in A&
Cosmic Ray Acceleration by Spiral Shocks in the Galactic Wind
Cosmic ray acceleration by shocks related with Slipping Interaction Regions
(SIRs) in the Galactic Wind is considered. SIRs are similar to Solar Wind
Corotating Interaction Regions. The spiral structure of our Galaxy results in a
strong nonuniformity of the Galactic Wind flow and in SIR formation at
distances of 50 to 100 kpc. SIRs are not corotating with the gas and magnetic
field because the angular velocity of the spiral pattern differs from that of
the Galactic rotation. It is shown that the collective reacceleration of the
cosmic ray particles with charge in the resulting shock ensemble can
explain the observable cosmic ray spectrum beyond the "knee" up to energies of
the order of eV. For the reaccelerated particles the Galactic Wind
termination shock acts as a reflecting boundary.Comment: LATEX, 14 pages, 7 figures, accepted to A&
New results from H.E.S.S. observations of galaxy clusters
Clusters of galaxies are believed to contain a significant population of
cosmic rays. From the radio and probably hard X-ray bands it is known that
clusters are the spatially most extended emitters of non-thermal radiation in
the Universe. Due to their content of cosmic rays, galaxy clusters are also
potential sources of VHE (>100 GeV) gamma rays. Recently, the massive, nearby
cluster Abell 85 has been observed with the H.E.S.S. experiment in VHE gamma
rays with a very deep exposure as part of an ongoing campaign. No significant
gamma-ray signal has been found at the position of the cluster. The
non-detection of this object with H.E.S.S. constrains the total energy of
cosmic rays in this system. For a hard spectral index of the cosmic rays of
-2.1 and if the cosmic-ray energy density follows the large scale gas density
profile, the limit on the fraction of energy in these non-thermal particles
with respect to the total thermal energy of the intra-cluster medium is 8% for
this particular cluster. This value is at the lower bounds of model
predictions.Comment: 4 pages, one figure, invited talk at the 2nd Heidelberg workshop:
"High-Energy Gamma-rays and Neutrinos from Extra-Galactic Sources", January
13 - 16, 2009, to be published in Int. J. Mod. Phys.
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