2,656 research outputs found
Magnetic and density spikes in cosmic ray shock precursors
In shock precursors populated by accelerated cosmic rays (CR), the CR return
current instability is believed to significantly enhance the pre-shock
perturbations of magnetic field. We have obtained fully-nonlinear exact ideal
MHD solutions supported by the CR return current. The solutions occur as
localized spikes of circularly polarized Alfven envelopes (solitons, or
breathers). As the conventional (undriven) solitons, the obtained magnetic
spikes propagate at a speed proportional to their amplitude,
. The sufficiently strong solitons run thus
ahead of the main shock and stand in the precursor, being supported by the
return current. This property of the nonlinear solutions is strikingly
different from the linear theory that predicts non-propagating (that is,
convected downstream) circularly polarized waves. The nonlinear solutions may
come either in isolated pulses (solitons) or in soliton-trains (cnoidal waves).
The morphological similarity of such quasi-periodic soliton chains with
recently observed X-ray stripes in Tycho supernova remnant (SNR) is briefly
discussed. The magnetic field amplification determined by the suggested
saturation process is obtained as a function of decreasing SNR blast wave
velocity during its evolution from the ejecta-dominated to the Sedov-Taylor
stage.Comment: 21 pages, 4 figure
Spatial chaos in weakly dispersive and viscous media: a nonperturbative theory of the driven KdV-Burgers equation
The asymptotic travelling wave solution of the KdV-Burgers equation driven by
the long scale periodic driver is constructed. The solution represents a
shock-train in which the quasi-periodic sequence of dispersive shocks or
soliton chains is interspersed by smoothly varying regions. It is shown that
the periodic solution which has the spatial driver period undergoes period
doublings as the governing parameter changes. Two types of chaotic behavior are
considered. The first type is a weak chaos, where only a small chaotic
deviation from the periodic solution occurs. The second type corresponds to the
developed chaos where the solution ``ignores'' the driver period and represents
a random sequence of uncorrelated shocks. In the case of weak chaos the shock
coordinate being repeatedly mapped over the driver period moves on a chaotic
attractor, while in the case of developed chaos it moves on a repellor. Both
solutions depend on a parameter indicating the reference shock position in the
shock-train. The structure of a one dimensional set to which this parameter
belongs is investigated. This set contains measure one intervals around the
fixed points in the case of periodic or weakly chaotic solutions and it becomes
a fractal in the case of strong chaos. The capacity dimension of this set is
calculated.Comment: 32 pages, 12 PostScript figures, useses elsart.sty and boxedeps.tex,
fig.11 is not included and can be requested from <[email protected]
Assessment of evolutionary status of eclipsing binaries using light-curve parameters and spectral classification
We have developed a procedure for the classification of eclipsing binaries
from their light-curve parameters and spectral type. The procedure was tested
on more than 1000 systems with known classification, and its efficiency was
estimated for every evolutionary status we use. The procedure was applied to
about 4700 binaries with no classification, and the vast majority of them was
classified successfully. Systems of relatively rare evolutionary classes were
detected in that process, as well as systems with unusual and/or contradictory
parameters. Also, for 50 previously unclassified cluster binaries evolutionary
classes were identified. These stars can serve as tracers for age and distance
estimation of their parent stellar systems. The procedure proved itself as
fast, flexible and effective enough to be applied to large ground based and
space born surveys, containing tens of thousands of eclipsing binaries.Comment: 12 pages, 6 tables, 2 figures, 3 appendixe
Probing Nearby CR Accelerators and ISM Turbulence with Milagro Hot Spots
Both the acceleration of cosmic rays (CR) in supernova remnant shocks and
their subsequent propagation through the random magnetic field of the Galaxy
deem to result in an almost isotropic CR spectrum. Yet the MILAGRO TeV
observatory discovered a sharp ( arrival anisotropy of CR
nuclei. We suggest a mechanism for producing a weak and narrow CR beam which
operates en route to the observer. The key assumption is that CRs are scattered
by a strongly anisotropic Alfven wave spectrum formed by the turbulent cascade
across the local field direction. The strongest pitch-angle scattering occurs
for particles moving almost precisely along the field line. Partly because this
direction is also the direction of minimum of the large scale CR angular
distribution, the enhanced scattering results in a weak but narrow particle
excess. The width, the fractional excess and the maximum momentum of the beam
are calculated from a systematic transport theory depending on a single scale
which can be associated with the longest Alfven wave, efficiently
scattering the beam. The best match to all the three characteristics of the
beam is achieved at pc. The distance to a possible source of the beam
is estimated to be within a few 100pc. Possible approaches to determination of
the scale from the characteristics of the source are discussed. Alternative
scenarios of drawing the beam from the galactic CR background are considered.
The beam related large scale anisotropic CR component is found to be energy
independent which is also consistent with the observations.Comment: 2 figures, ApJ accepted version2 minor changes and correction
Spectral universality of strong shocks accelerating charged particles
As a rule, the shock compression controls the spectrum of diffusively
accelerated particles. We argue that this is not so if the backreaction of
these particles on the shock structure is significant. We present a
self-similar solution in which the accelerated particles change the flow
structure near the shock so strongly that the total shock compression may
become arbitrarily large. Despite this, the energy spectrum behind the shock is
close to E^{-3/2} independently of anything at all.Comment: Submitted to ApJL, 4 pages, 1 figure, uses revtex and boxedep
- …