47,797 research outputs found
Magnetic field amplification by cosmic rays in supernova remnants
Magnetic field amplification is needed to accelerate cosmic cays to PeV
energies in supernova remants. Escaping cosmic rays trigger a return current in
the plasma that drives a non-resonant hybrid instability. We run simulations in
which we represent the escaping cosmic rays with the plasma return current,
keeping the maximum cosmic ray energy fixed, and evaluate its effects on the
upstream medium. In addition to magnetic field amplification, density
perturbations arise that, when passing through the shock, further increase
amplification levels downstream. As the growth rate of the instability is most
rapid for the smaller scales, the resolution is a limiting factor in the
amplification that can be reached with these simulations.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference
"370 years of Astronomy in Utrecht", eds. G. Pugliese, A. de Koter and M.
Wijbur
Cosmic ray acceleration in young supernova remnants
We investigate the appearance of magnetic field amplification resulting from
a cosmic ray escape current in the context of supernova remnant shock waves.
The current is inversely proportional to the maximum energy of cosmic rays, and
is a strong function of the shock velocity. Depending on the evolution of the
shock wave, which is drastically different for different circumstellar
environments, the maximum energy of cosmic rays as required to generate enough
current to trigger the non-resonant hybrid instability that confines the cosmic
rays follows a different evolution and reaches different values. We find that
the best candidates to accelerate cosmic rays to ~few PeV energies are young
remnants in a dense environment, such as a red supergiant wind, as may be
applicable to Cassiopeia A. We also find that for a typical background magnetic
field strength of 5 microG the instability is quenched in about 1000 years,
making SN1006 just at the border of candidates for cosmic ray acceleration to
high energies.Comment: 14 pages, 8 figures. Accepted for publication in MNRA
Confining the high-energy cosmic rays
Diffusive shock acceleration is the prime candidate for efficient
acceleration of cosmic rays. Galactic cosmic rays are believed to originate
predominantly from this process in supernova remnant shock waves. Confinement
of the cosmic rays in the shock region is key in making the mechanism
effective. It has been known that on small scales (smaller than the typical
gyroradius) high-amplitude non-resonant instabilities arise due to cosmic ray
streaming ahead of the shock. For the efficiency of scattering of the highest
energy cosmic rays it is of interest to determine the type of instabilities
that act on longer length scales, i.e. larger than the cosmic ray gyroradius.
We will present the results of our analysis of an instability that acts in this
regime and will discuss its driving mechanism and typical growth times.Comment: 4 pages, 1 figure. Proceedings for the conference on Cosmic Rays and
the Interstellar Medium (CRISM) in June 2011, Montpellier, France. To appear
in MSA
From cosmic ray source to the Galactic pool
The Galactic cosmic ray spectrum is a remarkably straight power law. Our
current understanding is that the dominant sources that accelerate cosmic rays
up to the knee ( eV) or perhaps even the ankle ( eV), are young Galactic supernova remnants. In theory, however, there
are various reasons why the spectrum may be different for different sources,
and may not even be a power law if nonlinear shock acceleration applies during
the most efficient stages of acceleration. We show how the spectrum at the
accelerator translates to the spectrum that make up the escaping cosmic rays
that replenish the Galactic pool of cosmic rays. We assume that cosmic ray
confinement, and thus escape, is linked to the level of magnetic field
amplification, and that the magnetic field is amplified by streaming cosmic
rays according to the non-resonant hybrid or resonant instability. When a fixed
fraction of the energy is transferred to cosmic rays, it turns out that a
source spectrum that is flatter than will result in a escape
spectrum, whereas a steeper source spectrum will result in an escape spectrum
with equal steepening. This alleviates some of the concern that may arise from
expected flat or concave cosmic ray spectra associated with nonlinear shock
modification.Comment: 5 pages, 1 figure. Accepted for publication in MNRA
Absence of Electron Surfing Acceleration in a Two-Dimensional Simulation
Electron acceleration in high Mach number perpendicular shocks is
investigated through two-dimensional electrostatic particle-in-cell (PIC)
simulation. We simulate the shock foot region by modeling particles that
consist of three components such as incident protons and electrons and
reflected protons in the initial state which satisfies the Buneman instability
condition. In contrast to previous one-dimensional simulations in which strong
surfing acceleration is realized, we find that surfing acceleration does not
occur in two-dimensional simulation. This is because excited electrostatic
potentials have a two-dimensional structure that makes electron trapping
impossible. Thus, the surfing acceleration does not work either in itself or as
an injection mechanism for the diffusive shock acceleration. We briefly discuss
implications of the present results on the electron heating and acceleration by
shocks in supernova remnants.Comment: 12 pages, 4 figures, accepted for publication in ApJ
The generalized Kochen-Specker theorem
A proof of the generalized Kochen-Specker theorem in two dimensions due to
Cabello and Nakamura is extended to all higher dimensions. A set of 18 states
in four dimensions is used to give closely related proofs of the generalized
Kochen-Specker, Kochen-Specker and Bell theorems that shed some light on the
relationship between these three theorems.Comment: 5 pages, 1 Table. A new third paragraph and an additional reference
have been adde
Detection of genuinely entangled and non-separable -partite quantum states
We investigate the detection of entanglement in -partite quantum states.
We obtain practical separability criteria to identify genuinely entangled and
non-separable mixed quantum states. No numerical optimization or eigenvalue
evaluation is needed, and our criteria can be evaluated by simple computations
involving components of the density matrix. We provide examples in which our
criteria perform better than all known separability criteria. Specifically, we
are able to detect genuine -partite entanglement which has previously not
been identified. In addition, our criteria can be used in today's experiment.Comment: 8 pages, one figur
Quantum Preferred Frame: Does It Really Exist?
The idea of the preferred frame as a remedy for difficulties of the
relativistic quantum mechanics in description of the non-local quantum
phenomena was undertaken by such physicists as J. S. Bell and D. Bohm. The
possibility of the existence of preferred frame was also seriously treated by
P. A. M. Dirac. In this paper, we propose an Einstein-Podolsky-Rosen-type
experiment for testing the possible existence of a quantum preferred frame. Our
analysis suggests that to verify whether a preferred frame of reference in the
quantum world exists it is enough to perform an EPR type experiment with pair
of observers staying in the same inertial frame and with use of the massive EPR
pair of spin one-half or spin one particles.Comment: 5 pp., 6 fig
Transition from spot to faculae domination -- An alternate explanation for the dearth of intermediate \textit{Kepler} rotation periods
The study of stellar activity cycles is crucial to understand the underlying
dynamo and how it causes activity signatures such as dark spots and bright
faculae. We study the appearance of activity signatures in contemporaneous
photometric and chromospheric time series. Lomb-Scargle periodograms are used
to search for cycle periods present in both time series. To emphasize the
signature of the activity cycle we account for rotation-induced scatter in both
data sets by fitting a quasi-periodic Gaussian process model to each observing
season. After subtracting the rotational variability, cycle amplitudes and the
phase difference between the two time series are obtained by fitting both time
series simultaneously using the same cycle period. We find cycle periods in 27
of the 30 stars in our sample. The phase difference between the two time series
reveals that the variability in fast rotating active stars is usually in
anti-phase, while the variability of slowly rotating inactive stars is in
phase. The photometric cycle amplitudes are on average six times larger for the
active stars. The phase and amplitude information demonstrates that active
stars are dominated by dark spots, whereas less active stars are dominated by
bright faculae. We find the transition from spot to faculae domination at the
Vaughan-Preston gap, and around a Rossby number equal to one. We conclude that
faculae are the dominant ingredient of stellar activity cycles at ages >2.55
Gyr. The data further suggest that the Vaughan-Preston gap can not explain the
previously detected dearth of Kepler rotation periods between 15-25 days.
Nevertheless, our results led us to propose an explanation for the rotation
period dearth to be due to the non-detection of periodicity caused by the
cancellation of dark spots and bright faculae at 800 Myr.Comment: 12+15 pages, 10+2 figures, accepted for publication in A&
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