4,079 research outputs found
Non-linear diffusive shock acceleration with free escape boundary
We present here a semi-analytical solution of the problem of particle
acceleration at non-linear shock waves with a free escape boundary at some
location upstream. This solution, besides allowing us to determine the spectrum
of particles accelerated at the shock front, including the shape of the cutoff
at some maximum momentum, also allows us to determine the spectrum of particles
escaping the system from upstream. This latter aspect of the problem is crucial
for establishing a connection between the accelerated particles in
astrophysical sources, such as supernova remnants, and the cosmic rays observed
at the Earth. An excellent approximate solution, which leads to a
computationally fast calculation of the structure of shocks with an arbitrary
level of cosmic ray modification, is also obtained.Comment: 11 pages, 2 figures, Accepted for publication in APh
On the escape of particles from cosmic ray modified shocks
Stationary solutions to the problem of particle acceleration at shock waves
in the non-linear regime, when the dynamical reaction of the accelerated
particles on the shock cannot be neglected, are known to show a prominent
energy flux escaping from the shock towards upstream infinity. On physical
grounds, the escape of particles from the upstream region of a shock has to be
expected in all those situations in which the maximum momentum of accelerated
particles, , decreases with time, as is the case for the Sedov-Taylor
phase of expansion of a shell Supernova Remnant, when both the shock velocity
and the cosmic ray induced magnetization decrease. In this situation, at each
time , particles with momenta larger than leave the system from
upstream, carrying away a large fraction of the energy if the shock is strongly
modified by the presence of cosmic rays. This phenomenon is of crucial
importance for explaining the cosmic ray spectrum detected at Earth. In this
paper we discuss how this escape flux appears in the different approaches to
non-linear diffusive shock acceleration, and especially in the quasi-stationary
semi-analytical kinetic ones. We apply our calculations to the Sedov-Taylor
phase of a typical supernova remnant, including in a self-consistent way
particle acceleration, magnetic field amplification and the dynamical reaction
on the shock structure of both particles and fields. Within this framework we
calculate the temporal evolution of the maximum energy reached by the
accelerated particles and of the escape flux towards upstream infinity. The
latter quantity is directly related to the cosmic ray spectrum detected at
Earth.Comment: Version accepted for publication in MNRA
Dogs display owner-specific expectations based on olfaction
Most current knowledge about dogs’ understanding of, and reacting to, their environment is limited to the visual or auditory modality, but it remains unclear how olfaction and cognition are linked together. Here we investigate how domestic dogs search for their owners using their excellent olfactory sense. We raise the question whether dogs have a representation of someone when they smell their track. The question is what they expect when they follow a trail or whether they perceive an odour as a relevant or non-relevant stimulus. We adopted a classical violation-of-expectation paradigm—and as targets we used two persons that were both important to the dog, usually the owners. In the critical condition subjects could track the odour trail of one target, but at the end of the trail they find another target. Dogs showed an increased activity when the person did not correspond with the trail compared to a control condition. Moreover, we found huge individual differences in searching behaviour supporting the assumption that dogs are only able to smell when they really sniff, and that the temperature has an influence on dogs performance. Results are discussed in the light of how cognitive abilities, motivation and odour perception influence each other.Methods - Subjects. - Setup. - Procedure. - Design. - Coding and analysis. Results Discussio
New insights on hadron acceleration at supernova remnant shocks
We outline the main features of nuclei acceleration at supernova remnant
forward shocks, stressing the crucial role played by self-amplified magnetic
fields in determining the energy spectrum observed in this class of sources. In
particular, we show how the standard predictions of the non-linear theory of
diffusive shock acceleration has to be completed with an additional ingredient,
which we propose to be the enhanced velocity of the magnetic irregularities
particles scatter against, to reconcile the theory of efficient particle
acceleration with recent observations of gamma-ray bright supernova remnants.Comment: 7 pages, 2 figures. To apper in "Cosmic-ray induced phenomenology in
star-forming environments: Proceedings of the 2nd Session of the Sant Cugat
Forum of Astrophysics" (April 16-19, 2012), Olaf Reimer and Diego F. Torres
(eds.
On the equipartition of thermal and non-thermal energy in clusters of galaxies
Clusters of galaxies are revealing themselves as powerful sources of non
thermal radiation in a wide range of wavelengths. In order to account for these
multifrequency observations equipartition of cosmic rays (CRs) with the thermal
gas in clusters of galaxies is often invoked. This condition might suggest a
dynamical role played by cosmic rays in the virialization of these large scale
structures and is now testable through gamma ray observations. We show here, in
the specific case of the Coma and Virgo clusters, for which upper limits on the
gamma ray emission exist, that equipartition implies gamma ray fluxes that are
close or even in excess of the EGRET limit, depending on the adopted model of
CR injection. We use this bound to limit the validity of the equipartition
condition. We also show that, contrary to what claimed in previous
calculations, the equipartition assumption implies gamma ray fluxes in the TeV
range which can be detectable even by currently operating gamma ray
observatories if the injection cosmic ray spectrum is flatter than .Comment: 20 pages + 2 figures. To appear in the Astrophysical Journa
Non-linear diffusive acceleration of heavy nuclei in supernova remnant shocks
We describe a semi-analytical approach to non-linear diffusive shock
acceleration in the case in which nuclei other than protons are also
accelerated. The structure of the shock is determined by the complex interplay
of all nuclei, and in turn this shock structure determines the spectra of all
components. The magnetic field amplification upstream is described as due to
streaming instability of all nuclear species. The amplified magnetic field is
then taken into account for its dynamical feedback on the shock structure as
well as in terms of the induced modification of the velocity of the scattering
centers that enters the particle transport equation. The spectra of accelerated
particles are steep enough to be compared with observed cosmic ray spectra only
if the magnetic field is sufficiently amplified and the scattering centers have
high speed in the frame of the background plasma. We discuss the implications
of this generalized approach on the structure of the knee in the all-particle
cosmic ray spectrum, which we interpret as due to an increasingly heavier
chemical composition above eV. The effects of a non trivial chemical
composition at the sources on the gamma ray emission from a supernova remnant
when gamma rays are of hadronic origin are also discussed.Comment: 23 pages, 5 figures, minor changes to reflect the published versio
Pierre Auger Data, Photons, and Top-Down Cosmic Ray Models
We consider the ultra-high energy cosmic ray (UHECR) spectrum as measured by
the Pierre Auger Observatory. Top-down models for the origin of UHECRs predict
an increasing photon component at energies above about eV. Here we
present a simple prescription to compare the Auger data with a prediction
assuming a pure proton component or a prediction assuming a changing primary
component appropriate for a top-down model. We find that the UHECR spectrum
predicted in top-down models is a good fit to the Auger data. Eventually, Auger
will measure a composition-independent spectrum and will be capable of either
confirming or excluding the quantity of photons predicted in top-down models.Comment: 8 pages, 3 figure
Vector Supersymmetry of 2D Yang-Mills Theory
The vector supersymmetry of the 2D topological BF model is extended to 2D
Yang-Mills. The consequences of the corresponding Ward identity on the
ultraviolet behavior of the theory are analyzed.Comment: Some references adde
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