1,021 research outputs found
Relativistic Fermi acceleration with shock compressed turbulence
This paper presents numerical simulations of test particle Fermi acceleration
at relativistic shocks of Lorentz factor Gamma_sh = 2-60, using a realistic
downstream magnetic structure obtained from the shock jump conditions. The
upstream magnetic field is described as pure Kolmogorov turbulence; the
corresponding downstream magnetic field lies predominantly in the plane
tangential to the shock surface and the coherence length is smaller along the
shock normal than in the tangential plane. Acceleration is nonetheless
efficient and leads to powerlaw spectra with index s = 2.6-2.7 at large shock
Lorentz factor Gamma_sh >> 1, markedly steeper than for isotropic scattering
downstream. The acceleration timescale t_acc in the upstream rest frame becomes
a fraction of Larmor time t_L in the ultra-relativistic limit, t_acc ~ 10
t_L/Gamma_sh. Astrophysical applications are discussed, in particular the
acceleration in gamma-ray bursts internal and external shocks.Comment: 11 pages; 10 figures; submitted to MNRA
Inhomogeneous extragalactic magnetic fields and the second knee in the cosmic ray spectrum
Various experiments indicate the existence of a second knee around energy
E=3.10^{17} eV in the cosmic ray spectrum. This feature could be the signature
of the end of the galactic component and of the emergence of the extragalactic
one, provided that the latter cuts off at low energies. Recent analytical
calculations have shown that this cut-off could be a consequence of the
existence of extragalactic magnetic fields: low energy protons diffuse on
extragalactic magnetic fields and cannot reach the observer within a given
time. We study the influence of inhomogeneous magnetic fields on the magnetic
horizon, using a new semi-analytical propagation code. Our results indicate
that, at a fixed value of the volume averaged magnetic field , the amplitude
of the low energy cut-off is mainly controled by the strength of magnetic
fields in the voids of the large scale structure distribution.Comment: 15 pages, 10 figures. Version to appear in PRD (minor changes
Anisotropy vs chemical composition at ultra-high energies
This paper proposes and discusses a test of the chemical composition of
ultra-high energy cosmic rays that relies on the anisotropy patterns measured
as a function of energy. In particular, we show that if one records an
anisotropy signal produced by heavy nuclei of charge Z above an energy E_{thr},
one should record an even stronger (possibly much stronger) anisotropy at
energies >E_{thr}/Z due to the proton component that is expected to be
associated with the sources of the heavy nuclei. This conclusion remains robust
with respect to the parameters characterizing the sources and it does not
depend at all on the modelling of astrophysical magnetic fields. As a concrete
example, we apply this test to the most recent data of the Pierre Auger
Observatory. Assuming that the anisotropy reported above 55EeV is not a
statistical accident, and that no significant anisotropy has been observed at
energies <10EeV, we show that the apparent clustering toward Cen A cannot be
attributed to heavy nuclei. Similar conclusions are drawn regarding the
apparent excess correlation with nearby active galactic nuclei. We then discuss
a robust lower bound to the magnetic luminosity that a source must possess in
order to be able to accelerate particles of charge Z up to 100EeV,
L_B>10^{45}Z^{-2}erg/s. Using this bound in conjunction with the above
conclusions, we argue that the current PAO data does not support the model of
cosmic ray origin in active radio-quiet or even radio-loud galaxies. Finally,
we demonstrate that the apparent clustering in the direction of Cen A can be
explained by the contribution of the last few gamma-ray bursts or magnetars in
the host galaxy thanks to the scattering of the cosmic rays on the magnetized
lobes.Comment: 10 pages, 3 figure
Generation of gravitational waves during early structure formation between cosmic inflation and reheating
In the pre-reheating era, following cosmic inflation and preceding radiation
domination, the energy density may be dominated by an oscillating massive
scalar condensate, such as is the case for quadratic chaotic inflation. We have
found in a previous paper that during this period, a wide range of sub-Hubble
scale perturbations are subject to a preheating instability, leading to the
growth of density perturbations ultimately collapsing to form non-linear
structures. We compute here the gravitational wave signal due to these
structures in the linear limit and present estimates for emission in the
non-linear limit due to various effects: the collapse of halos, the tidal
interactions, the evaporation during the conversion of the inflaton condensate
into radiation and finally the ensuing turbulent cascades. The gravitational
wave signal could be rather large and potentially testable by future detectors.Comment: 11 pages, 3 figure
On the efficiency of Fermi acceleration at relativistic shocks
It is shown that Fermi acceleration at an ultra-relativistic shock wave
cannot operate on a particle for more than 1 1/2 Fermi cycle (i.e., u -> d -> u
-> d) if the particle Larmor radius is much smaller than the coherence length
of the magnetic field on both sides of the shock, as is usually assumed. This
conclusion is shown to be in excellent agreement with recent numerical
simulations. We thus argue that efficient Fermi acceleration at
ultra-relativistic shock waves requires significant non-linear processing of
the far upstream magnetic field with strong amplification of the small scale
magnetic power. The streaming or transverse Weibel instabilities are likely to
play a key role in this respect.Comment: 4 pages, 2 figures; to appear in ApJ Letter
Distortion of the ultrahigh energy cosmic ray flux from rare transient sources in inhomogeneous extragalactic magnetic fields
Detecting and characterizing the anisotropy pattern of the arrival directions
of the highest energy cosmic rays are crucial steps towards the identification
of their sources. We discuss a possible distortion of the cosmic ray flux
induced by the anisotropic and inhomogeneous distribution of extragalactic
magnetic fields in cases where sources of ultrahigh energy cosmic rays are rare
transient phenomena, such as gamma-ray bursts and/or newly born magnetars. This
distortion does not involve an angular deflection but the modulation of the
flux related to the probability of seeing the source on an experiment lifetime.
To quantify this distortion, we construct sky maps of the arrival directions of
these highest energy cosmic rays for various magnetic field configurations and
appeal to statistical tests proposed in the literature. We conclude that this
distortion cannot affect present experiments but should be considered when
performing anisotropy studies with future large-scale experiments that record
as many as hundreds of events above 6x10^19 eV.Comment: 9 pages, 6 figures, accepted by A&
Relativistic Shocks: Particle Acceleration and Magnetization
We review the physics of relativistic shocks, which are often invoked as the
sources of non-thermal particles in pulsar wind nebulae (PWNe), gamma-ray
bursts (GRBs), and active galactic nuclei (AGN) jets, and as possible sources
of ultra-high energy cosmic-rays. We focus on particle acceleration and
magnetic field generation, and describe the recent progress in the field driven
by theory advances and by the rapid development of particle-in-cell (PIC)
simulations. In weakly magnetized or quasi parallel-shocks (where the magnetic
field is nearly aligned with the flow), particle acceleration is efficient. The
accelerated particles stream ahead of the shock, where they generate strong
magnetic waves which in turn scatter the particles back and forth across the
shock, mediating their acceleration. In contrast, in strongly magnetized
quasi-perpendicular shocks, the efficiencies of both particle acceleration and
magnetic field generation are suppressed. Particle acceleration, when
efficient, modifies the turbulence around the shock on a long time scale, and
the accelerated particles have a characteristic energy spectral index of ~ 2.2
in the ultra-relativistic limit. We discuss how this novel understanding of
particle acceleration and magnetic field generation in relativistic shocks can
be applied to high-energy astrophysical phenomena, with an emphasis on PWNe and
GRB afterglows.Comment: 32 pages; 9 figures; invited topical review, comments welcome;
submitted for publication in "The Strongest Magnetic Fields in the Universe"
(Space Sciences Series of ISSI, Springer), Space Science Review
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