2,686 research outputs found
Centaurus A as the Source of ultra-high energy cosmic rays?
We present numerical simulations for energy spectra and angular distributions
of nucleons above 10^{19} eV injected by the radio-galaxy Centaurus A at a
distance 3.4 Mpc and propagating in extra-galactic magnetic fields in the
sub-micro Gauss range. We show that field strengths B~0.3 micro Gauss, as
proposed by Farrar and Piran, cannot provide sufficient angular deflection to
explain the observational data. A magnetic field of intensity ~1 micro Gauss
could reproduce the observed large-scale isotropy and could marginally explain
the observed energy spectrum. However, it would not readily account for the
E=320 plusminus 93 EeV Fly's Eye event that was detected at an angle 136
degrees away from Cen-A. Such a strong magnetic field also saturates
observational upper limits from Faraday rotation observations and X-ray
bremsstrahlung emission from the ambient gas (assuming equipartition of
energy). This scenario may already be tested by improving magnetic field limits
with existing instruments. We also show that high energy cosmic ray experiments
now under construction will be able to detect the level of anisotropy predicted
by this scenario. We conclude that for magnetic fields B~0.1-0.5 micro Gauss,
considered as more reasonable for the local Supercluster environment, in all
likelihood at least a few sources within ~10 Mpc from the Earth should
contribute to the observed ultra high energy cosmic ray flux.Comment: 7 latex pages, 7 postscript figures included; for related numerical
simulations see also http://www.iap.fr/users/sigl/r2e.htm
Current-driven filamentation upstream of magnetized relativistic collisionless shocks
The physics of instabilities in the precursor of relativistic collisionless
shocks is of broad importance in high energy astrophysics, because these
instabilities build up the shock, control the particle acceleration process and
generate the magnetic fields in which the accelerated particles radiate. Two
crucial parameters control the micro-physics of these shocks: the magnetization
of the ambient medium and the Lorentz factor of the shock front; as of today,
much of this parameter space remains to be explored. In the present paper, we
report on a new instability upstream of electron-positron relativistic shocks
and we argue that this instability shapes the micro-physics at moderate
magnetization levels and/or large Lorentz factors. This instability is seeded
by the electric current carried by the accelerated particles in the shock
precursor as they gyrate around the background magnetic field. The compensation
current induced in the background plasma leads to an unstable configuration,
with the appearance of charge neutral filaments carrying a current of the same
polarity, oriented along the perpendicular current. This ``current-driven
filamentation'' instability grows faster than any other instability studied so
far upstream of relativistic shocks, with a growth rate comparable to the
plasma frequency. Furthermore, the compensation of the current is associated
with a slow-down of the ambient plasma as it penetrates the shock precursor (as
viewed in the shock rest frame). This slow-down of the plasma implies that the
``current driven filamentation'' instability can grow for any value of the
shock Lorentz factor, provided the magnetization \sigma <~ 10^{-2}. We argue
that this instability explains the results of recent particle-in-cell
simulations in the mildly magnetized regime.Comment: 14 pages, 8 figures; to appear in MNRA
Particle transport in tangled magnetic fields and Fermi acceleration at relativistic shocks
This paper presents a new method of Monte-Carlo simulations of test particle
Fermi acceleration at relativistic shocks. The particle trajectories in tangled
magnetic fields are integrated out exactly from entry to exit through the
shock, and the conditional probability of return as a function of ingress and
egress pitch angles is constructed by Monte-Carlo iteration. These upstream and
downstream probability laws are then used in conjunction with the energy gain
formula at shock crossing to reproduce Fermi acceleration. For pure Kolmogorov
magnetic turbulence upstream and downstream, the spectral index is found to
evolve smoothly from s=2.09 +/- 0.02 for mildly relativistic shocks with
Lorentz factor Gamma=2 to s=2.26 +/- 0.04 in the ultra-relativistic limit Gamma
>> 1. The energy gain is ~Gamma^2 at first shock crossing, and ~2 in all
subsequent cycles as anticipated by Gallant & Achterberg (1999). The
acceleration timescale is found to be as short as a fraction of Larmor time
when Gamma >> 1.Comment: 4 pages, 4 figures, v2: minor modifications, version to appear in
ApJ
Particle Transport in intense small scale magnetic turbulence with a mean field
Various astrophysical studies have motivated the investigation of the
transport of high energy particles in magnetic turbulence, either in the source
or en route to the observation sites. For strong turbulence and large rigidity,
the pitch-angle scattering rate is governed by a simple law involving a mean
free path that increases proportionally to the square of the particle energy.
In this paper, we show that perpendicular diffusion deviates from this behavior
in the presence of a mean field. We propose an exact theoretical derivation of
the diffusion coefficients and show that a mean field significantly changes the
transverse diffusion even in the presence of a stronger turbulent field. In
particular, the transverse diffusion coefficient is shown to reach a finite
value at large rigidity instead of increasing proportionally to the square of
the particle energy. Our theoretical derivation is corroborated by a dedicated
Monte Carlo simulation. We briefly discuss several possible applications in
astrophysics.Comment: (9 pages, 6 figures, revised version with minor changes
Constraints on cosmic-ray efficiency in the supernova remnant RCW 86 using multi-wavelength observations
Several young supernova remnants (SNRs) have recently been detected in the
high-energy and very-high-energy gamma-ray domains. As exemplified by RX
J1713.7-3946, the nature of this emission has been hotly debated, and direct
evidence for the efficient acceleration of cosmic-ray protons at the SNR shocks
still remains elusive. We analyzed more than 40 months of data acquired by the
Large Area Telescope (LAT) on-board the Fermi Gamma-Ray Space Telescope in the
HE domain, and gathered all of the relevant multi-wavelength (from radio to VHE
gamma-rays) information about the broadband nonthermal emission from RCW 86.
For this purpose, we re-analyzed the archival X-ray data from the ASCA/Gas
Imaging Spectrometer (GIS), the XMM-Newton/EPIC-MOS, and the RXTE/Proportional
Counter Array (PCA). Beyond the expected Galactic diffuse background, no
significant gamma-ray emission in the direction of RCW 86 is detected in any of
the 0.1-1, 1-10 and 10-100 GeV Fermi-LAT maps. In the hadronic scenario, the
derived HE upper limits together with the HESS measurements in the VHE domain
can only be accommodated by a spectral index Gamma <= 1.8, i.e. a value
in-between the standard (test-particle) index and the asymptotic limit of
theoretical particle spectra in the case of strongly modified shocks. The
interpretation of the gamma-ray emission by inverse Compton scattering of high
energy electrons reproduces the multi-wavelength data using a reasonable value
for the average magnetic field of 15-25 muG. For these two scenarios, we
assessed the level of acceleration efficiency. We discuss these results in the
light of existing estimates of the magnetic field strength, the effective
density and the acceleration efficiency in RCW 86.Comment: Accepted for publication in A&A; 10 pages and 4 figure
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