88 research outputs found
Cut-offs and pile-ups in shock acceleration spectra
We have examined cutoffs and pile-ups due to various processes in the spectra
of particles produced by shock acceleration, and found that, even in the
absence of energy losses, the shape of the spectrum of accelerated particles at
energies well below the nominal maximum energy depends strongly on the energy
dependence of the diffusion coefficient. This has implications in many areas,
for example, in fitting the observed cosmic ray spectrum with models based on
power-law source spectra and rigidity dependent diffusive escape from the
galaxy. With continuous energy losses, prominent pile-ups may arise, and these
should be included when modelling synchrotron X-ray and inverse Compton
gamma-ray spectra from a shock-accelerated electron population.
We have developed a Monte Carlo/numerical technique to model the shape of the
spectrum for the case of non-continuous energy losses such as inverse Compton
scattering in the Klein-Nishina regime. We find that the shapes of the
resulting cut-offs differ substantially from those arising from continuous
processes, and we suggest that such differences could be observable through
their effect on the spectrum of radiation emitted by a population of recently
accelerated electrons as, for example, may exist in young supernova remnants.Comment: 23 pages, 8 figures, submitted to Astroparticle Physic
The Directional Dependence of the Lunar Cherenkov Technique for UHE Neutrino Detection
The LUNASKA (Lunar UHE Neutrino Astrophysics with the Square Kilometre Array)
project is a theoretical and experimental project developing the lunar
Cherenkov technique for the next generation of giant radio-telescope arrays.
This contribution presents our simulation results on the directional dependence
of the technique for UHE neutrino detection. In particular, these indicate that
both the instantaneous sensitivities and time-integrated limits from lunar
Cherenkov experiments such as those at Parkes, Goldstone, Kalyazin and ATCA are
highly anisotropic. We study the regions of the sky which have not been probed
by either these or other experiments, and present the expected sky coverage of
future experiments with the SKA. Our results show how the sensitivity of Lunar
Cherenkov observations to potential astrophysical sources of UHE particles may
be maximised by choosing appropriate observations dates and antenna-beam
pointing positions.Comment: 4 pages, 4 figures, presented at ARENA 2008, Rome, Ital
Prospects for radio detection of ultra-high energy cosmic rays and neutrinos
The origin and nature of the highest energy cosmic ray events is currently
the subject of intense investigation by giant air shower arrays and fluorescent
detectors. These particles reach energies well beyond what can be achieved in
ground-based particle accelerators and hence they are fundamental probes for
particle physics as well as astrophysics. Because of the scarcity of these
high-energy particles, larger and larger ground-based detectors have been
built. The new generation of digital radio telescopes may play an important
role in this, if properly designed. Radio detection of cosmic ray showers has a
long history but was abandoned in the 1970's. Recent experimental developments
together with sophisticated air shower simulations incorporating radio emission
give a clearer understanding of the relationship between the air shower
parameters and the radio signal, and have led to resurgence in its use.
Observations of air showers by the SKA could, because of its large collecting
area, contribute significantly to measuring the cosmic ray spectrum at the
highest energies. Because of the large surface area of the moon, and the
expected excellent angular resolution of the SKA, using the SKA to detect radio
Cherenkov emission from neutrino-induced cascades in lunar regolith will be
potentially the most important technique for investigating cosmic ray origin at
energies above the photoproduction cut-off. (abridged)Comment: latex, 26 pages, 17 figures, to appear in: "Science with the Square
Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews,
(Elsevier: Amsterdam
Instant preheating mechanism and UHECR
Top-down models assume that the still unexplained Ultra High Energy Cosmic
Rays (UHECR's) are the decay products of superheavy particles. Such particles
may have been produced by one of the post-inflationary reheating mechanisms and
may account for a fraction of the cold dark matter. In this paper, we assess
the phenomenological applicability of the simplest instant preheating framework
not to describe a reheating process, but as a mechanism to generate relic
supermassive particles as possible sources of UHECR's. We use cosmic ray flux
and cold dark matter observational data to constrain the parameters of the
model.Comment: 7 pages, 2 figures, submitted to PR
BL Lac Objects in the Synchrotron Proton Blazar Model
We calculate the spectral energy distribution (SED) of electromagnetic
radiation and the spectrum of high energy neutrinos from BL Lac objects in the
context of the Synchrotron Proton Blazar Model. In this model, the high energy
hump of the SED is due to accelerated protons, while most of the low energy
hump is due to synchrotron radiation by co-accelerated electrons. To accelerate
protons to sufficiently high energies to produce the high energy hump, rather
high magnetic fields are required. Assuming reasonable emission region volumes
and Doppler factors, we then find that in low-frequency peaked BL Lacs (LBLs),
which have higher luminosities than high-frequency peaked BL Lacs (HBLs), there
is a significant contribution to the high frequency hump of the SED from pion
photoproduction and subsequent cascading, including synchrotron radiation by
muons. In contrast, in HBLs we find that the high frequency hump of the SED is
dominated by proton synchrotron radiation. We are able to model the SED of
typical LBLs and HBLs, and to model the famous 1997 flare of Markarian 501. We
also calculate the expected neutrino output of typical BL Lac objects, and
estimate the diffuse neutrino intensity due to all BL Lacs. Because pion
photoproduction is inefficient in HBLs, as protons lose energy predominantly by
synchrotron radiation, the contribution of LBLs dominates the diffuse neutrino
intensity. We suggest that nearby LBLs may well be observable with future
high-sensitivity TeV gamma-ray telescopes.Comment: 33 pages, 20 Figures. Astropart. Phys., accepte
A new estimate of the extragalactic radio background and implications for ultra-high-energy gamma-ray propagation
We make a new estimate of the extragalactic radio background down to kHz
frequencies based on the observed luminosity functions and radio spectra of
normal galaxies and radio galaxies. We have constructed models for the spectra
of these two classes of objects down to low frequencies based on observations
of our Galaxy, other normal galaxies and radio galaxies. We check that the
models and evolution of the luminosity functions give source counts consistent
with data and calculate the radio background expected from kHz to GHz
frequencies.
The motivation for this calculation is that the propagation of ultra-high
energy gamma-rays in the universe is limited by photon-photon pair production
on the radio background. Electromagnetic cascades involving photon-photon pair
production and subsequent synchrotron radiation in the intergalactic magnetic
field may develop. Such gamma-rays may be produced in acceleration sites of
ultra-high energy cosmic rays, as a result of interactions with the microwave
background, or emitted as a result of decay or annihilation of topological
defects. We find that photon-photon pair production on the radio background
remains the dominant attenuation process for gamma-rays from
GeV up to GUT scale energies.Comment: LaTeX, 21 pages, including 10 postscript figures, tar'd and gzip'
High-energy gamma-ray emission from the inner jet of LS I+61 303: the hadronic contribution revisited
LS I+61 303 has been detected by the Cherenkov telescope MAGIC at very high
energies, presenting a variable flux along the orbital motion with a maximum
clearly separated from the periastron passage. In the light of the new
observational constraints, we revisit the discussion of the production of
high-energy gamma rays from particle interactions in the inner jet of this
system. The hadronic contribution could represent a major fraction of the TeV
emission detected from this source. The spectral energy distribution resulting
from p-p interactions is recalculated. Opacity effects introduced by the photon
fields of the primary star and the stellar decretion disk are shown to be
essential in shaping the high-energy gamma-ray light curve at energies close to
200 GeV. We also present results of Monte Carlo simulations of the
electromagnetic cascades developed very close to the periastron passage. We
conclude that a hadronic microquasar model for the gamma-ray emission in LS I
+61 303 can reproduce the main features of its observed high-energy gamma-ray
flux.Comment: 6 pages. Sligth improvements made. Accepted version by Astrophysics
and Space Scienc
High Energy Cosmic Rays from Neutrinos
We discuss recent models in which neutrinos, which are assumed to have mass
in the eV range, originate the highest energy cosmic rays by interaction with
the enhanced density in the galactic halo of the relic cosmic neutrino
background. We make an analytical calculation of the required neutrino fluxes
to show that the parameter space for these models is constrained by horizontal
air shower searches and by the total number of background neutrinos, so that
only models which have fairly unnatural halo sizes and enhanced densities are
allowed.Comment: 14 pages, 3 ps figures. To appear in Phys. Rev.
Contribution of nuclei accelerated by gamma-ray pulsars to cosmic rays in the Galaxy
We consider the galactic population of gamma-ray pulsars as possible sources
of cosmic rays at and just above the ``knee'' in the observed cosmic ray
spectrum at -- eV. We suggest that iron nuclei may be
accelerated in the outer gaps of pulsars, and then suffer partial
photo-disintegration in the non-thermal radiation fields of the outer gaps. As
a result, protons, neutrons, and surviving heavier nuclei are injected into the
expanding supernova remnant. We compute the spectra of nuclei escaping from
supernova remnants into the interstellar medium, taking into account the
observed population of radio pulsars.
Our calculations, which include a realistic model for acceleration and
propagation of nuclei in pulsar magnetospheres and supernova remnants, predict
that heavy nuclei accelerated directly by gamma-ray pulsars could contribute
about 20% of the observed cosmic rays in the knee region. Such a contribution
of heavy nuclei to the cosmic ray spectrum at the knee can significantly
increase the average value of with increasing energy as is suggested
by recent observations.Comment: 21 pages, 5 figures, accepted for publication in Astroparticle
Physic
A lower bound on the local extragalactic magnetic field
Assuming that the hard gamma-ray emission of Cen A is a result of synchrotron
radiation of ultra-relativistic electrons, we derive a lower bound on the local
extragalactic magnetic field, G. This result is consistent with
(and close to) upper bounds on magnetic fields derived from consideration of
cosmic microwave background distortions and Faraday rotation measurements.Comment: Includes extensive discussion of particle acceleration above 10^20 eV
in the hot spot-like region of Cen
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