525 research outputs found
TeV Cherenkov Events as Bose-Einstein Gamma Condensations
The recent detection of gamma radiation from Mkn 501 at energies as high as
25 TeV suggests stringent upper bounds on the diffuse, far infrared,
extragalactic radiation density. The production of electron-positron pairs
through photon-photon collisions would prevent gamma photons of substantially
higher energies from reaching us across distances of order 100 Mpc. However,
coherently arriving TeV or sub-TeV gammas - Bose-Einstein condensations of
photons at these energies - could mimic the Cherenkov shower signatures of
extremely energetic gammas. To better understand such events, we describe their
observational traits and discuss how they might be generated.Comment: 12 pages, 2 figures, accepted for publication in Ap.J.(Lett.
Neutrino telescopes under the ocean: The case for ANTARES
Neutrino telescopes offer an alternative way to explore the Universe. Several
projects are in operation or under construction. A detector under the ocean is
very promising because of the very accurate angular resolution that it
provides. The ANTARES project is intended to demonstrate the feasibilty of such
a detector.Comment: Talk given at the Neutrino98 conference, Takayama, Japan, June 4-9,
199
On the dependence of the spectral parameters on the observational conditions in homogeneous time dependent models of the TeV blazars
Most of current models of TeV blazars emission assume a Synchrotron
Self-Compton mechanism where relativistic particles emit both synchrotron
radiation and Inverse Compton photons. For sake of simplicity, these models
usually consider only steady state emission. The spectral features are thus
only related to the shape of the particle distribution, and do not depend on
the timing of observations. In this letter, we study the effect of, firstly,
the lag between the beginning of the injection of the fresh particles and the
trigger of the observation, and secondly, of a finite injection duration. We
illustrate these effects considering an analytical time-dependent model of the
synchrotron emission by a monoenergetic distribution of leptons. We point out
that the spectral shape can be in fact very dependent on observational
conditions if the particle injection term is time-dependent, particularly
taking into account the effect of the time averaging procedure on the final
shape of the SED. Consequences on the acceleration process are also discussed.Comment: Letter to Editor, accepted for publication in A&
Propagation of ultra-high energy protons in the nearby universe
We present a new calculation of the propagation of protons with energies
above eV over distances of up to several hundred Mpc. The calculation
is based on a Monte Carlo approach using the event generator
SOPHIA for the simulation of hadronic nucleon-photon interactions and a
realistic integration of the particle trajectories in a random extragalactic
magnetic field. Accounting for the proton scattering in the magnetic field
affects noticeably the nucleon energy as a function of the distance to their
source and allows us to give realistic predictions on arrival energy, time
delay, and arrival angle distributions and correlations as well as secondary
particle production spectra.Comment: 12 pages, 9 figures, ReVTeX. Physical Review D, accepte
Limits on models of the ultrahigh energy cosmic rays based on topological defects
An erratum exists for this article. Please see the description link below for details.Using the propagation of ultrahigh energy nucleons, photons, and electrons in the universal radiation backgrounds, we obtain limits on the luminosity of topological defect scenarios for the origin of the highest energy cosmic rays. The limits are set as a function of the mass of the X particles emitted by the cosmic strings or other defects, the cosmological evolution of the topological defects, and the strength of the extragalactic magnetic fields. The existing data on the cosmic ray spectrum and on the isotropic 100 MeV gamma-ray background limit significantly the parameter space in which topological defects can generate the flux of the highest energy cosmic rays, and rule out models with the standard X-particle mass of 10¹⁶GeV and higher.R. J. Protheroe and Todor Stane
Very high energy gamma-ray emission from X-ray transients during major outbursts
Context: Some high mass X-ray binaries (HMXB) have been recently confirmed as
gamma-ray sources by ground based Cherenkov telescopes. In this work, we
discuss the gamma-ray emission from X-ray transient sources formed by a Be star
and a highly magnetized neutron star. This kind of systems can produce variable
hadronic gamma-ray emission through the mechanism proposed by Cheng and
Ruderman, where a proton beam accelerated in the pulsar magnetosphere impacts
the transient accretion disk. We choose as case of study the best known system
of this class: A0535+26. Aims: We aim at making quantitative predictions about
the very high-energy radiation generated in Be-X ray binary systems with
strongly magnetized neutron stars. Methods: We study the gamma-ray emission
generated during a major X-ray outburst of a HMXB adopting for the model the
parameters of A0535+26. The emerging photon signal from the disk is determined
by the grammage of the disk that modulates the optical depth. The
electromagnetic cascades initiated by photons absorbed in the disk are
explored, making use of the so-called "Approximation A" to solve the cascade
equations. Very high energy photons induce Inverse Compton cascades in the
photon field of the massive star. We implemented Monte Carlo simulations of
these cascades, in order to estimate the characteristics of the resulting
spectrum. Results: TeV emission should be detectable by Cherenkov telescopes
during a major X-ray outburst of a binary formed by a Be star and a highly
magnetized neutron star. The gamma-ray light curve is found to evolve in
anti-correlation with the X-ray signal.Comment: 8 pages, 7 figures, accepted for publication in Astronomy and
Astrophysical journa
p-Branes and the GZK Paradox
In spacetimes with asymmetric extra dimensions, cosmic neutrino interactions
may be extraordinarily enhanced by p-brane production. Brane formation and
decay may then initiate showers deep in the Earth's atmosphere at rates far
above the standard model rate. We explore the p-brane discovery potential of
cosmic ray experiments. The absence of deeply penetrating showers at AGASA
already provides multi-TeV bounds on the fundamental Planck scale that
significantly exceed those obtained from black hole production in symmetric
compactification scenarios. This sensitivity will be further enhanced at the
Auger Observatory. We also examine the possibility that p-brane formation
resolves the GZK paradox. For flat compactifications, astrophysical bounds
exclude this explanation. For warped scenarios, a solution could be consistent
with the absence of deep showers only for extra dimensions with fine-tuned
sizes well below the fundamental Planck length. In addition, it requires
moderately penetrating showers, so far not reported, and ~100% modifications to
standard model phenomenology at 100 GeV energies.Comment: 8 pages, 6 figure
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
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