488 research outputs found
Cosmic ray propagation in the local superbubble
It is suggested that a ring of HI gas lying in the galactic plane is part of a supershell which formed some 3 x to the 7th power years ago. The consequences of a closed magnetic supershell for cosmic ray propagation are examined and it is concluded that there is no evidence which precludes the production and trapping of cosmic rays in such a region. A consequence of superbubble confinement is that the mean age of cosmic rays would be independent of energy. This can be tested by high energy observations of the isotopic composition of Be
Dynamical QCD Predictions for Ultrahigh Energy Neutrino Cross Sections
Neutrino-nucleon total cross sections for neutrino energies up to ultrahigh
energies (UHE), E_\nu=10^12 GeV, are evaluated within the framework of the
dynamical (radiative) parton model. The expected uncertainties of these
predictions do not exceed the level of about 20 % at the highest energies where
contributions of parton distributions in the yet unmeasured region around
x\simeq 10^-8 to 10^-9 are non-negligible. This is far more accurate than
estimated uncertainties of about 2^+-1 due to ad hoc extrapolations of parton
distributions to x<10^-5 required for calculating UHE cosmic neutrino event
rates.Comment: 16 pages, LaTeX, 4 figures, uses epsfig and amssymb styl
Kiloparsec-Scale Jets in FR I Radio Galaxies and the Gamma-Ray Background
We discuss the contribution of kiloparsec-scale jets in FR I radio galaxies
to the diffuse gamma-ray background radiation. The analyzed gamma-ray emission
comes from inverse-Compton scattering of starlight photon fields by the
ultrarelativistic electrons whose synchrotron radiation is detected from such
sources at radio, optical and X-ray energies. We find that these objects, under
the minimum-power hypothesis (corresponding to a magnetic field of 300 muG in
the brightest knots of these jets), can contribute about one percent to the
extragalactic gamma-ray background measured by EGRET. We point out that this
result already indicates that the magnetic fields in kpc-scale jets of
low-power radio galaxies are not likely to be smaller than 10 muG on average,
as otherwise the extragalactic gamma-ray background would be overproduced.Comment: 18 pages, 3 figures included. ApJ accepte
Electromagnetic Cascades and Cascade Nucleosynthesis in the Early Universe
We describe a calculation of electromagnetic cascading in radiation and
matter in the early universe initiated by the decay of massive particles or by
some other process. We have used a combination of Monte Carlo and numerical
techniques which enables us to use exact cross sections, where known, for all
the relevant processes. In cascades initiated after the epoch of big bang
nucleosynthesis -rays in the cascades will photodisintegrate He,
producing He and deuterium. Using the observed He and deuterium
abundances we are able to place constraints on the cascade energy deposition as
a function of cosmic time. In the case of the decay of massive primordial
particles, we place limits on the density of massive primordial particles as a
function of their mean decay time, and on the expected intensity of decay
neutrinos.Comment: compressed and uuencoded postscript. We now include a comparison with
previous work of the photon spectrum in the cascade and the limits we
calculate for the density of massive particles. The method of calculation of
photon spectra at low energies has been improved. Most figures are revised.
Our conclusions are substantially unchange
Search for bursts in air shower data
There have been reports in recent years of the possible observation of bursts in air shower data. If such events are truly of an astrophysical nature then, they represent an important new class of phemonenon since no other bursts have been observed above the MeV level. The spectra of conventional gamma ray bursts are unknown at higher energies but their observed spectra at MeV energies appear generally to exhibit a steepening in the higher MeV range and are thus unlikely to extrapolate to measurable fluxes at air shower energies. An attempt has been made to look for deviations from randomness in the arrival times of air showers above approx. 10 to the 14th power eV with a number of systems and results so far are presented here. This work will be continued for a substantial period of ime with a system capable of recording bursts with multiple events down to a spacing of 4 microns. Earlier data have also been searched for the possible association of air shower events with a glitch of the Vela pulsar
GeV Photons from Ultra High Energy Cosmic Rays accelerated in Gamma Ray Bursts
Gamma-ray bursts are produced by the dissipation of the kinetic energy of a
highly relativistic fireball, via the formation of a collisionless shock. When
this happens, Ultra High Energy Cosmic Rays up to 10^20 eV are produced. I show
in this paper that these particles produce, via synchrotron emission as they
cross the acceleration region, photons up to 300 GeV which carry away a small,
~0.01, but non-negligible fraction of the total burst energy. I show that, when
the shock occurs with the interstellar medium, the optical depth to
photon-photon scattering, which might cause energy degradation of the photons,
is small. The burst thusly produced would be detected at Earth simultaneoulsy
with the parent gamma-ray burst, although its duration may differ significantly
from that of the lower energy photons. The expected fluences, ~10^{-5}-10^{-6}
erg/cm^2 are well within the range of planned detectors. A new explanation for
the exceptional burst GRB 940217 is discussed.Comment: Accepted for publication in The Physical Review Letters. 4 pages,
RevTeX needed, no figure
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
Ultra high energy neutrinos from gamma ray bursts
Protons accelerated to high energies in the relativistic shocks that generate
gamma ray bursts photoproduce pions, and then neutrinos in situ. I show that
ultra high energy neutrinos (> 10^19 eV) are produced during the burst and the
afterglow. A larger flux, also from bursts, is generated via photoproduction
off CMBR photons in flight but is not correlated with currently observable
bursts, appearing as a bright background. Adiabatic/synchrotron losses from
protons/pions/muons are negligible. Temporal and directional coincidences with
bursts detected by satellites can separate correlated neutrinos from the
background.Comment: Adiabatic/synchrotron losses from protons/pions/muons shown to be
negligible. Accepted for publication in Phys. Rev. Letters. RevTe
Extremely High Energy Neutrinos, Neutrino Hot Dark Matter, and the Highest Energy Cosmic Rays
Extremely high energy (up to 10**(22) eV) cosmic neutrino beams initiate high
energy particle cascades in the background of relic neutrinos from the Big
Bang. We perform numerical calculations to show that such cascades could
contribute more than 10% to the observed cosmic ray flux above 10**(19) eV if
neutrinos have masses in the electron volt range. The required intensity of
primary neutrinos could be consistent with astrophysical models for their
production if the maximum neutrino energy reaches to 10**(22) eV and the
massive neutrino dark matter is locally clustered. Future observations of ultra
high energy cosmic rays will lead to an indirect but practical search for
neutrino dark matter.Comment: 4 latex pages, 3 postscript figures included, uses revtex.sty and
psfig.sty. Submitted to Physical Review Letter
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