231 research outputs found
Very Strong TeV Emission as Gamma-Ray Burst Afterglows
Gamma-ray bursts (GRBs) and following afterglows are considered to be
produced by dissipation of kinetic energy of a relativistic fireball and
radiation process is widely believed as synchrotron radiation or inverse
Compton scattering of electrons. We argue that the transfer of kinetic energy
of ejecta into electrons may be inefficient process and hence the total energy
released by a GRB event is much larger than that emitted in soft gamma-rays, by
a factor of \sim (m_p/m_e). We show that, in this case, very strong emission of
TeV gamma-rays is possible due to synchrotron radiation of protons accelerated
up to \sim 10^{21} eV, which are trapped in the magnetic field of afterglow
shock and radiate their energy on an observational time scale of \sim day. This
suggests a possibility that GRBs are most energetic in TeV range and such TeV
gamma-rays may be detectable from GRBs even at cosmological distances, i.e., z
\sim 1, by currently working ground-based telescopes. Furthermore, this model
gives a quantitative explanation for the famous long-duration GeV photons
detected from GRB940217. If TeV gamma-ray emission which is much more energetic
than GRB photons is detected, it provides a strong evidence for acceleration of
protons up to \sim 10^{21} eV.Comment: 10 pages, no figure. To appear in ApJ Letter
Invariance Violation Extends the Cosmic Ray Horizon ?
We postulate in the present paper that the energy-momentum relation is
modified for very high energy particles to violate Lorentz invariance and the
speed of photon is changed from the light velocity c. The violation effect is
amplified, in a sensitive way to detection, through the modified kinematical
constraints on the conservation of energy and momentum, in the absorption
process of gamma-rays colliding against photons of longer wavelengths and
converting into an electron-positron pair. For gamma-rays of energies higher
than 10 TeV, the minimum energy of the soft photons for the reaction and then
the absorption mean free path of gamma-rays are altered by orders of magnitude
from the ones conventionally estimated. Consideration is similarly applied to
high energy cosmic ray protons. The consequences may require the standard
assumptions on the maximum distance that very high energy radiation can travel
from to be revised.Comment: 14 pages, 1 figure, to be published in Ap J Letter
Ultra-high energy cosmic rays threshold in Randers-Finsler space
Kinematics in Finsler space is used to study the propagation of ultra high
energy cosmic rays particles through the cosmic microwave background radiation.
We find that the GZK threshold is lifted dramatically in Randers-Finsler space.
A tiny deformation of spacetime from Minkowskian to Finslerian allows more
ultra-high energy cosmic rays particles arrive at the earth. It is suggested
that the lower bound of particle mass is related with the negative second
invariant speed in Randers-Finsler space
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. II: anisotropy
We investigate the effects of stochasticity in the spatial and temporal
distribution of supernova remnants on the anisotropy of cosmic rays observed at
Earth. The calculations are carried out for different choices of the diffusion
coefficient D(E) for propagation in the Galaxy. The propagation and spallation
of nuclei are taken into account. At high energies we assume that
, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars with and without
accounting for the spiral structure of the Galaxy. Our calculations allow us to
determine the contribution to anisotropy resulting from both the large scale
distribution of SNRs in the Galaxy and the random distribution of the nearest
remnants. The naive expectation that the anisotropy amplitude scales as D(E) is
shown to be an oversimplification which does not reflect in the predicted
anisotropy for any realistic distribution of the sources. The fluctuations in
the anisotropy pattern are dominated by nearby sources, so that predicting or
explaining the observed anisotropy amplitude and phase becomes close to
impossible. We find however that the very weak energy dependence of the
anisotropy amplitude below GeV and the rise at higher energies, can
best be explained if the diffusion coefficient is . Faster
diffusion, for instance with , leads in general to an exceedingly
large anisotropy amplitude. The spiral structure introduces interesting trends
in the energy dependence of the anisotropy pattern, which qualitatively reflect
the trend seen in the data. For large values of the halo size we find that the
anisotropy becomes dominated by the large scale regular structure of the source
distribution, leading indeed to a monotonic increase of with energy.Comment: 21 Pages, to appear in JCA
Quantum theory's last challenge
Quantum mechanics is now 100 years old and still going strong. Combining
general relativity with quantum mechanics is the last hurdle to be overcome in
the "quantum revolution".Comment: (9 pages, LaTex) This is the preprint version of an article that
appeared in the issue 6813 (volume 408) of Nature, as part of a 3-article
celebration of the 100th anniversary of Planck's solution of the
black-body-radiation proble
Formation of hard VHE gamma-ray spectra of blazars due to internal photon-photon absorption
The energy spectra of TeV gamma-rays from blazars, after being corrected for
intergalatic absorption in the Extragalactic Background Light (EBL), appear
unusually hard, a fact that poses challenges to the conventional models of
particle acceleration in TeV blazars and/or to the EBL models. In this paper we
show that the internal absorption of gamma-rays caused by interactions with
dense narrow-band radiation fields in the vicinity of compact gamma-ray
production regions can lead to the formation of gamma-ray spectra of an almost
arbitrary hardness. This allows significant relaxation of the current tight
constraints on particle acceleration and radiation models, although at the
expense of enhanced requirements to the available nonthermal energy budget. The
latter, however, is not a critical issue, as long as it can be largely
compensated by the Doppler boosting, assuming very large () Doppler
factors of the relativistically moving gamma-ray production regions. The
suggested scenario of formation of hard gamma-ray spectra predicts detectable
synchrotron radiation of secondary electron-positron pairs which might require
a revision of the current ``standard paradigm'' of spectral energy
distributions of gamma-ray blazars. If the primary gamma-rays are of hadronic
origin related to or interactions, the ``internal gamma-ray
absorption'' model predicts neutrino fluxes close to the detection threshold of
the next generation high energy neutrino detectors.Comment: 10 pages, 8 figures, submitted to MNRA
Interplay between curvature and Planck-scale effects in astrophysics and cosmology
Several recent studies have considered the implications for astrophysics and
cosmology of some possible nonclassical properties of spacetime at the Planck
scale. The new effects, such as a Planck-scale-modified energy-momentum
(dispersion) relation, are often inferred from the analysis of some quantum
versions of Minkowski spacetime, and therefore the relevant estimates depend
heavily on the assumption that there could not be significant interplay between
Planck-scale and curvature effects. We here scrutinize this assumption, using
as guidance a quantum version of de Sitter spacetime with known Inonu-Wigner
contraction to a quantum Minkowski spacetime. And we show that, contrary to
common (but unsupported) beliefs, the interplay between Planck-scale and
curvature effects can be significant. Within our illustrative example, in the
Minkowski limit the quantum-geometry deformation parameter is indeed given by
the Planck scale, while in the de Sitter picture the parameter of quantization
of geometry depends both on the Planck scale and the curvature scalar. For the
much-studied case of Planck-scale effects that intervene in the observation of
gamma-ray bursts we can estimate the implications of "quantum spacetime
curvature" within robust simplifying assumptions. For cosmology at the present
stage of the development of the relevant mathematics one cannot go beyond
semiheuristic reasoning, and we here propose a candidate approximate
description of a quantum FRW geometry, obtained by patching together pieces
(with different spacetime curvature) of our quantum de Sitter. This
semiheuristic picture, in spite of its limitations, provides rather robust
evidence that in the early Universe the interplay between Planck-scale and
curvature effects could have been particularly significant.Comment: 26 pages
Probing the quantum-gravity realm with slow atoms
For the study of Planck-scale modifications of the energy-momentum dispersion
relation, which had been previously focused on the implications for
ultrarelativistic (ultrafast) particles, we consider the possible role of
experiments involving nonrelativistic particles, and particularly atoms. We
extend a recent result establishing that measurements of "atom-recoil
frequency" can provide insight that is valuable for some theoretical models.
And from a broader perspective we analyze the complementarity of the
nonrelativistic and the ultrarelativistic regimes in this research area.Comment: LaTex, 13 page
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