10 research outputs found
The Origin of Galactic Cosmic Rays
Motivated by recent measurements of the major components of the cosmic
radiation around 10 TeV/nucleon and above, we discuss the phenomenology of a
model in which there are two distinct kinds of cosmic ray accelerators in the
galaxy. Comparison of the spectra of hydrogen and helium up to 100 TeV per
nucleon suggests that these two elements do not have the same spectrum of
magnetic rigidity over this entire region and that these two dominant elements
therefore receive contributions from different sources.Comment: To be published in Physical Review D, 13 pages, with 3 figures,
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Superheavy dark matter and ultrahigh energy cosmic rays
The phase of inflationary expansion in the early universe produces superheavy
relics in a mass window between 10^{12} GeV and 10^{14} GeV. Decay or
annihilation of these superheavy relics can explain the observed ultrahigh
energy cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff. We emphasize that
the pattern of cosmic ray arrival directions with energies beyond 20 EeV will
decide between the different proposals for the origin of ultrahigh energy
cosmic rays.Comment: Based on an invited talk given by RD at Theory Canada 1, Vancouver,
June 2-5, 200
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
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'
UHECR as Decay Products of Heavy Relics? The Lifetime Problem
The essential features underlying the top-down scenarii for UHECR are
discussed, namely, the stability (or lifetime) imposed to the heavy objects
(particles) whatever they be: topological and non-topological solitons,
X-particles, cosmic defects, microscopic black-holes, fundamental strings. We
provide an unified formula for the quantum decay rate of all these objects as
well as the particle decays in the standard model. The key point in the
top-down scenarii is the necessity to adjust the lifetime of the heavy object
to the age of the universe. This ad-hoc requirement needs a very high
dimensional operator to govern its decay and/or an extremely small coupling
constant. The natural lifetimes of such heavy objects are, however, microscopic
times associated to the GUT energy scale (sim 10^{-28} sec. or shorter). It is
at this energy scale (by the end of inflation) where they could have been
abundantly formed in the early universe and it seems natural that they decayed
shortly after being formed.Comment: 11 pages, LaTex, no figures, updated versio
Particle acceleration in relativistic outows
In this review we confront the current theoretical understanding of particle
acceleration at relativistic outflows with recent observational results on
various source classes thought to involve such outflows, e.g. gamma-ray bursts,
active galactic nuclei, and pulsar wind nebulae. We highlight the possible
contributions of these sources to ultra-high-energy cosmic rays.Comment: Accepted for publication in Space Science Reviews, will also appear
as chapter of an ISSI publicatio
Planck 2018 results: IX. Constraints on primordial non-Gaussianity
We analyse the Planck full-mission cosmic microwave background (CMB) temperature and E-mode polarization maps to obtain constraints on primordial non-Gaussianity (NG). We compare estimates obtained from separable template-fitting, binned, and optimal modal bispectrum estimators, finding consistent values for the local, equilateral, and orthogonal bispectrum amplitudes. Our combined temperature and polarization analysis produces the following final results: flocalNL= -0.9 ± 5.1; fequilNL= -26 ± 47; and forthoNL= -38 ± 24 (68% CL, statistical). These results include low-multipole (4 †â < 40) polarization data that are not included in our previous analysis. The results also pass an extensive battery of tests (with additional tests regarding foreground residuals compared to 2015), and they are stable with respect to our 2015 measurements (with small fluctuations, at the level of a fraction of a standard deviation, which is consistent with changes in data processing). Polarizationonly bispectra display a significant improvement in robustness; they can now be used independently to set primordial NG constraints with a sensitivity comparable to WMAP temperature-based results and they give excellent agreement. In addition to the analysis of the standard local, equilateral, and orthogonal bispectrum shapes, we consider a large number of additional cases, such as scale-dependent feature and resonance bispectra, isocurvature primordial NG, and parity-breaking models, where we also place tight constraints but do not detect any signal. The nonprimordial lensing bispectrum is, however, detected with an improved significance compared to 2015, excluding the null hypothesis at 3.5Ï. Beyond estimates of individual shape amplitudes, we also present model-independent reconstructions and analyses of the Planck CMB bispectrum. Our final constraint on the local primordial trispectrum shape is glocalNL= (-5.8 ± 6.5) Ă 104(68% CL, statistical), while constraints for other trispectrum shapes are also determined. Exploiting the tight limits on various bispectrum and trispectrum shapes, we constrain the parameter space of different early-Universe scenarios that generate primordial NG, including general single-field models of inflation, multi-field models (e.g. curvaton models), models of inflation with axion fields producing parity-violation bispectra in the tensor sector, and inflationary models involving vector-like fields with directionally-dependent bispectra. Our results provide a high-precision test for structure-formation scenarios, showing complete agreement with the basic picture of the CDM cosmology regarding the statistics of the initial conditions, with cosmic structures arising from adiabatic, passive, Gaussian, and primordial seed perturbations