2,488 research outputs found
The Effect of a Non-Thermal Tail on the Sunyaev-Zeldovich Effect in clusters of galaxies
We study the spectral distortions of the cosmic microwave background
radiation induced by the Sunyaev-Zel'dovich (SZ) effect in clusters of galaxies
when the target electrons have a modified Maxwell-Boltzmann distribution with a
high-energy non-thermal tail. Bremsstrahlung radiation from this type of \\
electron distribution may explain the supra-thermal X-ray emission observed in
some clusters such as the Coma cluster and A2199 and serve as an alternative to
the classical but problematic inverse Compton scattering interpretation. We
show that the SZ effect can be used as a powerful tool to probe the electron
distribution in clusters of galaxies and discriminate among these different
interpretations of the X-ray excess. The existence of a non-thermal tail can
have important consequences for cluster based estimators of cosmological
parameters.Comment: 14 pages, 3 figures, version to be published in ApJ. Let
Pierre Auger Data, Photons, and Top-Down Cosmic Ray Models
We consider the ultra-high energy cosmic ray (UHECR) spectrum as measured by
the Pierre Auger Observatory. Top-down models for the origin of UHECRs predict
an increasing photon component at energies above about eV. Here we
present a simple prescription to compare the Auger data with a prediction
assuming a pure proton component or a prediction assuming a changing primary
component appropriate for a top-down model. We find that the UHECR spectrum
predicted in top-down models is a good fit to the Auger data. Eventually, Auger
will measure a composition-independent spectrum and will be capable of either
confirming or excluding the quantity of photons predicted in top-down models.Comment: 8 pages, 3 figure
Reduced neural sensitivity to social stimuli in infants at risk for autism
In the hope of discovering early markers of autism, attention has recently turned to the study of infants at risk owing to being the younger siblings of children with autism. Because the condition is highly heritable, later-born siblings of diagnosed children are at substantially higher risk for developing autism or the broader autism phenotype than the general population. Currently, there are no strong predictors of autism in early infancy and diagnosis is not reliable until around 3 years of age. Because indicators of brain functioning may be sensitive predictors, and atypical social interactions are characteristic of the syndrome, we examined whether temporal lobe specialization for processing visual and auditory social stimuli during infancy differs in infants at risk. In a functional near-infrared spectroscopy study, infants aged 4–6 months at risk for autism showed less selective neural responses to social stimuli (auditory and visual) than low-risk controls. These group differences could not be attributed to overall levels of attention, developmental stage or chronological age. Our results provide the first demonstration of specific differences in localizable brain function within the first 6 months of life in a group of infants at risk for autism. Further, these differences closely resemble known patterns of neural atypicality in children and adults with autism. Future work will determine whether these differences in infant neural responses to social stimuli predict either later autism or the broader autism phenotype frequently seen in unaffected family members
Identifying Nearby UHECR Accelerators using UHE (and VHE) Photons
Ultra-high energy photons (UHE, E > 10^19 eV) are inevitably produced during
the propagation of 10^20 eV protons in extragalactic space. Their short
interaction lengths (<20 Mpc) at these energies, combined with the impressive
sensitivity of the Pierre Auger Observatory detector to these particles, makes
them an ideal probe of nearby ultra-high-energy cosmic ray (UHECR) sources. We
here discuss the particular case of photons from a single nearby (within 30
Mpc) source in light of the possibility that such an object might be
responsible for several of the UHECR events published by the Auger
collaboration. We demonstrate that the photon signal accompanying a cluster of
a few > 6x10^19 eV UHECRs from such a source should be detectable by Auger in
the near future. The detection of these photons would also be a signature of a
light composition of the UHECRs from the nearby source.Comment: 4 pages, 2 figures, accepted for publication in PR
Ultra-High Energy Cosmic Rays from Young Neutron Star Winds
The long-held notion that the highest-energy cosmic rays are of distant
extragalactic origin is challenged by observations that events above eV do not exhibit the expected high-energy cutoff from photopion
production off the cosmic microwave background. We suggest that these
unexpected ultra-high-energy events are due to iron nuclei accelerated from
young strongly magnetized neutron stars through relativistic MHD winds. We find
that neutron stars whose initial spin periods are shorter than ms, where is the surface magnetic field, can
accelerate iron cosmic rays to greater than eV. These ions can
pass through the remnant of the supernova explosion that produced the neutron
star without suffering significant spallation reactions. For plausible models
of the Galactic magnetic field, the trajectories of the iron ions curve
sufficiently to be consistent with the observed arrival directions of the
highest energy events.Comment: 11 pages, 1 figure, replaced with revised version, some references
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