771 research outputs found
The Anisotropy of the Ultra-High Energy Cosmic Rays
Ultra-high energy cosmic rays (UHECRs) may originate from the decay of
massive relic particles in the dark halo of the Galaxy, or they may be produced
by supermassive black holes in the nuclei of nearby galaxies. The anisotropy in
the arrival directions is studied in four dark halo models (cusped, isothermal,
triaxial and tilted) and in four galaxy samples (galaxies intrinsically
brighter than Centaurus A within 50 and 100 Mpc, and brighter than M32 within
50 and 100 Mpc). In decaying dark matter models, the amplitude of the
anisotropy is controlled by the size of the Galactic halo, while the phase is
controlled by the shape. In the northern hemisphere, the amplitude is about 0.5
for cusped haloes, but falls to roughly 0.3 for isothermal haloes. The phase
points in the direction of the Galactic Centre, with deviations of up to 30
degrees possible for triaxial and tilted haloes. The effect of the halo of M31
is too weak to provide conclusive evidence for the decaying dark matter origin
of UHECRs. In extragalactic models, samples of galaxies brighter than Centaurus
A produce substantial anisotropies (roughly 1.8), much larger than the limits
set by the available data. If all galaxies brighter than M32 contribute, then
the anisotropy is more modest (roughly 0.5) and is directed toward mass
concentrations in the supergalactic plane, like the Virgo cluster. Predictions
are made for the southern hemisphere station of the Pierre Auger Observatory.
If the UHECRs have a Galactic origin, then the phase points towards the
Galactic Centre. If they have an extragalactic origin, then it points in the
rough direction of the Fornax cluster. This provides an unambiguous
discriminant and requires about 350-500 events at South Auger.Comment: 38 pages, 8 figures. This version is identical to the one in press at
Astroparticle Physic
Fitting oscillating string gas cosmology to supernova data
In string gas cosmology, extra dimensions are stabilised by a gas of strings.
In the matter-dominated era, competition between matter pushing the extra
dimensions to expand and the string gas pulling them back can lead to
oscillations of the extra dimensions and acceleration in the visible
dimensions. We fit this model to supernova data, taking into account the Big
Bang Nucleosynthesis constraint on the energy density of the string gas. The
fit to the Union set of supernova data is acceptable, but the fit to the
ESSENCE data is poor.Comment: 17 pages, 4 figures. v2: published version. Important correction in
the calculation of distances, added reference
Dark matter distributions around massive black holes: A general relativistic analysis
The cold dark matter at the center of a galaxy will be redistributed by the
presence of a massive black hole. The redistribution may be determined using an
approach pioneered by Gondolo and Silk: begin with a model distribution
function for the dark matter, and ``grow'' the black hole adiabatically,
holding the adiabatic invariants of the motion constant. Unlike the approach of
Gondolo and Silk, which adopted Newtonian theory together with ad hoc
correction factors to mimic general relativistic effects, we carry out the
calculation fully relativistically, using the exact Schwarzschild geometry of
the black hole. We find that the density of dark matter generically vanishes at
r=2R_S, not 4R_S as found by Gondolo and Silk, where R_S is the Schwarzschild
radius, and that the spike very close to the black hole reaches significantly
higher densities. We apply the relativistic adiabatic growth framework to
obtain the final dark matter density for both cored and cusped initial
distributions. Besides the implications of these results for indirect detection
estimates, we show that the gravitational effects of such a dark matter spike
are significantly smaller than the relativistic effects of the black hole,
including frame dragging and quadrupolar effects, for stars orbiting close to
the black hole that might be candidates for testing the black hole no-hair
theorems.Comment: 18 pages, 5 figures, submitted to Phys. Rev
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