122 research outputs found
TeV cosmic-ray proton and helium spectra in the myriad model II
Recent observations show that the cosmic ray nuclei spectra start to harden
above 100 GeV, in contradiction with the conventional steady-state cosmic ray
model. We had suggested that this anomaly is due to the propagation effect of
cosmic rays released from local young cosmic ray sources, the total flux of the
cosmic ray should be computed with the myriad model, where contribution from
sources in local catalog is added to the background. However, while the
hardening could be elegantly explained in this model, the model parameters
obtained from the fit skew toward a region with fast diffusion and low
supernova rate in the Galaxy, in tension with other observations. In this
paper, we further explore this model in order to set up a concordant picture.
Two possible improvements related to the cosmic ray sources have been
considered. Firstly, instead of the usual axisymmetric disk model, we
considered a spiral model of source distribution. Secondly, for the nearby and
young sources which are paramount to the hardening, we allow for an
energy-dependent escape time. We find that major improvement comes from the
energy-dependent escape time of the local sources, and with both modifications,
not only the cosmic ray proton and helium anomalies are solved, but also the
parameters attain reasonable range values compatible with other analysis.Comment: 13 pages, 7 figures, 1 table, accepted for publication in RA
Quintessence and the Relic Density of Neutralinos
The archetypal model for the recently discovered dark energy component of the
universe is based on the existence of a scalar field whose dynamical evolution
comes down today to a non-vanishing cosmological constant. In the past - before
big-bang nucleosynthesis for that matter - that scalar field could have gone
through a period of kination during which the universe has expanded at a much
higher pace than what is currently postulated in the standard radiation
dominated cosmology. I examine here the consequences of such a period of
kination on the relic abundance of neutralinos and find that the latter could
be much higher - by three orders of magnitude - than what is estimated in the
canonical derivation. I shortly discuss the implications of this scenario for
the dark matter candidates and their astrophysical signatures.
This new version contains a discussion - see section 2 - of the overshooting
problem and offers perspectives to reconcile an initial period of violent
kination with the existence of a cosmological constant today.Comment: Latex 12 pages, 3 figure
Neutralino Clumps and Cosmic Rays
The halo of the Miky Way might contain numerous and dense substructures
inside which the putative weakly interacting massive particles (suggested as
the main constituent of the astronomical dark matter) would produce a stronger
annihilation signal than in the smooth regions. The closer the nearest clump,
the larger the positron and antiproton cosmic ray fluxes at the Earth. But the
actual distribution of these substructures is not known. The predictions on the
antimatter yields at the Earth are therefore affected by a kind of cosmic
variance whose analysis is the subject of this contribution. The statistical
tools to achieve that goal are presented and Monte Carlo simulations are
compared to analytic results.Comment: 8 pages, proceedings of the 6th International Workshop on the
Identification of Dark Matter, Rhodes Island, Greece, September 11-16, 200
The galactic antiproton spectrum at high energies: background expectation vs. exotic contributions
A new generation of upcoming space-based experiments will soon start to probe
the spectrum of cosmic ray antiparticles with an unprecedented accuracy and, in
particular, will open up a window to energies much higher than those accessible
so far. It is thus timely to carefully investigate the expected antiparticle
fluxes at high energies. Here, we perform such an analysis for the case of
antiprotons. We consider both standard sources as the collision of other cosmic
rays with interstellar matter, as well as exotic contributions from dark matter
annihilations in the galactic halo. Up to energies well above 100 GeV, we find
that the background flux in antiprotons is almost uniquely determined by the
existing low-energy data on various cosmic ray species; for even higher
energies, however, the uncertainties in the parameters of the underlying
propagation model eventually become significant. We also show that if the dark
matter is composed of particles with masses at the TeV scale, which is
naturally expected in extra-dimensional models as well as in certain parameter
regions of supersymmetric models, the annihilation flux can become comparable
to - or even dominate - the antiproton background at the high energies
considered here.Comment: 17 pages revtex4, 7 figures; minor changes (to match the published
version
A fussy revisitation of antiprotons as a tool for Dark Matter searches
Antiprotons are regarded as a powerful probe for Dark Matter (DM) indirect
detection and indeed current data from PAMELA have been shown to lead to
stringent constraints. However, in order to exploit their
constraining/discovery power properly and especially in anticipation of the
exquisite accuracy of upcoming data from AMS, great attention must be put into
effects (linked to their propagation in the Galaxy) which may be perceived as
subleasing but actually prove to be quite relevant. We revisit the computation
of the astrophysical background and of the DM antiproton fluxes fully including
the effects of: diffusive reacceleration, energy losses including tertiary
component and solar modulation (in a force field approximation). We show that
their inclusion can somewhat modify the current bounds, even at large DM
masses, and that a wrong interpretation of the data may arise if they are not
taken into account. The numerical results for the astrophysical background are
provided in terms of fit functions; the results for Dark Matter are
incorporated in the new release of the PPPC4DMID.Comment: v3: small comments and references added, matches journal versio
Systematic effects in the estimate of the local gamma-ray emissivity
We show in this letter that estimates of the local emissivity of
{\gamma}-rays in the GeV-TeV range suffer uncertainties which are of the same
order of magnitude as the current Fermi results. Primary cosmic-ray fluxes,
cosmic-ray propagation, interstellar helium abundance and {\gamma}-ray
production crosssections all affect the estimate of this quantity. We also show
that the so-called nuclear enhancement factor -- though widely used so far to
model the {\gamma}-ray emissivity -- is no longer a relevant quantity given the
latest measurements of the primary cosmic ray proton and helium spectra
Intermediate Mass Black Holes and Nearby Dark Matter Point Sources: A Critical Reassessment
The proposal of a galactic population of intermediate mass black holes
(IMBHs), forming dark matter (DM) ``mini-spikes'' around them, has received
considerable attention in recent years. In fact, leading in some scenarios to
large annihilation fluxes in gamma rays, neutrinos and charged cosmic rays,
these objects are sometimes quoted as one of the most promising targets for
indirect DM searches. In this letter, we apply a detailed statistical analysis
to point out that the existing EGRET data already place very stringent limits
on those scenarios, making it rather unlikely that any of these objects will be
observed with, e.g., the Fermi/GLAST satellite or upcoming Air Cherenkov
telescopes. We also demonstrate that prospects for observing signals in
neutrinos or charged cosmic rays seem even worse. Finally, we address the
question of whether the excess in the cosmic ray positron/electron flux
recently reported by PAMELA/ATIC could be due to a nearby DM point source like
a DM clump or mini-spike; gamma-ray bounds, as well as the recently released
Fermi cosmic ray electron and positron data, again exclude such a possibility
for conventional DM candidates, and strongly constrain it for DM purely
annihilating into light leptons.Comment: 4 pages revtex4, 4 figures. Improved analysis and discussion, added
constraints from Fermi data, corrected figures and updated reference
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