65 research outputs found
Phenomenological Constraints on Axion Models of Dynamical Dark Matter
In two recent papers (arXiv:1106.4546, arXiv:1107.0721), we introduced
"dynamical dark matter" (DDM), a new framework for dark-matter physics in which
the requirement of stability is replaced by a delicate balancing between
lifetimes and cosmological abundances across a vast ensemble of individual
dark-matter components whose collective behavior transcends that normally
associated with traditional dark-matter candidates. We also presented an
explicit model involving axions in large extra spacetime dimensions, and
demonstrated that this model has all of the features necessary to constitute a
viable realization of the general DDM framework. In this paper, we complete our
study by performing a general analysis of all phenomenological constraints
which are relevant to this bulk-axion DDM model. Although the analysis in this
paper is primarily aimed at our specific DDM model, many of our findings have
important implications for bulk axion theories in general. Our analysis can
also serve as a prototype for phenomenological studies of theories in which
there exist large numbers of interacting and decaying particles.Comment: 48 pages, LaTeX, 13 figures, 1 tabl
Cosmological Constraints on Neutrino Injection
We derive general constraints on the relic abundances of a long-lived
particle which mainly decays into a neutrino (and something else) at
cosmological time scales. Such an exotic particle may show up in various
particle-physics models based on physics beyond the standard model. The
constraints are obtained from big-bang nucleosynthesis, cosmic microwave
background and diffuse neutrino and photon fluxes, depending on the lifetime
and the electromagnetic and hadronic branching ratios.Comment: 33 pages, 23 figure
The COMPTEL instrumental line background
The instrumental line background of the Compton telescope COMPTEL onboard the
Compton Gamma-Ray Observatory is due to the activation and/or decay of many
isotopes. The major components of this background can be attributed to eight
individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl.
The identification of instrumental lines with specific isotopes is based on the
line energies as well as on the variation of the event rate with time,
cosmic-ray intensity, and deposited radiation dose during passages through the
South-Atlantic Anomaly. The characteristic variation of the event rate due to a
specific isotope depends on its life-time, orbital parameters such as the
altitude of the satellite above Earth, and the solar cycle. A detailed
understanding of the background contributions from instrumental lines is
crucial at MeV energies for measuring the cosmic diffuse gamma-ray background
and for observing gamma-ray line emission in the interstellar medium or from
supernovae and their remnants. Procedures to determine the event rate from each
background isotope are described, and their average activity in spacecraft
materials over the first seven years of the mission is estimated.Comment: accepted for publication in A&A, 22 pages, 21 figure
Compton Heating of the Intergalactic Medium by the Hard X-ray Background
High-resolution hydrodynamics simulations of the Ly-alpha forest in cold dark
matter dominated cosmologies appear to predict line widths that are
substantially narrower than those observed. Here we point out that Compton
heating of the intergalactic gas by the hard X-ray background (XRB), an effect
neglected in all previous investigations, may help to resolve this discrepancy.
The rate of gain in thermal energy by Compton scattering will dominate over the
energy input from hydrogen photoionization if the XRB energy density is
0.2x/ times higher than the energy density of the UV background at a
given epoch, where x is the hydrogen neutral fraction in units of 1e-6 and
is the mean X-ray photon energy in units of m_ec^2. The numerical
integration of the time-dependent rate equations shows that the intergalactic
medium approaches a temperature of about 1.5e4 K at z>3 in popular models for
the redshift evolution of the extragalactic background radiation. The
importance of Compton heating can be tested experimentally by measuring the
Ly-alpha line-width distribution as a function of redshift, thus the
Lyman-alpha forest may provide a useful probe of the evolution of the XRB at
high redshifts.Comment: LaTeX, 10 pages, 2 figures, final version to be published in the Ap
The Contribution of Blazars to the Extragalactic Diffuse Gamma-ray Background and Their Future Spatial Resolution
We examine the constraints on the luminosity-dependent density evolution
model for the evolution of blazars given the observed spectrum of the diffuse
gamma-ray background (DGRB), blazar source-count distribution, and the blazar
spectral energy distribution sequence model, which relates the observed the
blazar spectrum to its luminosity. We show that the DGRB observed by the Large
Area Telescope (LAT) aboard the Fermi Gamma Ray Space Telescope can be produced
entirely by gamma-ray emission from blazars and nonblazar active galactic
nuclei, and that our blazar evolution model is consistent with and constrained
by the spectrum of the DGRB and flux source-count distribution function of
blazars observed by Fermi-LAT. Our results are consistent with previous work
that used EGRET spectral data to forecast the Fermi-LAT DGRB. The model
includes only three free parameters, and forecasts that >~ 95% of the flux from
blazars will be resolved into point sources by Fermi-LAT with 5 years of
observation, with a corresponding reduction of the flux in the DGRB by a factor
of ~2 to 3 (95% confidence level), which has implications for the Fermi-LAT's
sensitivity to dark matter annihilation photons.Comment: 13 pages, 7 figures; v3: minor changes, matches version to appear in
Phys. Rev.
MeV sterile neutrinos in low reheating temperature cosmological scenarios
It is commonly assumed that the cosmological and astrophysical bounds on the
mixings of sterile with active neutrinos are much more stringent than those
obtained from laboratory measurements. We point out that in scenarios with a
very low reheating temperature T_RH << 100 MeV at the end of (the last episode
of) inflation or entropy creation, the abundance of sterile neutrinos becomes
largely suppressed with respect to that obtained within the standard framework.
Thus, in this case cosmological bounds become much less stringent than usually
assumed, allowing sterile neutrinos to be ``visible'' in future experiments.
Here, we concentrate on massive (mostly sterile) neutrinos heavier than 1 MeV.Comment: 14 pp, 7 fig
MeV measurements of gamma-ray bursts by CGRO-COMPTEL
Since the launch of the Compton Gamma-Ray Observatory in April 1991, the imaging COMPTEL telescope has accumulated positions and 0.75–30 MeV spectra of more than thirty gamma-ray bursts within its ∼π sr field of view. In an ongoing collaboration with BACODINE/GCN, COMPTEL positions are relayed to a global network of multiwavelength observers in near real time (∼10 minutes). Here we summarize the MeV properties, and present spatial, spectral, and temporal data for the latest of these events, GRB 970807. In concurrence with earlier SMM and current BATSE, OSSE, and EGRET measurements, COMPTEL data add to the accumulating evidence that GRB spectra do seem to have a characteristic shape: a peak (inE2F(E) ) around several hundred keV; and a power law above (spectral index 1.5–3.5) extending beyond the COMPTEL energy range
The Diffuse Gamma-Ray Background from Supernovae
The Cosmic Gamma-ray Background (CGB) in the MeV region is believed to be due
to photons from radioactivity produced in SNe throughout the history of
galaxies in the universe. In particular, gamma-ray line emission from the decay
chain 56Ni-> 56Co->56Fe provides the dominant photon source. Although iron
synthesis occurs in all types of SNe, the contribution to the CGB is dominated
by SNIa events due to their higher photon escape probabilities. Estimates of
the star formation history in the universe suggest a rapid increase by a factor
\~ 10 from the present to a redshift z_p ~ 1.5, beyond which it either remains
constant or decreases slowly. We integrate the observed star formation history
to determine the CGB from the corresponding SN rate history. In addition to
gamma-rays from short-lived radioactivity in SNIa and SNII/Ibc we also
calculate the minor contributions from long-lived radioactivities (26Al, 44Ti,
60Co, and electron-positron pair annihilation). Although progenitor evolution
for SNIa is not yet fully understood, various arguments suggest delays of order
1-2 Gy between star formation and the production of SNIa's. The effect of this
delay on the CGB is discussed. We emphasize the value of gamma-ray observations
of the CGB in the MeV range as an independent tool for studies of the cosmic
star formation history. If the delay between star formation and SNIa activity
exceeds 1 Gy substantially, and/or the peak of the cosmic star formation rate
occurs at a redshift much larger than unity, the gamma-ray production of SNIa
would be insufficient to explain the observed CGB. Alternatively, the cosmic
star formation rate would have to be higher (by a factor 2-3) than commonly
assumed, which is in accord with several upward revisions reported in the
recent literature.Comment: Minor changes, 26 pages, 9 figures, Accepted by Ap
Cosmological Constraints on Theories with Large Extra Dimensions
In theories with large extra dimensions, constraints from cosmology lead to
non-trivial lower bounds on the fundamental scale M_F, corresponding to upper
bounds on the radii of the compact extra dimensions. These constraints are
especially relevant to the case of two extra dimensions, since only if M_F is
10 TeV or less do deviations from the standard gravitational force law become
evident at distances accessible to planned sub-mm gravity experiments. By
examining the graviton decay contribution to the cosmic diffuse gamma
radiation, we derive, for the case of two extra dimensions, a conservative
bound M_F > 110 TeV, corresponding to r_2 < 5.1 times 10^-5 mm, well beyond the
reach of these experiments. We also consider the constraint coming from
graviton overclosure of the universe and derive an independent bound M_F > 6.5
h^(-1/2) TeV, or r_2 < .015 h mm.Comment: 10 pages, references adde
Cosmic constraints rule out s-wave annihilation of light dark matter
Light dark matter annihilating into electron-positron pairs emits a
significant amount of internal bremsstrahlung that may contribute to the cosmic
gamma-ray background. The amount of emitted gamma-rays depends on the dark
matter clumping factor. Recent calculations indicate that this value should be
of order . That allows us to calculate the expected gamma-ray
background contribution from dark matter annihilation. We find that the light
dark matter model can be ruled out if a constant thermally-averaged cross
section is assumed (s-wave annihilation). For more massive dark matter
candidates like neutralinos, however, cosmic constraints are weaker.Comment: 5 pages, 2 figures, accepted at PR
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