168 research outputs found
New constraints for heavy axion-like particles from supernovae
We derive new constraints on the coupling of heavy pseudoscalar (axion-like)
particles to photons, based on the gamma ray flux expected from the decay of
these particles into photons. After being produced in the supernova core, these
heavy axion-like particles would escape and a fraction of them would decay into
photons before reaching the Earth. We have calculated the expected flux on
Earth of these photons from the supernovae SN 1987A and Cassiopeia A and
compared our results to data from the Fermi Large Area Telescope. This analysis
provides strong constraints on the parameter space for axion-like particles.
For a particle mass of 100 MeV, we find that the Peccei-Quinn constant, f_a,
must be greater than about 10^{15} GeV. Alternatively, for fa=10^{12} GeV, we
exclude the mass region between approximately 100 eV and 1 GeV.Comment: 14 pages, 4 figures. Version published in JCAP. Major changes in the
exposition. Added a figure. Added appendix. Minor changes in the results.
Some changes in the bibliograph
Prospective dark matter annihilation signals from the Sagittarius Dwarf Spheroidal
The Sagittarius Dwarf Spheroidal galaxy (Sgr) is investigated as a target for dark matter (DM) annihilation searches utilizing J-factor distributions calculated directly from a high-resolution hydrodynamic simulation of the infall and tidal disruption of Sgr around the Milky Way. In contrast to past studies, the simulation incorporates DM, stellar and gaseous components for both the Milky Way and the Sgr progenitor galaxy. The simulated distributions account for significant tidal disruption affecting the DM density profile. Our estimate of the J-factor value for Sgr, JSgr = 1.48 × 1010 M2☉ kpc−5 (6.46 × 1016 GeV cm−5), is significantly lower than found in prior studies. This value, while formally a lower limit, is likely close to the true J-factor value for Sgr. It implies a DM cross-section incompatibly large in comparison with existing constraints would be required to attribute recently observed gamma-ray emission from Sgr to DM annihilation. We also calculate a J-factor value using a NFW profile fitted to the simulated DM density distribution to facilitate comparison with past studies. This NFW J-factor value supports the conclusion that most past studies have overestimated the dark matter density of Sgr on small scales. This, together with the fact that the Sgr has recently been shown to emit gamma-rays of astrophysical origin, complicate the use of Sgr in indirect DM detection searches
The accretion history of dark matter haloes - II. The connections with the mass power spectrum and the density profile
Large scale structure and cosmolog
quasiharmonic equations of state for dynamically-stabilized soft-mode materials
We introduce a method for treating soft modes within the analytical framework
of the quasiharmonic equation of state. The corresponding double-well
energy-displacement relation is fitted to a functional form that is harmonic in
both the low- and high-energy limits. Using density-functional calculations and
statistical physics, we apply the quasiharmonic methodology to solid periclase.
We predict the existence of a B1--B2 phase transition at high pressures and
temperatures
Challenges for room temperature operation of electrically pumped GeSn lasers
Recent demonstrations of room-temperature lasing in optically pumped GeSn show promise for
future CMOS compatible lasers for Si-photonics applications. However, challenges remain for
electrically pumped devices. Investigation of the processes that limit device performance is therefore
vital in aiding the production of future commercial devices. In this work, a combined experimental
and modelling approach is utilised to explore the dominant loss processes in current devices. By
manipulating the band structure of functioning devices using high hydrostatic pressure techniques
at low temperature, the dominant carrier recombination pathways are identifed. This reveals that
93±5% of the threshold current is attributable to defect-related recombination at a temperature,
T = 85 K. Furthermore, carrier occupation of L-valley states (carrier leakage) is responsible for 1.1±
0.3% of the threshold current, but this sharply increases to 50% with a decrease of just 30 meV in
the L-r separation energy. This indicates that thermal broadening of a similar order may reproduce
these adverse efects, limiting device performance at higher temperatures. Temperature dependent
calculations show that carrier occupation of indirect valley L-states strongly afects the transparency
carrier density and is therefore very sensitive to the Sn composition, leading to an efective
operational temperature range for given Sn compositions and strain values. Recommendations for
future device designs are proposed based on band structure and growth optimisations
Semi-empirical catalog of early-type galaxy-halo systems: dark matter density profiles, halo contraction and dark matter annihilation strength
With SDSS galaxy data and halo data from up-to-date N-body simulations we
construct a semi-empirical catalog (SEC) of early-type systems by making a
self-consistent bivariate statistical match of stellar mass (M_star) and
velocity dispersion (sigma) with halo virial mass (M_vir). We then assign
stellar mass profile and velocity dispersion profile parameters to each system
in the SEC using their observed correlations with M_star and sigma.
Simultaneously, we solve for dark matter density profile of each halo using the
spherical Jeans equation. The resulting dark matter density profiles deviate in
general from the dissipationless profile of NFW or Einasto and their mean inner
density slope and concentration vary systematically with M_vir. Statistical
tests of the distribution of profiles at fixed M_vir rule out the null
hypothesis that it follows the distribution predicted by N-body simulations for
M_vir ~< 10^{13.5-14.5} M_solar. These dark matter profiles imply that dark
matter density is, on average, enhanced significantly in the inner region of
halos with M_vir ~< 10^{13.5-14.5} M_solar supporting halo contraction. The
main characteristics of halo contraction are: (1) the mean dark matter density
within the effective radius has increased by a factor varying systematically up
to ~ 3-4 at M_vir = 10^{12} M_solar, and (2) the inner density slope has a mean
of ~ 1.3 with rho(r) ~ r^{-alpha} and a halo-to-halo rms scatter of
rms(alpha) ~ 0.4-0.5 for 10^{12} M_solar ~< M_vir ~< 10^{13-14} M_solar steeper
than the NFW profile (alpha=1). Based on our results we predict that halos of
nearby elliptical and lenticular galaxies can, in principle, be promising
targets for gamma-ray emission from dark matter annihilation.Comment: 43 pages, 20 figures, JCAP, revised and accepted versio
Astrophysical Axion Bounds
Axion emission by hot and dense plasmas is a new energy-loss channel for
stars. Observational consequences include a modification of the solar
sound-speed profile, an increase of the solar neutrino flux, a reduction of the
helium-burning lifetime of globular-cluster stars, accelerated white-dwarf
cooling, and a reduction of the supernova SN 1987A neutrino burst duration. We
review and update these arguments and summarize the resulting axion
constraints.Comment: Contribution to Axion volume of Lecture Notes in Physics, 20 pages, 3
figure
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