369 research outputs found
Very-high energy gamma-ray astronomy: A 23-year success story in high-energy astroparticle physics
Very-high energy (VHE) gamma quanta contribute only a minuscule fraction -
below one per million - to the flux of cosmic rays. Nevertheless, being neutral
particles they are currently the best "messengers" of processes from the
relativistic/ultra-relativistic Universe because they can be extrapolated back
to their origin. The window of VHE gamma rays was opened only in 1989 by the
Whipple collaboration, reporting the observation of TeV gamma rays from the
Crab nebula. After a slow start, this new field of research is now rapidly
expanding with the discovery of more than 150 VHE gamma-ray emitting sources.
Progress is intimately related with the steady improvement of detectors and
rapidly increasing computing power. We give an overview of the early attempts
before and around 1989 and the progress after the pioneering work of the
Whipple collaboration. The main focus of this article is on the development of
experimental techniques for Earth-bound gamma-ray detectors; consequently, more
emphasis is given to those experiments that made an initial breakthrough rather
than to the successors which often had and have a similar (sometimes even
higher) scientific output as the pioneering experiments. The considered energy
threshold is about 30 GeV. At lower energies, observations can presently only
be performed with balloon or satellite-borne detectors. Irrespective of the
stormy experimental progress, the success story could not have been called a
success story without a broad scientific output. Therefore we conclude this
article with a summary of the scientific rationales and main results achieved
over the last two decades.Comment: 45 pages, 38 figures, review prepared for EPJ-H special issue "Cosmic
rays, gamma rays and neutrinos: A survey of 100 years of research
Dark Matter Detection with Hard X-ray Telescopes
We analyze the impact of future hard X-ray observations on the search for
indirect signatures of particle dark matter in large extragalactic systems such
as nearby clusters or groups of galaxies. We argue that the hard X-ray energy
band falls squarely at the peak of the inverse Compton emission from electrons
and positrons produced by dark matter annihilation or decay for a large class
of dark matter models. Specifically, the most promising are low-mass models
with a hard electron-positron annihilation final state spectrum and
intermediate-mass models with a soft electron-positron spectrum. We find that
constraints on dark matter models similar to the current constraints from the
Fermi Gamma-Ray Space Telescope will be close to the sensitivity limit of the
near-term hard X-ray telescopes NuSTAR and ASTRO-H for relatively long
observations. An instrument like the Wide Field Imager (WFI) proposed for
ATHENA would instead give a significant gain in sensitivity to dark matter if
placed in a low background orbit similar to NuSTAR's; however, given the higher
expected background level for ATHENA's proposed orbit at L2, its sensitivity
will be similar to that of NuSTAR.Comment: 18 pages, 3 figures, accepted to MNRAS, updated with predictions for
ATHENA and some additional discussion of systematic
Dark Matter and Fundamental Physics with the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) is a project for a next-generation
observatory for very high energy (GeV-TeV) ground-based gamma-ray astronomy,
currently in its design phase, and foreseen to be operative a few years from
now. Several tens of telescopes of 2-3 different sizes, distributed over a
large area, will allow for a sensitivity about a factor 10 better than current
instruments such as H.E.S.S, MAGIC and VERITAS, an energy coverage from a few
tens of GeV to several tens of TeV, and a field of view of up to 10 deg. In the
following study, we investigate the prospects for CTA to study several science
questions that influence our current knowledge of fundamental physics. Based on
conservative assumptions for the performance of the different CTA telescope
configurations, we employ a Monte Carlo based approach to evaluate the
prospects for detection. First, we discuss CTA prospects for cold dark matter
searches, following different observational strategies: in dwarf satellite
galaxies of the Milky Way, in the region close to the Galactic Centre, and in
clusters of galaxies. The possible search for spatial signatures, facilitated
by the larger field of view of CTA, is also discussed. Next we consider
searches for axion-like particles which, besides being possible candidates for
dark matter may also explain the unexpectedly low absorption by extragalactic
background light of gamma rays from very distant blazars. Simulated
light-curves of flaring sources are also used to determine the sensitivity to
violations of Lorentz Invariance by detection of the possible delay between the
arrival times of photons at different energies. Finally, we mention searches
for other exotic physics with CTA.Comment: (31 pages, Accepted for publication in Astroparticle Physics
Prospects for Observations of Pulsars and Pulsar Wind Nebulae with CTA
The last few years have seen a revolution in very-high gamma-ray astronomy
(VHE; E>100 GeV) driven largely by a new generation of Cherenkov telescopes
(namely the H.E.S.S. telescope array, the MAGIC and MAGIC-II large telescopes
and the VERITAS telescope array). The Cherenkov Telescope Array (CTA) project
foresees a factor of 5 to 10 improvement in sensitivity above 0.1 TeV,
extending the accessible energy range to higher energies up to 100 TeV, in the
Galactic cut-off regime, and down to a few tens GeV, covering the VHE photon
spectrum with good energy and angular resolution. As a result of the fast
development of the VHE field, the number of pulsar wind nebulae (PWNe) detected
has increased from one PWN in the early '90s to more than two dozen firm
candidates today. Also, the low energy threshold achieved and good sensitivity
at TeV energies has resulted in the detection of pulsed emission from the Crab
Pulsar (or its close environment) opening new and exiting expectations about
the pulsed spectra of the high energy pulsars powering PWNe. Here we discuss
the physics goals we aim to achieve with CTA on pulsar and PWNe physics
evaluating the response of the instrument for different configurations.Comment: accepted for publication in Astroparticle Physic
A genetic algorithm for the atomistic design and global optimisation of substitutionally disordered materials
We present a genetic algorithm for the atomistic design and global
optimisation of substitutionally disordered bulk materials and surfaces.
Premature convergence which hamper conventional genetic algorithms due to
problems with synchronisation is avoided using a symmetry adapted crossover.
The algorithm outperforms previously reported Monte Carlo and genetic algorithm
simulations for finding low energy minima of two simple alloy models without
the need for any redesign.Comment: 9 pages, 4 figures, presented on E-MRS Fall Meeting 200
Pulsar-wind nebulae and magnetar outflows: observations at radio, X-ray, and gamma-ray wavelengths
We review observations of several classes of neutron-star-powered outflows:
pulsar-wind nebulae (PWNe) inside shell supernova remnants (SNRs), PWNe
interacting directly with interstellar medium (ISM), and magnetar-powered
outflows. We describe radio, X-ray, and gamma-ray observations of PWNe,
focusing first on integrated spectral-energy distributions (SEDs) and global
spectral properties. High-resolution X-ray imaging of PWNe shows a bewildering
array of morphologies, with jets, trails, and other structures. Several of the
23 so far identified magnetars show evidence for continuous or sporadic
emission of material, sometimes associated with giant flares, and a few
possible "magnetar-wind nebulae" have been recently identified.Comment: 61 pages, 44 figures (reduced in quality for size reasons). Published
in Space Science Reviews, "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray
Bursts and Blazars: Physics of Extreme Energy Release
Sommerfeld Enhancement from Multiple Mediators
We study the Sommerfeld enhancement experienced by a scattering object that
couples to a tower of mediators. This can occur in, e.g., models of secluded
dark matter when the mediator scale is generated naturally by hidden-sector
confinement. Specializing to the case of a confining CFT, we show that
off-resonant values of the enhancement can be increased by ~ 20% for cases of
interest when (i) the (strongly-coupled) CFT admits a weakly-coupled dual
description and (ii) the conformal symmetry holds up to the Planck scale.
Larger enhancements are possible for lower UV scales due to an increase in the
coupling strength of the tower.Comment: 17p, 2 figures; v2 JHEP version (inconsequential typo fixed,
references added
Secluded Dark Matter Coupled to a Hidden CFT
Models of secluded dark matter offer a variant on the standard WIMP picture
and can modify our expectations for hidden sector phenomenology and detection.
In this work we extend a minimal model of secluded dark matter, comprised of a
U(1)'-charged dark matter candidate, to include a confining hidden-sector CFT.
This provides a technically natural explanation for the hierarchically small
mediator-scale, with hidden-sector confinement generating m_{gamma'}>0.
Furthermore, the thermal history of the universe can differ markedly from the
WIMP picture due to (i) new annihilation channels, (ii) a (potentially) large
number of hidden-sector degrees of freedom, and (iii) a hidden-sector phase
transition at temperatures T << M_{dm} after freeze out. The mediator allows
both the dark matter and the Standard Model to communicate with the CFT, thus
modifying the low-energy phenomenology and cosmic-ray signals from the secluded
sector.Comment: ~50p, 8 figs; v2 JHEP versio
gamma-rays from annihilating dark matter in galaxy clusters: stacking vs single source analysis
Clusters of galaxies are potentially important targets for indirect searches
for dark matter annihilation. Here we reassess the detection prospects for
annihilation in massive halos, based on a statistical investigation of 1743
clusters in the new Meta-Catalog of X-ray Clusters. We derive a new limit for
the extra-galactic dark matter annihilation background of at least 20% of that
originating from the Galaxy for an integration angle of 0.1 deg. The number of
clusters scales as a power law with their brightness, suggesting that stacking
may provide a significant improvement over a single target analysis. The mean
angle containing 80% of the dark-matter signal for the sample is ~0.15 deg,
indicating that instruments with this angular resolution or better would be
optimal for a cluster annihilation search based on stacking. A detailed study
based on the Fermi-LAT performance and position-dependent background, suggests
that stacking may result in a factor ~2 improvement in sensitivity, depending
on the source selection criteria. Based on the expected performance of CTA, we
find no improvement with stacking, due to the requirement for pointed
observations. We note that several potentially important targets: Opiuchius,
A2199, A3627 (Norma) and CIZAJ1324.7-5736 may be disfavoured due to a poor
contrast with respect to the Galactic dark-matter signal. The use of the
homogenised MCXC meta-catalogue provides a robust ranking of the targets,
although the absolute value of their signal depends on the exact dark matter
substructure content. For conservative assumptions, we find that galaxy
clusters (with or without stacking) can probe down to 1e-25-1e-24
cm3/s for dark matter masses in the range 10 GeV-100 GeV. For more favourable
substructure configurations, ~1e-26 cm3/s may be reached.Comment: 11 pages, 6+2(new) figures, impact of substructures discussed in new
Sec 3.4 (matches accepted MNRAS version). Supplementary file available at
http://lpsc.in2p3.fr/clumpy/downloads.htm
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