57 research outputs found
Isospin-Breaking quark condensates in Chiral Perturbation Theory
We analyze the isospin-breaking corrections to quark condensates within
one-loop SU(2) and SU(3) Chiral Perturbation Theory including as
well as electromagnetic (EM) contributions. The explicit expressions are given
and several phenomenological aspects are studied. We analyze the sensitivity of
recent condensate determinations to the EM low-energy constants (LEC). If the
explicit chiral symmetry breaking induced by EM terms generates a
ferromagnetic-like response of the vacuum, as in the case of quark masses, the
increasing of the order parameter implies constraints for the EM LEC, which we
check with different estimates in the literature. In addition, we extend the
sum rule relating quark condensate ratios in SU(3) to include EM corrections,
which are of the same order as the ones, and we use that sum rule
to estimate the vacuum asymmetry within ChPT. We also discuss the matching
conditions between the SU(2) and SU(3) LEC involved in the condensates, when
both isospin-breaking sources are taken into account.Comment: 16 pages, 1 figure, final version accepted for publication in Journal
of Physics
LOFAR discovery of a double radio halo system in Abell 1758 and radio/X-ray study of the cluster pair
Radio halos and radio relics are diffuse synchrotron sources that extend over Mpc-scales and are found in a number of merger galaxy clusters. They are believed to form as a consequence of the energy that is dissipated by turbulence and shocks in the intra-cluster medium (ICM). However, the precise physical processes that generate these steep synchrotron spectrum sources are still poorly constrained. We present a new LOFAR observation of the double galaxy cluster Abell 1758. This system is composed of A1758N, a massive cluster hosting a known giant radio halo, and A1758S, which is a less massive cluster whose diffuse radio emission is confirmed here for the first time. Our observations have revealed a radio halo and a candidate radio relic in A1758S, and a suggestion of emission along the bridge connecting the two systems which deserves confirmation. We combined the LOFAR data with archival VLA and GMRT observations to constrain the spectral properties of the diffuse emission. We also analyzed a deep archival Chandra observation and used this to provide evidence that A1758N and A1758S are in a pre-merger phase. The ICM temperature across the bridge that connects the two systems shows a jump which might indicate the presence of a transversal shock generated in the initial stage of the merger
First evidence of diffuse ultra-steep-spectrum radio emission surrounding the cool core of a cluster
Diffuse synchrotron radio emission from cosmic-ray electrons is observed at the center of a number of galaxy clusters. These sources can be classified either as giant radio halos, which occur in merging clusters, or as mini halos, which are found only in cool-core clusters. In this paper, we present the first discovery of a cool-core cluster with an associated mini halo that also shows ultra-steep-spectrum emission extending well beyond the core that resembles radio halo emission. The large-scale component is discovered thanks to LOFAR observations at 144 MHz. We also analyse GMRT observations at 610 MHz to characterise the spectrum of the radio emission. An X-ray analysis reveals that the cluster is slightly disturbed, and we suggest that the steep-spectrum radio emission outside the core could be produced by a minor merger that powers electron re-acceleration without disrupting the cool core. This discovery suggests that, under particular circumstances, both a mini and giant halo could co-exist in a single cluster, opening new perspectives for particle acceleration mechanisms in galaxy clusters
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
Search for extended gamma-ray emission from the Virgo galaxy cluster with Fermi-LAT
Galaxy clusters are one of the prime sites to search for dark matter (DM)
annihilation signals. Depending on the substructure of the DM halo of a galaxy
cluster and the cross sections for DM annihilation channels, these signals
might be detectable by the latest generation of -ray telescopes. Here
we use three years of Fermi Large Area Telescope (LAT) data, which are the most
suitable for searching for very extended emission in the vicinity of nearby
Virgo galaxy cluster. Our analysis reveals statistically significant extended
emission which can be well characterized by a uniformly emitting disk profile
with a radius of 3\deg that moreover is offset from the cluster center. We
demonstrate that the significance of this extended emission strongly depends on
the adopted interstellar emission model (IEM) and is most likely an artifact of
our incomplete description of the IEM in this region. We also search for and
find new point source candidates in the region. We then derive conservative
upper limits on the velocity-averaged DM pair annihilation cross section from
Virgo. We take into account the potential -ray flux enhancement due to
DM sub-halos and its complex morphology as a merging cluster. For DM
annihilating into , assuming a conservative sub-halo model
setup, we find limits that are between 1 and 1.5 orders of magnitude above the
expectation from the thermal cross section for
. In a more optimistic scenario, we
exclude
for for the same channel. Finally, we
derive upper limits on the -ray-flux produced by hadronic cosmic-ray
interactions in the inter cluster medium. We find that the volume-averaged
cosmic-ray-to-thermal pressure ratio is less than .Comment: 15 pages, 11 figures, 4 tables, accepted for publication in ApJ;
corresponding authors: T. Jogler, S. Zimmer & A. Pinzk
Extragalactic Inverse Compton Light from Dark Matter Annihilation and the Pamela Positron Excess
We calculate the extragalactic diffuse emission originating from the
up-scattering of cosmic microwave photons by energetic electrons and positrons
produced in particle dark matter annihilation events at all redshifts and in
all halos. We outline the observational constraints on this emission and we
study its dependence on both the particle dark matter model (including the
particle mass and its dominant annihilation final state) and on assumptions on
structure formation and on the density profile of halos. We find that for
low-mass dark matter models, data in the X-ray band provide the most stringent
constraints, while the gamma-ray energy range probes models featuring large
masses and pair-annihilation rates, and a hard spectrum for the injected
electrons and positrons. Specifically, we point out that the all-redshift,
all-halo inverse Compton emission from many dark matter models that might
provide an explanation to the anomalous positron fraction measured by the
Pamela payload severely overproduces the observed extragalactic gamma-ray
background.Comment: Version accepted for publication in JCAP, one new figure and text
added; 19 pages, 5 figure
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
Probing the origin of giant radio halos through radio and gamma-ray data : the case of the Coma cluster
We combine all available information about the spectral shape and morphology
of the radio halo of the Coma cluster with the gamma-ray upper limits obtained
by the Fermi-LAT and with the magnetic field strength derived from Faraday
rotation measures (RM). We explore the possibility that the radio halo is due
to synchrotron emission of secondary electrons generated via p-p collisions in
the intra-cluster-medium (ICM). First we investigate the case of pure secondary
models. We use the observed spatial distribution of the halo's radio brightness
to constrain the amount of cosmic rays (CRs) and their spatial distribution in
the cluster that are required by the model. Under the canonical assumption that
the spectrum of CRs is a power-law in momentum and that the spectrum of
secondaries is stationary, we find that the combination of the steep spectrum
of CRs necessary to explain the spectrum of the halo and their very broad
spatial distribution (and large energy density) result in a gamma-ray emission
in excess of present limits, unless the cluster magnetic field is sufficiently
large. However such a field appears inconsistent with constraints from RM.
Second we investigate more complex models based on secondary particles in which
CR protons and their secondaries are all reaccelerated by MHD turbulence. We
show that under these conditions it is possible to reproduce the radio data and
to predict gamma-rays in agreement with the Fermi-LAT limits without tension
with constraints on the cluster magnetic field. Reacceleration of secondaries
by MHD turbulence also requires a spatial distribution of CRs much flatter than
that of the ICM, if both the turbulent and magnetic field energy densities
scale with that of the ICM. However broader spatial distributions of turbulence
and field and/or the reacceleration of additional primary electrons in the ICM
greatly alleviate this requirement.Comment: 17 pages, 7 figures, MNRAS in press. Line added in Fig.2 to show the
substantial increase of the expected gamma-ray flux, deriving from the
adoption of the new brightness profile of the halo from Brown & Rudnick
(2011), with respect to calculations based on other profiles derived from
older/shallower radio observations (including recent calculations in
arXiv:1207.6410 and 1208.0676
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