466 research outputs found
Stochastic conversions of TeV photons into axion-like particles in extragalactic magnetic fields
Very-high energy photons emitted by distant cosmic sources are absorbed on
the extragalactic background light (EBL) during their propagation. This effect
can be characterized in terms of a photon transfer function at Earth. The
presence of extragalactic magnetic fields could also induce conversions between
very high-energy photons and hypothetical axion-like particles (ALPs). The
turbulent structure of the extragalactic magnetic fields would produce a
stochastic behaviour in these conversions, leading to a statistical
distribution of the photon transfer functions for the different realizations of
the random magnetic fields. To characterize this effect, we derive new
equations to calculate the mean and the variance of this distribution. We find
that, in presence of ALP conversions, the photon transfer functions on
different lines of sight could have relevant deviations with respect to the
mean value, producing both an enhancement or a suppression in the observable
photon flux with respect to the expectations with only absorption. As a
consequence, the most striking signature of the mixing with ALPs would be a
reconstructed EBL density from TeV photon observations which appears to vary
over different directions of the sky: consistent with standard expectations in
some regions, but inconsistent in others.Comment: v2: 22 pages, 5 eps figures. Minor changes. A reference added.
Matches the version published on JCA
Contribution of the lung to the genesis of cheyne-stokes respiration in heart failure: Plant gain beyond chemoreflex gain and circulation time
Background-The contribution of the lung or the plant gain (PG; ie, change in blood gases per unit change in ventilation) to Cheyne-Stokes respiration (CSR) in heart failure has only been hypothesized by mathematical models, but never been directly evaluated.Methods and Results-Twenty patients with systolic heart failure (age, 72.4 +/- 6.4 years; left ventricular ejection fraction, 31.5 +/- 5.8%), 10 with relevant CSR (24-hour apnea-hypopnea index [AHI] >= 10 events/h) and 10 without (AHI <10 events/h) at 24-hour cardiorespiratory monitoring underwent evaluation of chemoreflex gain (CG) to hypoxia (CG(O2)) and hypercapnia (CG(CO2)) by rebreathing technique, lung-to-finger circulation time, and PG assessment through a visual system. PG test was feasible and reproducible (intraclass correlation coefficient, 0.98; 95% CI, 0.91-0.99); the best-fitting curve to express the PG was a hyperbola (R-2 >= 0.98). Patients with CSR showed increased PG, CG(CO2) (but not CG(O2)), and lung-to-finger circulation time, compared with patients without CSR (all P<0.05). PG was the only predictor of the daytime AHI (R=0.56, P=0.01) and together with the CG(CO2) also predicted the nighttime AHI (R=0.81, P=0.0003) and the 24-hour AHI (R=0.71, P=0.001). Lung-to-finger circulation time was the only predictor of CSR cycle length (R=0.82, P=0.00006).Conclusions-PG is a powerful contributor of CSR and should be evaluated together with the CG and circulation time to individualize treatments aimed at stabilizing breathing in heart failure
Strange quark matter in explosive astrophysical systems
Explosive astrophysical systems, such as supernovae or compact star binary
mergers, provide conditions where strange quark matter can appear. The high
degree of isospin asymmetry and temperatures of several MeV in such systems may
cause a transition to the quark phase already around saturation density.
Observable signals from the appearance of quark matter can be predicted and
studied in astrophysical simulations. As input in such simulations, an equation
of state with an integrated quark matter phase transition for a large
temperature, density and proton fraction range is required. Additionally,
restrictions from heavy ion data and pulsar observation must be considered. In
this work we present such an approach. We implement a quark matter phase
transition in a hadronic equation of state widely used for astrophysical
simulations and discuss its compatibility with heavy ion collisions and pulsar
data. Furthermore, we review the recently studied implications of the QCD phase
transition during the early post-bounce evolution of core-collapse supernovae
and introduce the effects from strong interactions to increase the maximum mass
of hybrid stars. In the MIT bag model, together with the strange quark mass and
the bag constant, the strong coupling constant provides a parameter
to set the beginning and extension of the quark phase and with this the mass
and radius of hybrid stars.Comment: 6 pages, 5 figures, talk given at the International Conference on
Strangeness in Quark Matter (SQM2009), Buzios, Brasil, September 28 - October
2, 2009, to be published in Journal Phys.
Comparison of electrohysterogram signal measured by surface electrodes with different designs: A computational study with dipole band and abdomen models
Non-invasive measurement of uterine activity using electrohysterogram (EHG) surface electrodes has been attempted to monitor uterine contraction. This study aimed to computationally compare the performance of acquiring EHG signals using monopolar electrode and three types of Laplacian concentric ring electrodes (bipolar, quasi-bipolar and tri-polar). With the implementation of dipole band model and abdomen model, the performances of four electrodes in terms of the local sensitivity were quantifed by potential attenuation. Furthermore, the efects of fat and muscle thickness on potential attenuation were evaluated using the bipolar and tri-polar electrodes with diferent radius. The results showed that all the four types of electrodes detected the simulated EHG signals with consistency. That the bipolar and tri-polar electrodes had greater attenuations than the others, and the shorter distance between the origin and location of dipole band at 20dB attenuation, indicating that they had relatively better local sensitivity. In addition, ANOVA analysis showed that, for all the electrodes with diferent outer ring radius, the efects of fat and muscle on potential attenuation were signifcant (all p<0.01). It is therefore concluded that the bipolar and tri-polar electrodes had higher local sensitivity than the others, indicating that they can be applied to detect EHG efectively
The RGB tip of galactic globular clusters and the revision of the bound of the axion-electron coupling
By combining Hubble Space Telescope (HST) and ground based optical and
near-infrared photometric samples, we derive the RGB tip absolute magnitude of
22 galactic globular clusters (GGCs). The effects of varying the distance and
the metallicity scales are also investigated. Then we compare the observed tip
luminosities with those predicted by state-of-the-art stellar models that
include the energy-loss due to the axion production in the degenerate core of
red giant stars. We find that theoretical predictions including only the
energy-loss by plasma neutrinos are, in general, in good agreement with the
observed tip bolometric magnitudes, even though the latter are about 0.04 mag
brighter, on the average. This small shift may be the result of systematic
errors affecting the evaluation of the RGB tip bolometric magnitudes or,
alternatively, it could be ascribed to an axion-electron coupling causing a
non-negligible thermal production of axions. In order to estimate the strength
of this possible axion sink, we perform a cumulative likelihood analysis using
the RGB tips of the whole set of 22 GGCs. All the possible source of
uncertainties affecting both the measured bolometric magnitudes and the
corresponding theoretical predictions are carefully considered. As a result, we
find that the value of the axion-electron coupling parameter that maximizes the
likelihood probability is gae/10^13=0.60(+0.32;-0.58). This hint is valid,
however, if the dominant energy sinks operating in the core of red giant stars
are standard neutrinos and axions coupled with electrons. Any additional
energy-loss process, not included in the stellar models, would reduce such a
hint. Nevertheless, we find that values gae/10^13 > 1.48 can be excluded with a
95% of confidence.Comment: accepted by Astronomy and Astrophysic
Preliminary analysis of the ICRF launcher for DTT
The paper reports the preliminary analysis of different typologies of ICRH launchers for chosing the most efficient solution for the ICRH system of the Divertor Tokamak Test facility (DTT), designed by the Italian DTT Limited Liability Consortium (S.C. a r.l.). In its final configuration this system will couple to the DTT plasma a nominal power of 6 MW in the 60 – 90 MHz frequency range by means of four launchers. This very preliminary analysis has been done with the ANSYS HFSS code
Damping of supernova neutrino transitions in stochastic shock-wave density profiles
Supernova neutrino flavor transitions during the shock wave propagation are
known to encode relevant information not only about the matter density profile
but also about unknown neutrino properties, such as the mass hierarchy (normal
or inverted) and the mixing angle theta_13. While previous studies have
focussed on "deterministic" density profiles, we investigate the effect of
possible stochastic matter density fluctuations in the wake of supernova shock
waves. In particular, we study the impact of small-scale fluctuations on the
electron (anti)neutrino survival probability, and on the observable spectra of
inverse-beta-decay events in future water-Cherenkov detectors. We find that
such fluctuations, even with relatively small amplitudes, can have significant
damping effects on the flavor transition pattern, and can partly erase the
shock-wave imprint on the observable time spectra, especially for
sin^2(theta_13) > O(10^-3).Comment: v2 (23 pages, including 6 eps figures). Typos removed, references
updated, matches the published versio
A complete 3D numerical study of the effects of pseudoscalar-photon mixing on quasar polarizations
We present the results of three-dimensional simulations of quasar
polarizations in the presence of pseudoscalar-photon mixing in the
intergalactic medium. The intergalactic magnetic field is assumed to be
uncorrelated in wave vector space but correlated in real space. Such a field
may be obtained if its origin is primordial. Furthermore we assume that the
quasars, located at cosmological distances, have negligible initial
polarization. In the presence of pseudoscalar-photon mixing we show, through a
direct comparison with observations, that this may explain the observed large
scale alignments in quasar polarizations within the framework of big bang
cosmology. We find that the simulation results give a reasonably good fit to
the observed data.Comment: 15 pages, 8 figures, significant changes, to appear in EPJ
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