500 research outputs found
Meta-analysis of Randomized Trials Comparing Carotid Endarterectomy and Endovascular Treatment
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
Dark Energy Survey year 1 results: Cosmological constraints from galaxy clustering and weak lensing
We present cosmological results from a combined analysis of galaxy clustering and weak gravitational lensing, using 1321ââdeg2 of griz imaging data from the first year of the Dark Energy Survey (DES Y1). We combine three two-point functions: (i) the cosmic shear correlation function of 26 million source galaxies in four redshift bins, (ii) the galaxy angular autocorrelation function of 650,000 luminous red galaxies in five redshift bins, and (iii) the galaxy-shear cross-correlation of luminous red galaxy positions and source galaxy shears. To demonstrate the robustness of these results, we use independent pairs of galaxy shape, photometric-redshift estimation and validation, and likelihood analysis pipelines. To prevent confirmation bias, the bulk of the analysis was carried out while âblindâ to the true results; we describe an extensive suite of systematics checks performed and passed during this blinded phase. The data are modeled in flat ÎCDM and wCDM cosmologies, marginalizing over 20 nuisance parameters, varying 6 (for ÎCDM) or 7 (for wCDM) cosmological parameters including the neutrino mass density and including the 457Ă457 element analytic covariance matrix. We find consistent cosmological results from these three two-point functions and from their combination obtain S8âĄÏ8(Ωm/0.3)0.5=0.773+0.026â0.020 and Ωm=0.267+0.030â0.017 for ÎCDM; for wCDM, we find S8=0.782+0.036â0.024, Ωm=0.284+0.033â0.030, and w=â0.82+0.21â0.20 at 68% C.L. The precision of these DES Y1 constraints rivals that from the Planck cosmic microwave background measurements, allowing a comparison of structure in the very early and late Universe on equal terms. Although the DES Y1 best-fit values for S8 and Ωm are lower than the central values from Planck for both ÎCDM and wCDM, the Bayes factor indicates that the DES Y1 and Planck data sets are consistent with each other in the context of ÎCDM. Combining DES Y1 with Planck, baryonic acoustic oscillation measurements from SDSS, 6dF, and BOSS and type Ia supernovae from the Joint Lightcurve Analysis data set, we derive very tight constraints on cosmological parameters: S8=0.802±0.012 and Ωm=0.298±0.007 in ÎCDM and w=â1.00+0.05â0.04 in wCDM. Upcoming Dark Energy Survey analyses will provide more stringent tests of the ÎCDM model and extensions such as a time-varying equation of state of dark energy or modified gravity
When will 'open science' become simply 'science'?
Open science describes the practice of carrying out scientific research in a completely transparent manner, and making the results of that research available to everyone. Isnât that just âscienceâ
TReND in Africa: Toward a Truly Global (Neuro)science Community.
TReND is a volunteer-scientist run charity dedicated to promoting research and education on the African continent. Focusing on neuroscience, we discuss approaches to address some of the factors that currently stifle Africa's scientific development and our experience in implementing them
The major upgrade of the MAGIC telescopes, Part II: A performance study using observations of the Crab Nebula
MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes located in
the Canary island of La Palma, Spain. During summer 2011 and 2012 it underwent
a series of upgrades, involving the exchange of the MAGIC-I camera and its
trigger system, as well as the upgrade of the readout system of both
telescopes. We use observations of the Crab Nebula taken at low and medium
zenith angles to assess the key performance parameters of the MAGIC stereo
system. For low zenith angle observations, the standard trigger threshold of
the MAGIC telescopes is ~50GeV. The integral sensitivity for point-like sources
with Crab Nebula-like spectrum above 220GeV is (0.66+/-0.03)% of Crab Nebula
flux in 50 h of observations. The angular resolution, defined as the sigma of a
2-dimensional Gaussian distribution, at those energies is < 0.07 degree, while
the energy resolution is 16%. We also re-evaluate the effect of the systematic
uncertainty on the data taken with the MAGIC telescopes after the upgrade. We
estimate that the systematic uncertainties can be divided in the following
components: < 15% in energy scale, 11-18% in flux normalization and +/-0.15 for
the energy spectrum power-law slope.Comment: 21 pages, 25 figures, accepted for publication in Astroparticle
Physic
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
Lead Telluride Doped with Au as a Very Promising Material for Thermoelectric Applications
PbTe single crystals doped with monovalent Au or Cu were grown using the Bridgman method. Far infrared reflectivity spectra were measured at room temperature for all samples and plasma minima were registered. These experimental spectra were numerically analyzed and optical parameters were calculated. All the samples of PbTe doped with Au or Cu were of the ânâ type. The properties of these compositions were analyzed and compared with PbTe containing other dopants. The samples of PbTe doped with only 3.3âat% Au were the best among the PbTe + Au samples having the lowest plasma frequency and the highest mobility of free carriers-electrons, while PbTe doped with Cu was the opposite. Samples with the lowest Cu concentration of 0.23âat% Cu had the best properties. Thermal diffusivity and electronic transport properties of the same PbTe doped samples were also investigated using a photoacoustic (PA) method with the transmission detection configuration. The results obtained with the far infrared and photoacoustic characterization of PbTe doped samples were compared and discussed. Both methods confirmed that when PbTe was doped with 3.3âat% Au, thermoelectric and electrical properties of this doped semiconductor were both significantly improved, so Au as a dopant in PbTe could be used as a new high quality thermoelectric material
Eliminating the mystery from the concept of emergence
While some branches of complexity theory are advancing rapidly, the same cannot be said for our understanding of emergence. Despite a complete knowledge of the rules underlying the interactions between the parts of many systems, we are often baffled by their sudden transitions from simple to complex. Here I propose a solution to this conceptual problem. Given that emergence is often the result of many interactions occurring simultaneously in time and space, an ability to intuitively grasp it would require the ability to consciously think in parallel. A simple exercise is used to demonstrate that we do not possess this ability. Our surprise at the behaviour of cellular automata models, and the natural cases of pattern formation they mimic, is then explained from this perspective. This work suggests that the cognitive limitations of the mind can be as significant a barrier to scientific progress as the limitations of our senses
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