2,799 research outputs found
On the equipartition of thermal and non-thermal energy in clusters of galaxies
Clusters of galaxies are revealing themselves as powerful sources of non
thermal radiation in a wide range of wavelengths. In order to account for these
multifrequency observations equipartition of cosmic rays (CRs) with the thermal
gas in clusters of galaxies is often invoked. This condition might suggest a
dynamical role played by cosmic rays in the virialization of these large scale
structures and is now testable through gamma ray observations. We show here, in
the specific case of the Coma and Virgo clusters, for which upper limits on the
gamma ray emission exist, that equipartition implies gamma ray fluxes that are
close or even in excess of the EGRET limit, depending on the adopted model of
CR injection. We use this bound to limit the validity of the equipartition
condition. We also show that, contrary to what claimed in previous
calculations, the equipartition assumption implies gamma ray fluxes in the TeV
range which can be detectable even by currently operating gamma ray
observatories if the injection cosmic ray spectrum is flatter than .Comment: 20 pages + 2 figures. To appear in the Astrophysical Journa
Three dimensional imaging of short pulses
We exploit a slightly noncollinear second-harmonic cross-correlation scheme
to map the 3D space-time intensity distribution of an unknown complex-shaped
ultrashort optical pulse. We show the capability of the technique to
reconstruct both the amplitude and the phase of the field through the coherence
of the nonlinear interaction down to a resolution of 10 m in space and 200
fs in time. This implies that the concept of second-harmonic holography can be
employed down to the sub-ps time scale, and used to discuss the features of the
technique in terms of the reconstructed fields.Comment: 16 pages, 6 figure
Vector Supersymmetry of 2D Yang-Mills Theory
The vector supersymmetry of the 2D topological BF model is extended to 2D
Yang-Mills. The consequences of the corresponding Ward identity on the
ultraviolet behavior of the theory are analyzed.Comment: Some references adde
Over-reliance on English hinders cognitive science
English is the dominant language in the study of human cognition and behavior: the individuals studied by cognitive scientists, as well as most of the scientists themselves, are frequently English speakers. However, English differs from other languages in ways that have consequences for the whole of the cognitive sciences, reaching far beyond the study of language itself. Here, we review an emerging body of evidence that highlights how the particular characteristics of English and the linguistic habits of English speakers bias the field by both warping research programs (e.g., overemphasizing features and mechanisms present in English over others) and overgeneralizing observations from English speakers’ behaviors, brains, and cognition to our entire species. We propose mitigating strategies that could help avoid some of these pitfalls
Are the school prevention programmes - aimed at de-normalizing smoking among youths - beneficial in the long term? An example from the Smoke Free Class Competition in Italy
Tobacco smoking by young people is of great concern because it usually leads to regular smoking, nicotine addiction and quitting difficulties. Young people "hooked" by tobacco maintain the profits of the tobacco industry by replacing smokers who quit or die. If new generations could be tobacco-free, as supported by tobacco endgame strategies, the tobacco epidemic could end within decades. Smoking prevention programmes for teens are offered by schools with the aim to prevent or delay smoking onset. Among these, the Smoke Free Class Competition (SFC) was widely implemented in Europe. Its effectiveness yielded conflicting results, but it was only evaluated at short/medium term (6 - 18 months). The aim of this study is to evaluate its effectiveness after a longer follow-up (3 to 5 years) in order to allow enough time for the maturing of the students and the internalization of the experience and its contents. Fifteen classes were randomly sampled from two Italian high schools of Bologna province that regularly offered the SFC to first year students; 382 students (174 participating in the SFC and 208 controls) were retrospectively followed-up and provided their "smoking histories". At the end of their last year of school (after 5 years from the SFC), the percentage of students who stated that they were regular smokers was lower among the SFC students than in controls: 13.5% vs 32.9% (p=0.03). From the students' "smoking histories", statistically significant protective ORs were observed for SFC students at the end of 1st and 5th year: 0.42 (95% CI 0.19-0.93) and 0.32 (95% CI 0.11-0.91) respectively. Absence of smokers in the family was also a strongly statistically significant factor associated with being a non-smoker student. These results suggest that SFC may have a positive impact on lowering the prevalence of smoking in the long term (5 years)
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. I: spectrum and chemical composition
In this paper we investigate the effect of stochasticity in the spatial and
temporal distribution of supernova remnants on the spectrum and chemical
composition of cosmic rays observed at Earth. The calculations are carried out
for different choices of the diffusion coefficient D(E) experienced by cosmic
rays during propagation in the Galaxy. In particular, at high energies we
assume that D(E)\sim E^{\delta}, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars, with and without
accounting for the spiral structure of the Galaxy. We find that the stochastic
fluctuations induced by the spatial and temporal distribution of supernovae,
together with the effect of spallation of nuclei, lead to mild but sensible
violations of the simple, leaky-box-inspired rule that the spectrum observed at
Earth is with , where
is the slope of the cosmic ray injection spectrum at the sources. Spallation of
nuclei, even with the small rates appropriate for He, may account for slight
differences in spectral slopes between different nuclei, providing a possible
explanation for the recent CREAM observations. For we find that
the slope of the proton and helium spectra are and
respectively at energies above 1 TeV (to be compared with the measured values
of and ). For the hardening of the He
spectra is not observed. We also comment on the effect of time dependence of
the escape of cosmic rays from supernova remnants, and of a possible clustering
of the sources in superbubbles. In a second paper we will discuss the
implications of these different scenarios for the anisotropy of cosmic rays.Comment: 28 pages, To appear in JCA
Origin of TeV Galactic Cosmic Rays
We consider a possibility of identification of sources of cosmic rays (CR) of
the energy above 1 TeV via observation of degree-scale extended gamma-ray
emission which traces the locations of recent sources in the Galaxy. Such
emission in the energy band above 100 GeV is produced by CR nuclei and
electrons released by the sources and spreading into the interstellar medium.
We use the data from the Fermi gamma-ray telescope to locate the degree-scale
100 GeV gamma-ray sources. We find that the number of such sources and their
overall power match to those expected when CRs injection events happen every
~100 yr in portions of ~1e50 erg. We find that most of the sources are
associated to pulsars with spin down age less than ~30 kyr and hence to the
recent supernova explosions. This supports the hypothesis of supernova origin
of Galactic CRs. We notice that the degree-scale extended emission does not
surround shell-like supernova remnants without pulsars. Based on this
observation, we argue that the presence of the pulsar is essential for the CR
acceleration process. We expect that a significant fraction of the degree-scale
sources should be detectable as extended sources with km3-scale neutrino
detectors.Comment: 14 pages, 14 figures, accepted for publication in Phys.Rev.
The GZK horizon and constraints on the cosmic ray source spectrum from observations in the GZK regime
We discuss the GZK horizon of protons and present a method to constrain the
injection spectrum of ultrahigh energy cosmic rays (UHECRs) from supposedly
identified extragalactic sources. This method can be applied even when only one
or two events per source are observed and is based on the analysis of the
probability for a given source to populate different energy bins, depending on
the actual CR injection spectral index. In particular, we show that for a
typical source density of , a data set of 100 events
above eV allows one in 97% of all cases to distinguish a
source spectrum from one with at 95%
confidence level.Comment: v2: 5 pages, 3 figures; shortened, title changed, matches version to
be publishe
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