18,884 research outputs found

    The flat X-ray spectrum of the LINER NGC1052

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    We report on ROSAT and ASCA observations of the LINER NGC1052, which is the first one where broad optical lines in polarized light have been observed. The 2-10 keV spectrum is very flat, with an observed photon index (Gamma) ~0.1. A model where a nuclear source is - partly or totally - obscured by a screen of matter with column density ~10^23 atom/cm/cm is the most convincing explanation for the observed flatness. This agrees with the hypothesis that the LINERs are a population of low-luminosity AGN, to which the Seyfert unification scenario applies. The intrinsic spectral index is still rather flat (1.0-1.4), as observed in a few type-2 Seyferts so far or predicted if the accretion occurs in an advection-dominated flow.Comment: 5 pages, Latex, 2 Postscript figures, accepted for publication in MNRA

    Beyond similarity: A network approach for identifying and delimiting biogeographical regions

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    Biogeographical regions (geographically distinct assemblages of species and communities) constitute a cornerstone for ecology, biogeography, evolution and conservation biology. Species turnover measures are often used to quantify biodiversity patterns, but algorithms based on similarity and clustering are highly sensitive to common biases and intricacies of species distribution data. Here we apply a community detection approach from network theory that incorporates complex, higher order presence-absence patterns. We demonstrate the performance of the method by applying it to all amphibian species in the world (c. 6,100 species), all vascular plant species of the USA (c. 17,600), and a hypothetical dataset containing a zone of biotic transition. In comparison with current methods, our approach tackles the challenges posed by transition zones and succeeds in identifying a larger number of commonly recognised biogeographical regions. This method constitutes an important advance towards objective, data derived identification and delimitation of the world's biogeographical regions.Comment: 5 figures and 1 supporting figur

    Full QCD on APE100 Machines

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    We present the first tests and results from a study of QCD with two flavours of dynamical Wilson fermions using the Hybrid Monte Carlo Algorithm (HMCA) on APE100 machines.Comment: 23 pages, LaTeX, 13 PS figures not include

    Solar Neutrinos

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    The study of solar neutrinos has given since ever a fundamental contribution both to astroparticle and to elementary particle physics, offering an ideal test of solar models and offering at the same time relevant indications on the fundamental interactions among particles. After reviewing the striking results of the last two decades, which were determinant to solve the long standing solar neutrino puzzle and refine the Standard Solar Model, we focus our attention on the more recent results in this field and on the experiments presently running or planned for the near future. The main focus at the moment is to improve the knowledge of the mass and mixing pattern and especially to study in detail the lowest energy part of the spectrum, which represents most of solar neutrino spectrum but is still a partially unexplored realm. We discuss this research project and the way in which present and future experiments could contribute to make the theoretical framemork more complete and stable, understanding the origin of some "anomalies" that seem to emerge from the data and contributing to answer some present questions, like the exact mechanism of the vacuum to matter transition and the solution of the so called solar metallicity problem.Comment: 51 pages, to be published in Special Issue on Neutrino Physics, Advances in High Energy Physics Hindawi Publishing Corporation 201

    Numerical simulations of X-rays Free Electron Lasers (XFEL)

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    We study a nonlinear Schr\"odinger equation which arises as an effective single particle model in X-ray Free Electron Lasers (XFEL). This equation appears as a first-principles model for the beam-matter interactions that would take place in an XFEL molecular imaging experiment in \cite{frat1}. Since XFEL is more powerful by several orders of magnitude than more conventional lasers, the systematic investigation of many of the standard assumptions and approximations has attracted increased attention. In this model the electrons move under a rapidly oscillating electromagnetic field, and the convergence of the problem to an effective time-averaged one is examined. We use an operator splitting pseudo-spectral method to investigate numerically the behaviour of the model versus its time-averaged version in complex situations, namely the energy subcritical/mass supercritical case, and in the presence of a periodic lattice. We find the time averaged model to be an effective approximation, even close to blowup, for fast enough oscillations of the external field. This work extends previous analytical results for simpler cases \cite{xfel1}.Comment: 14 page

    Extragalactic gamma-ray background from AGN winds and star-forming galaxies in cosmological galaxy formation models

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    We derive the contribution to the extragalactic gamma-ray background (EGB) from AGN winds and star-forming galaxies by including a physical model for the gamma-ray emission produced by relativistic protons accelerated by AGN-driven and supernova-driven shocks into a state-of-the-art semi-analytic model of galaxy formation. This is based on galaxy interactions as triggers of AGN accretion and starburst activity and on expanding blast wave as the mechanism to communicate outwards the energy injected into the interstellar medium by the active nucleus. We compare the model predictions with the latest measurement of the EGB spectrum performed by the Fermi-LAT in the range between 100 MeV and 820 GeV. We find that AGN winds can provide ~35±\pm15% of the observed EGB in the energy interval E_{\gamma}=0.1-1 GeV, for ~73±\pm15% at E_{\gamma}=1-10 GeV, and for ~60±\pm20% at E_{\gamma}>10 GeV. The AGN wind contribution to the EGB is predicted to be larger by a factor of 3-5 than that provided by star-forming galaxies (quiescent plus starburst) in the hierarchical clustering scenario. The cumulative gamma-ray emission from AGN winds and blazars can account for the amplitude and spectral shape of the EGB, assuming the standard acceleration theory, and AGN wind parameters that agree with observations. We also compare the model prediction for the cumulative neutrino background from AGN winds with the most recent IceCube data. We find that for AGN winds with accelerated proton spectral index p=2.2-2.3, and taking into account internal absorption of gamma-rays, the Fermi-LAT and IceCube data could be reproduced simultaneously.Comment: 12 pages, 8 figures, accepted for publication in A&

    Study of the performance of the NA62 Small-Angle Calorimeter at the DAΦ\PhiNE Linac

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    The measurement of BR(K+→π+ννˉ)BR(K^+\to\pi^+\nu\bar{\nu}) with 10% precision by the NA62 experiment requires extreme background suppression. The Small Angle Calorimeter aims to provide an efficient veto for photons flying at angles down to zero with respect to the kaon flight direction. The initial prototype was upgraded and tested at the Beam Test Facility of the DAΦ\PhiNE Linac at Frascati. The energy resolution and the efficiency were measured and are presented.Comment: 5 pages, 7 figure

    A common distributed language approach to software integration

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    An important objective in software integration is the development of techniques to allow programs written in different languages to function together. Several approaches are discussed toward achieving this objective and the Common Distributed Language Approach is presented as the approach of choice
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