International School for Advanced Studies

About: International School for Advanced Studies is a education organization based out in Trieste, Friuli-Venezia Giulia, Italy. It is known for research contribution in the topics: Galaxy & Dark matter. The organization has 3751 authors who have published 13433 publications receiving 588454 citations. The organization is also known as: SISSA & Scuola Internazionale Superiore di Studi Avanzati.

A New Technique for the Detection of Periodic Signals in ``Colored'' Power Spectra

Abstract: The light curves from a variety of celestial objects display aperiodic variations, often giving rise to red–noise components in their power spectra. Searching for a narrow power spectrum peak resulting from a periodic modulation over the frequency range in which these “coloured” noise components are dominant has proven a very complex task. Commonly used methods rely upon spectral smoothing or incoherent summation of sample spectra in order to decrease the variance of the power estimates. The consequent reduction in frequency resolution causes also a reduction of sensitivity to periodic signals. We develop here a technique aimed at detecting periodicities in the presence of “coloured” power spectrum components, while mantaining the highest Fourier frequency resolution. First we introduce a simple approximation to the statistical properties of the “coloured” power spectra from celestial objects, based on a few examples and the theory of linear processes. We then estimate the continuum components in the power spectrum through an ad hoc smoothing technique. This involves averaging the spectral estimates adjacent to each frequency over a suitably chosen interval, in order to follow steep red–noise features and produce estimates that are locally unaffected by the possible presence of a sharp peak. By dividing the sample spectrum by the smoothed one, a white–noise like spectrum is obtained, Affiliated to the International Center for Relativistic Astrophysics Now at the Astronomical Observatory of Rome, Via dell’Osservatorio 2, I–00040, Monteporzio Catone (Roma), Italy

CRASH: A radiative transfer scheme

Abstract: We present a largely improved version of crash, a 3D radiative transfer code that treats the effects of ionizing radiation propagating through a given inhomogeneous H/He cosmological density field on the physical conditions of the gas. The code, based on a Monte Carlo technique, self-consistently calculates the time evolution of gas temperature and ionization fractions due to an arbitrary number of point/extended sources and/or diffuse background radiation with given spectra. In addition, the effects of diffuse ionizing radiation following recombinations of ionized atoms have been included. After a complete description of the numerical scheme, to demonstrate the performance, accuracy, convergence and robustness of the code, we present four different test cases designed to investigate specific aspects of radiative transfer: (i) a pure-hydrogen isothermal Stromgren sphere; (ii) realistic Stromgren spheres; (iii) multiple overlapping point sources; and (iv) shadowing of background radiation by an intervening optically thick layer. When possible, detailed quantitative comparison of the results against either analytical solutions or 1D standard photoionization codes has been made, and shows a good level of agreement. For more complicated tests the code yields physically plausible results, which could be eventually checked only by comparison with other similar codes. Finally, we briefly discuss future possible developments and cosmological applications of the code.

Low power BL Lacertae objects and the blazar sequence Clues on the particle acceleration process

Abstract: The spectral properties of blazars seem to follow a phenomenological sequence according to the source luminosity. By inferring the source physical parameters through (necessarily) modeling of the blazar spectra, we have previously proposed that the sequence arises because the particles responsible for most of the emission suer increasing radiative losses as the luminosity increases. Here we extend those results by considering the widest possible range of blazar spectral properties. We nd a new important ingredient for shaping the spectra of the lowest power objects, namely the role of a nite timescale for the injection of relativistic particles. Only high energy particles radiatively cool over such a timescale leading to a break in the particle distribution: particles with this break energy are those emitting most of the power, and this gives rise to a link between blazar spectra and total energy density inside the source, which controls the cooling timescale. The emerging picture requires two phases for the particle acceleration: a rst pre{heating phase in which particles reach a characteristic energy as the result of balancing heating and radiative cooling, and a more rapid acceleration phase which further accelerate these particles to form a power law distribution. While in agreement with standard shock theory, this scenario also agrees with the idea that the luminosity of blazars is produced through internal shocks, which naturally lead to shocks lasting for a nite time.

Eulerian Calculus for the Displacement Convexity in the Wasserstein Distance

Abstract: In this paper we give a new proof of the (strong) displacement convexity of a class of integral functionals defined on a compact Riemannian manifold satisfying a lower Ricci curvature bound. Our approach does not rely on existence and regularity results for optimal transport maps on Riemannian manifolds, but it is based on the Eulerian point of view recently introduced by Otto and Westdickenberg [SIAM J. Math. Anal., 37 (2005), pp. 1227–1255] and on the metric characterization of the gradient flows generated by the functionals in the Wasserstein space.

Minimal supersymmetric left-right model

Abstract: We construct the minimal left-right symmetric model by utilizing only the fields dictated by supersymmetry and automatic R-parity conservation. Allowing for nonrenormalizable operators in the superpotential, we show that parity can be spontaneously broken while preserving electromagnetic gauge invariance. The scale of parity breakdown is predicted in the intermediate region ${M}_{R}g{10}^{10}--{10}^{11}\mathrm{GeV},$ and R-parity remains exact. The theory contains a number of charged and doubly charged Higgs scalars with a low mass of order ${M}_{R}^{2}{/M}_{\mathrm{Planck}},$ accessible to experiment.