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 complete sample of bright swift long gamma-ray bursts. I. Sample presentation, luminosity function and evolution

Abstract: We present a carefully selected sub-sample of Swift long gamma-ray bursts (GRBs) that is complete in redshift. The sample is constructed by considering only bursts with favorable observing conditions for ground-based follow-up searches, which are bright in the 15-150 keV Swift/BAT band, i.e., with 1-s peak photon fluxes in excess to 2.6 photons s{sup -1} cm{sup -2}. The sample is composed of 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in an unbiased way. We find that strong evolution in luminosity ({delta}{sub l} = 2.3 {+-} 0.6) or in density ({delta}{sub d} = 1.7 {+-} 0.5) is required in order to account for the observations. The derived redshift distributions in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distributions. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies.more » In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs.« less

The Baryon content of the Universe

Abstract: We estimate the baryon mass density of the Universe due to the stars in galaxies and the hot gas in clusters and groups of galaxies. The galaxy contribution is computed by using the Efstathiou, Ellis & Peterson luminosity function, together with van der Marel and Persic & Salucci’s mass-to-light versus luminosity relationships. We find stars ≃ 0.002. For clusters and groups we use the Edge et al. X -ray luminosity function, and Edge & Stewart and Kriss, Cioffi & Canizares’ (gas mass )-luminosity relations. We find gas ≃ 0.001. The total amount of visible baryons is then b ≃ 0.003, i.e. less than 10 per cent of the lower limit predicted by standard primordial nucleosynthesis, implying that the great majority of baryons in the Universe are unseen.

The emergence of grid cells: Intelligent design or just adaptation?

Abstract: Individual medial entorhinal cortex (mEC) 'grid' cells provide a representation of space that appears to be essentially invariant across environments, modulo simple transformations, in contrast to multiple, rapidly acquired hippocampal maps; it may therefore be established gradually during rodent development. We explore with a simplified mathematical model the possibility that the self-organization of multiple grid fields into a triangular grid pattern may be a single-cell process, driven by firing rate adaptation and slowly varying spatial inputs. A simple analytical derivation indicates that triangular grids are favored asymptotic states of the self-organizing system, and computer simulations confirm that such states are indeed reached during a model learning process, provided it is sufficiently slow to effectively average out fluctuations. The interactions among local ensembles of grid units serve solely to stabilize a common grid orientation. Spatial information, in the real mEC network, may be provided by any combination of feedforward cortical afferents and feedback hippocampal projections from place cells, since either input alone is likely sufficient to yield grid fields.

Absence of a Spin Liquid Phase in the Hubbard Model on the Honeycomb Lattice

Abstract: A spin liquid is a novel quantum state of matter with no conventional order parameter where a finite charge gap exists even though the band theory would predict metallic behavior. Finding a stable spin liquid in two or higher spatial dimensions is one of the most challenging and debated issues in condensed matter physics. Very recently, it has been reported that a model of graphene, i.e., the Hubbard model on the honeycomb lattice, can show a spin liquid ground state in a wide region of the phase diagram, between a semi-metal (SM) and an antiferromagnetic insulator (AFMI). Here, by performing numerically exact quantum Monte Carlo simulations, we extend the previous study to much larger clusters (containing up to 2592 sites), and find, if any, a very weak evidence of this spin liquid region. Instead, our calculations strongly indicate a direct and continuous quantum phase transition between SM and AFMI.