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.

From Band Insulator to Mott Insulator in One Dimension

Abstract: We derive the phase diagram for the one-dimensional model of a ferroelectric perovskite recently analyzed by Egami, Ishihara, and Tachiki [Science 261, 1307 (1993)]. We show that the interplay between covalency, ionicity, and strong correlations results in a spontaneously dimerized phase which separates the weak-coupling band insulator from the strong-coupling Mott insulator. The transition from the band insulator to the dimerized phase is identified as an Ising critical point. The charge gap vanishes at this single point with the optical conductivity diverging as $\ensuremath{\sigma}(\ensuremath{\omega})\ensuremath{\sim}{\ensuremath{\omega}}^{\ensuremath{-}3/4}$. The spin excitations are gapless above the second transition to the Mott insulator phase.

A unifying view of gamma-ray burst afterglows

Abstract: We selected a sample of 33 Gamma‐Ray Bursts (GRBs) detected by Swift, with known redshift and optical extinction at the host frame. For these, we constructed the de‐absorbed and K‐corrected X‐ray and optical rest frame light curves. These are modelled as the sum of two components: emission from the forward shock due to the interaction of a fireball with the circum‐burst medium and an additional component, treated in a completely phenomenological way. The latter can be identified, among other possibili ties, as “late prompt” emission produced by a long lived central engine with mechanisms similar to those responsible for the production of the “standard” early prompt radiation. Apart from flares or re‐brightenings, that we do not model, we find a good agreement with the data, despite of their complexity and diversity. Although based in part on a phenomenological model with a relatively large number of free parameters, we believe that our findings are a first ste p towards the construction of a more physical scenario. Our approach allows us to interpret the behaviour of the optical and X‐ray afterglows in a coherent way, by a relatively simple scenario. Within this context it is possible to explain why sometimes no jet break is observed; why, even if a jet break is observed, it is often chromatic; why the steepening after th e jet break time is often shallower than predicted. Finally, the decay slope of the late prompt emission after the shallow phase is found to be remarkably similar to the time profile expected by the accretion rate of fall‐back material (i.e. / t 5/3 ), suggesting that this can be the reason why the central engine can be active for a long time.

Nearby early-type galaxies with ionized gas - IV. Origin and powering mechanism of the ionized gas

Abstract: Aims. A significant fraction of early-type galaxies (ETGs) show em ission lines in their optical spectra. We aim at understandi ng the powering mechanism and the origin of the ionized gas in ETGs, and its connection with the host galaxy evolution. Methods. We analyzed intermediate-resolution optical spectra of a s ample of 65 ETGs, mostly located in low density environments and biased toward the presence of ISM traces, for which we already derived in the previous papers of the series the stellar population properties. To extract the emission lines from the galaxy sp ectra, we developed a new fitting procedure that properly sub tracts the underlying stellar continuum, and that accounts for the unc ertainties due to the age-metallicity degeneracy. The emis sion line luminosities derived in annuli of increasing galacto-centric d istance were used to constrain the excitation mechanism and the metallicity of the ionized gas. Results. Optical emission lines are detected in 89% of the sample. The detection fraction drops to 57% if only the galaxies with EW(Hα + [NII])>3 A are considered. The incidence and strength of emission do not correlate either with the E/S0 classification, or with the fast/slow rotator classification. Comparing the nuclear r