Topological properties of the bond-modulated honeycomb lattice

We study the combined effects of lattice deformation, e-e interaction and spin-orbit coupling in a two-dimensional (2D) honeycomb lattice. We adopt different kinds of hopping modulation--generalized dimerization and a Kekule distortion--and calculate topological invariants for the non-interacting system and for the interacting system. We identify the parameter range (Hubbard U, hopping modulation, spin-orbit coupling) where the 2D system behaves as a trivial insulator or Quantum Spin Hall Insulator.Comment: 8 pages, 4 figures: discussion improved, typos corrected, references updated. Matches version published in PR

Topological invariants in interacting Quantum Spin Hall: a Cluster Perturbation Theory approach

Using Cluster Perturbation Theory we calculate Green's functions, quasi-particle energies and topological invariants for interacting electrons on a 2-D honeycomb lattice, with intrinsic spin-orbit coupling and on-site e-e interaction. This allows to define the parameter range (Hubbard U vs spin-orbit coupling) where the 2D system behaves as a trivial insulator or Quantum Spin Hall insulator. This behavior is confirmed by the existence of gapless quasi-particle states in honeycomb ribbons. We have discussed the importance of the cluster symmetry and the effects of the lack of full translation symmetry typical of CPT and of most Quantum Cluster approaches. Comments on the limits of applicability of the method are also provided.Comment: 7 pages, 7 figures: discussion improved, one figure added, references updated. Matches version published in New J. Phy

High-energy neutrinos from FR0 radio-galaxies?

The sources responsible for the emission of high-energy ($\gtrsim$ 100 TeV) neutrinos detected by IceCube are still unknown. Among the possible candidates, active galactic nuclei with relativistic jets are often examined, since the outflowing plasma seems to offer the ideal environment to accelerate the required parent high-energy cosmic rays. The non-detection of single point sources or -- almost equivalently -- the absence, in the IceCube events, of multiplets originating from the same sky position, constrains the cosmic density and the neutrino output of these sources, pointing to a numerous population of faint sources. Here we explore the possibility that FR0 radiogalaxies, the population of compact sources recently identified in large radio and optical surveys and representing the bulk of radio-loud AGN population, can represent suitable candidates for neutrino emission. Modeling the spectral energy distribution of a FR0 radiogalaxy recently associated to a $\gamma$-ray source detected by the Large Area Telescope onboard Fermi, we derive the physical parameters of its jet, in particular the power carried by it. We consider the possible mechanisms of neutrino production, concluding that $p\gamma$ reactions in the jet between protons and ambient radiation is too inefficient to sustain the required output. We propose an alternative scenario, in which protons, accelerated in the jet, escape from it and diffuse in the host galaxy, producing neutrinos as a result of $pp$ scattering with the interstellar gas, in strict analogy with the processes taking place in star-forming galaxies.Comment: 5 pages, 3 figures, accepted for publication in MNRA

Vortex solutions of the Lifshitz-Chern-Simons theory

We study vortex-like solutions to the Lifshitz-Chern-Simons theory. We find that such solutions exists and have a logarithmically divergent energy, which suggests that a Kostelitz-Thouless transition may occur, in which voxtex-antivortex pairs are created above a critical temperature. Following a suggestion made by Callan and Wilzcek for the global U(1) scalar field model, we study vortex solutions of the Lifshitz-Chern-Simons model formulated on the hyperbolic plane, finding that, as expected, the resulting configurations have finite energy. For completeness, we also explore Lifshitz-Chern-Simons vortex solutions on the sphere.Comment: Published version, added appendix on electromagnetic duality in Lifshitz system

Noncommutative Maxwell-Chern-Simons theory, duality and a new noncommutative Chern-Simons theory in d=3

Noncommutative Maxwell-Chern-Simons theory in 3-dimensions is defined in terms of star product and noncommutative fields. Seiberg-Witten map is employed to write it in terms of ordinary fields. A parent action is introduced and the dual action is derived. For spatial noncommutativity it is studied up to second order in the noncommutativity parameter \theta. A new noncommutative Chern-Simons action is defined in terms of ordinary fields, inspired by the dual action. Moreover, a transformation between noncommuting and ordinary fields is proposed.Comment: 7 pages. Some comments, new eqs. and references added. The version to be published in Phys. Lett.

Radiative cooling, heating and thermal conduction in M87

The crisis of the standard cooling flow model brought about by Chandra and XMM-Newton observations of galaxy clusters, has led to the development of several models which explore different heating processes in order to assess if they can quench the cooling flow. Among the most appealing mechanisms are thermal conduction and heating through buoyant gas deposited in the ICM by AGNs. We combine Virgo/M87 observations of three satellites (Chandra, XMM-Newton and Beppo-SAX) to inspect the dynamics of the ICM in the center of the cluster. Using the spectral deprojection technique, we derive the physical quantities describing the ICM and determine the extra-heating needed to balance the cooling flow assuming that thermal conduction operates at a fixed fraction of the Spitzer value. We assume that the extra-heating is due to buoyant gas and we fit the data using the model developed by Ruszkowski and Begelman (2002). We derive a scale radius for the model of $\sim 5$ kpc, which is comparable with the M87 AGN jet extension, and a required luminosity of the AGN of a $few \times 10^{42}$ erg s$^{-1}$, which is comparable to the observed AGN luminosity. We discuss a scenario where the buoyant bubbles are filled of relativistic particles and magnetic field responsible for the radio emission in M87. The AGN is supposed to be intermittent and to inject populations of buoyant bubbles through a succession of outbursts. We also study the X-ray cool component detected in the radio lobes and suggest that it is structured in blobs which are tied to the radio buoyant bubbles.Comment: 25 pages, 10 figures and 2 tables. Accepted for publication in Ap

Time-dependent Circulation Flows: Iron Enrichment in Cooling Flows with Heated Return Flows

We describe a new type of dynamical model for hot gas in galaxy groups and clusters in which gas moves simultaneously in both radial directions. Circulation flows are consistent with (1) the failure to observe cooling gas in X-ray spectra, (2) multiphase gas observed near the centers of these flows and (3) the accumulation of iron in the hot gas from Type Ia supernovae in the central galaxy. Dense inflowing gas cools, producing a positive central temperature gradient, as in normal cooling flows. Bubbles of hot, buoyant gas flow outward. Circulation flows eventually cool catastrophically if the outward flowing gas transports mass but no heat; to maintain the circulation both mass and energy must be supplied to the inflowing gas over a large volume, extending to the cooling radius. The rapid radial recirculation of gas produces a flat central core in the gas iron abundance, similar to many observations. We believe the circulation flows described here are the first gasdynamic, long-term evolutionary models that are in good agreement with all essential features observed in the hot gas: little or no gas cools as required by XMM spectra, the gas temperature increases outward near the center, and the gaseous iron abundance is about solar near the center and decreases outward.Comment: 17 pages (emulateapj5) with 6 figures; accepted by The Astrophysical Journa