Divergence of the Thermal Conductivity in Uniaxially Strained Graphene

We investigate the effect of strain and isotopic disorder on thermal transport in suspended graphene by equilibrium molecular dynamics simulations. We show that the thermal conductivity of unstrained graphene, calculated from the fluctuations of the heat current at equilibrium is finite and converges with size at finite temperature. In contrast, the thermal conductivity of strained graphene diverges logarithmically with the size of the models, when strain exceeds a relatively large threshold value of 2%. An analysis of phonon populations and lifetimes explains the divergence of the thermal conductivity as a consequence of changes in the occupation of low-frequency out-of-plane phonons and an increase in their lifetimes due to strain.Comment: 6 pages, 7 figures. Accepted for publication in Physical Review

Electrovac Universes with a Cosmological Constant

We present the extension of the Einstein-Maxwell system called electrovac universes by introducing a cosmological constant $\Lambda$. In the absence of the $\Lambda$ term, the crucial equation in solving the Einstein-Maxwell system is the Laplace equation. The cosmological constant modifies this equation to become in a non-linear partial differential equation which takes the form $\Delta U=2\Lambda U^3$. We offer special solutions of this equation.Comment: 7 page

Black-hole kicks from numerical-relativity surrogate models

Binary black holes radiate linear momentum in gravitational waves as they merge. Recoils imparted to the black-hole remnant can reach thousands of km/s, thus ejecting black holes from their host galaxies. We exploit recent advances in gravitational waveform modeling to quickly and reliably extract recoils imparted to generic, precessing, black hole binaries. Our procedure uses a numerical-relativity surrogate model to obtain the gravitational waveform given a set of binary parameters, then from this waveform we directly integrate the gravitational-wave linear momentum flux. This entirely bypasses the need of fitting formulae which are typically used to model black-hole recoils in astrophysical contexts. We provide a thorough exploration of the black-hole kick phenomenology in the parameter space, summarizing and extending previous numerical results on the topic. Our extraction procedure is made publicly available as a module for the Python programming language named SURRKICK. Kick evaluations take ~0.1s on a standard off-the-shelf machine, thus making our code ideal to be ported to large-scale astrophysical studies.Comment: More: https://davidegerosa.com/surrkick - Source: https://github.com/dgerosa/surrkick - pypi: https://pypi.python.org/pypi/surrkick - Published in PR

Non-Gaussian wave functionals in Coulomb gauge Yang--Mills theory

A general method to treat non-Gaussian vacuum wave functionals in the Hamiltonian formulation of a quantum field theory is presented. By means of Dyson--Schwinger techniques, the static Green functions are expressed in terms of the kernels arising in the Taylor expansion of the exponent of the vacuum wave functional. These kernels are then determined by minimizing the vacuum expectation value of the Hamiltonian. The method is applied to Yang--Mills theory in Coulomb gauge, using a vacuum wave functional whose exponent contains up to quartic terms in the gauge field. An estimate of the cubic and quartic interaction kernels is given using as input the gluon and ghost propagators found with a Gaussian wave functional.Comment: 27 pages, 21 figure

Convergence of approximate deconvolution models to the mean Magnetohydrodynamics Equations: Analysis of two models

We consider two Large Eddy Simulation (LES) models for the approximation of large scales of the equations of Magnetohydrodynamics (MHD in the sequel). We study two $\alpha$-models, which are obtained adapting to the MHD the approach by Stolz and Adams with van Cittert approximate deconvolution operators. First, we prove existence and uniqueness of a regular weak solution for a system with filtering and deconvolution in both equations. Then we study the behavior of solutions as the deconvolution parameter goes to infinity. The main result of this paper is the convergence to a solution of the filtered MHD equations. In the final section we study also the problem with filtering acting only on the velocity equation

A kpc-scale X-ray jet in the BL Lac source S5 2007+777

X-ray jets in AGN are commonly observed in FRII and FRI radio-galaxies, but rarely in BL Lacs, most probably due to their orientation close to the line of sight and the ensuing foreshortening effects. Only three BL Lacs are known so far to contain a kpc-scale X-ray jet. In this paper, we present the evidence for the existence of a fourth extended X-ray jet in the classical radio-selected source S5 2007+777, which for its hybrid FRI/II radio morphology has been classified as a HYMOR (HYbrid MOrphology Radio source). Our Chandra ACIS-S observations of this source revealed an X-ray counterpart to the 19"-long radio jet. Interestingly, the X-ray properties of the kpc-scale jet in S5 2007+777 are very similar to those observed in FRII jets. First, the X-ray morphology closely mirrors the radio one, with the X-rays being concentrated in the discrete radio knots. Second, the X-ray continuum of the jet/brightest knot is described by a very hard power law, with photon index Gamma_x~1, although the uncertainties are large. Third, the optical upper limit from archival HST data implies a concave radio-to-X-ray SED. If the X-ray emission is attributed to IC/CMB with equipartition, strong beaming (delta=13) is required, implying a very large scale (Mpc) jet. The beaming requirement can be somewhat relaxed assuming a magnetic field lower than equipartition. Alternatively, synchrotron emission from a second population of very high-energy electrons is viable. Comparison to other HYMOR jets detected with Chandra is discussed, as well as general implications for the origin of the FRI/II division.Comment: Accepted for publication in ApJ, 19 pages, 3 figure

Thick fireballs and the steep decay in the early X-ray afterglow of gamma-ray bursts

We study the early afterglows of gamma-ray bursts produced by geometrically thick fireballs, following the development of the external shock as energy is continually supplied to the shocked material. We study the dependence of the early afterglow slope on the luminosity history of the central engine. The resulting light curves are modeled with power-law functions and the importance of a correct choice of the reference time t_0 is investigated. We find that deviations from a simple power-law are observed only if a large majority of the energy is released at late times. The light curve in this case can be described as a simple power-law if the reference time is set to be close to the end of the burst. We applied our analysis to the cases of GRB 050219a and GRB 050315. We show that the early steep decay of the afterglow cannot result from the interaction of the fireball with the ambient medium. We conclude that the early X-ray afterglow emission is associated with the prompt phase and we derive limits on the radius at which the prompt radiation is produced.Comment: Minor revisions, accepted for publication in Ap

Contribution of agronomy to land management issues - A Comparison of five interdisciplinary PhD theses

An introductory literature review highlights the growing attention within the processes taking place at farming region and landscape scale beside the classical spatial scales at cultivated/experimental plot level. This recent evolution in agronomy finds its origin in newly emerging land management issues. Meanwhile, geography and other disciplines are stressing the need for a greater integration of multifunctional agricultural activities into the decision-making processes at the various levels of land management, such as provinces, municipalities or watersheds. This requires also that studies on farmland management include explicitly the different environmental and social contexts influencing farming activities. In this paper we aim to analyse how recent agronomic oriented research are facing and supporting various land management issues. We have compared five interdisciplinary PhD theses examining their definitions and methods of analysis for: the farming system, the local land management issues at stake, the spatial scale selected for the study, the stakeholders' involvement and the interaction with other disciplines. Common issues which emerged from this comparison are delivery of agro-environmental services, sustainable land management and landscape conservation. Multiple spatial levels were considered, which included at least one administrative unit of policy decision/implementation. Consequently, the explicit (re)definition of some agronomic concepts and methods was needed. Regarding the interdisciplinary framework, the theses have stressed the interactions among agronomy, geography and ecology. All theses aimed at delivering tools for decision-making support, mainly in the form of cartography. Nevertheless the participation of local stakeholders was generally included as a final step; herewith the settings of stakeholders' involvement were various. In conclusion, we discuss how the produced knowledge has enhanced the land management issues in local planning tools. On these bases, we stress finally the issues at stake to strengthen the roles and contributions of agronomic oriented education and research to agricultural land management and development. (Résumé d'auteur