Bardeen-Petterson effect and the disk structure of the Seyfert galaxy NGC 1068

VLBA high spatial resolution observations of the disk structure of the active galactic nucleus NGC 1068 has recently revealed that the kinematics and geometry of this AGN is well characterized by an outer disk of H2O maser emission having a compact milliarcsecond (parsec) scale structure, which is encircling a thin rotating inner disk surrounding a ~10^7 M_\sun compact mass, likely a black hole. A curious feature in this source is the occurrence of a misalignment between the inner and outer parts of the disk, with the galaxy's radio jet being orthogonal to the inner disk. We interpret this peculiar configuration as due to the Bardeen-Petterson effect, a general relativistic effect that warps an initially inclined (to the black hole equator) viscous disk, and drives the angular momentum vector of its inner part into alignment with the rotating black hole spin. We estimate the time-scale for both angular momenta to get aligned as a function the spin parameter of the Kerr black hole. We also reproduce the shape of the parsec and kiloparsec scale jets, assuming a model in which the jet is precessing with a period and aperture angle that decrease exponentially with time, as expected from the Bardeen-Petterson effect.Comment: 12 pages, 3 figures, accepted for publication in The Astrophysical Journa

Hot and repulsive traffic flow

We study a message passing model, applicable also to traffic problems. The model is implemented in a discrete lattice, where particles move towards their destination, with fluctuations around the minimal distance path. A repulsive interaction between particles is introduced in order to avoid the appearance of traffic jam. We have studied the parameter space finding regions of fluid traffic, and saturated ones, being separated by abrupt changes. The improvement of the system performance is also explored, by the introduction of a non-constant potential acting on the particles. Finally, we deal with the behavior of the system when temporary failures in the transmission occurs.Comment: 22 pages, uuencoded gzipped postscript file. 11 figures include

Observational evidence of spin-induced precession in active galactic nuclei

We show that it is possible to explain the physical origin of jet precession in active galactic nuclei (AGNs) through the misalignment between the rotation axes of the accretion disk and of the Kerr black hole. We apply this scenario to quasars, Seyfert galaxies and also to the Galactic Center black hole Sgr A*, for which signatures of either jet or disk precession have been found. The formalism adopted is parameterized by the ratio of the precession period to the black hole mass and can be used to put constraints to the physical properties of the accretion disk as well as to the black hole spin in those systems.Comment: 10 pages, 1 figure, accepted for publication in ApJ Letter

Theoretical approach to two-dimensional traffic flow models

In this paper we present a theoretical analysis of a recently proposed two-dimensional Cellular Automata model for traffic flow in cities with the novel ingredient of turning capability. Numerical simulations of this model show that there is a transition between a freely moving phase with high velocity to a jammed state with low velocity. We study the dynamics of such a model starting with the microscopic evolution equation, which will serve as a basis for further analysis. It is shown that a kinetic approach, based on the Boltzmann assumption, is able to provide a reasonably good description of the jamming transition. We further introduce a space-time continuous phenomenological model leading to a couple of partial differential equations whose preliminary results agree rather well with the numerical simulations.Comment: 15 pages, REVTeX 3.0, 7 uuencoded figures upon request to [email protected]

METRIC-GIS: An advanced energy balance model for computing crop evapotranspiration in a GIS environment

A novel ArcGIS toolbox that applies the Mapping Evapotranspiration with Internalized Calibration model was developed and tested in a semi-arid environment. The tool, named METRIC-GIS, facilitates the pre-processing operations and the automatic identification of potential calibration and pixels review. The energy balance components obtained from METRIC-GIS were contrasted with those from the original METRIC version (R2 = 1; RMSE = 0 W m–2 or mm day–1 for ETc) Additionally, an irrigated scheme located at southern Spain was considered for assessing Kc variability in the maize fields with METRIC-GIS. The identified spatial variability was mainly due to differences in irrigation regimes, crop management practices, and planting and harvesting dates. This information is critical for developing irrigation advisory strategies that contribute to the area sustainability. The developed tool facilitates data input introduction and reduces computational time by up to 50%, providing a more user-friendly alternative to other existing platforms that use METRIC

Melting of Hard Cubes

The melting transition of a system of hard cubes is studied numerically both in the case of freely rotating cubes and when there is a fixed orientation of the particles (parallel cubes). It is shown that freelly rotating cubes melt through a first-order transition, whereas parallel cubes have a continuous transition in which positional order is lost but bond-orientational order remains finite. This is interpreted in terms of a defect-mediated theory of meltingComment: 5 pages, 3 figures included. To appear in Phys. Rev.

Optimised Traffic Flow at a Single Intersection: Traffic Responsive signalisation

We propose a stochastic model for the intersection of two urban streets. The vehicular traffic at the intersection is controlled by a set of traffic lights which can be operated subject to fix-time as well as traffic adaptive schemes. Vehicular dynamics is simulated within the framework of the probabilistic cellular automata and the delay experienced by the traffic at each individual street is evaluated for specified time intervals. Minimising the total delay of both streets gives rise to the optimum signalisation of traffic lights. We propose some traffic responsive signalisation algorithms which are based on the concept of cut-off queue length and cut-off density.Comment: 10 pages, 11 eps figs, to appear in J. Phys.

Ab initio calculations for bromine adlayers on the Ag(100) and Au(100) surfaces: the c(2x2) structure

Ab initio total-energy density-functional methods with supercell models have been employed to calculate the c(2x2) structure of the Br-adsorbed Ag(100) and Au(100) surfaces. The atomic geometries of the surfaces and the preferred bonding sites of the bromine have been determined. The bonding character of bromine with the substrates has also been studied by analyzing the electronic density of states and the charge transfer. The calculations show that while the four-fold hollow-site configuration is more stable than the two-fold bridge-site topology on the Ag(100) surface, bromine prefers the bridge site on the Au(100) surface. The one-fold on-top configuration is the least stable configuration on both surfaces. It is also observed that the second layer of the Ag substrate undergoes a small buckling as a consequence of the adsorption of Br. Our results provide a theoretical explanation for the experimental observations that the adsorption of bromine on the Ag(100) and Au(100) surfaces results in different bonding configurations.Comment: 10 pages, 4 figure, 5 tables, Phys. Rev. B, in pres