The Outburst of the Blazar AO 0235+164 in 2006 December: Shock-in-Jet Interpretation

We present the results of polarimetric ($R$ band) and multicolor photometric ($BVRIJHK$) observations of the blazar AO 0235+16 during an outburst in 2006 December. The data reveal a short timescale of variability (several hours), which increases from optical to near-IR wavelengths; even shorter variations are detected in polarization. The flux density correlates with the degree of polarization, and at maximum degree of polarization the electric vector tends to align with the parsec-scale jet direction. We find that a variable component with a steady power-law spectral energy distribution and very high optical polarization (30-50%) is responsible for the variability. We interpret these properties of the blazar withina model of a transverse shock propagating down the jet. In this case a small change in the viewing angle of the jet, by $\lesssim 1^o$, and a decrease in the shocked plasma compression by a factor of $\sim$1.5 are sufficient to account for the variability.Comment: 22 pages, 8 figures, accepted for Ap

Transport in Graphene superimposed by a moving Electrical Superlattice Potential

We calculate dc-conductivities of ballistic graphene undulated by a overlying moving unidirectional electrical superlattice (SL) potential whose SL-velocity is smaller than the electron velocity. We obtain no dependence of the conductivity on the velocity along the direction of the superlattice wavevector. In the orthogonal direction however, the dependence is strong on the velocity especially at voltages where a new Dirac point emerges for zero velocity. It is shown that the infinite graphene system can serve as an ideal motion detector at potentials where the first new Dirac point emerges. There the conductivity is zero at vanishing SL velocities and jumps to infinity when the SL starts moving. For finite systems at voltages where the number of new Dirac points is of the order of the ratio of the electron velocity by the SL-velocity, the modifications to the conductivity of a moving SL is at least of similar magnitude as the conductivity of the stagnant SL.Comment: 10 pages, 3 figures, Appendix B added, version published in PR

Color Variability of the Blazar AO 0235+16

Multicolor (UBVRIJHK) observations of the blazar AO 0235+16 are analyzed. The light curves were compiled at the Turin Observatory from literature data and the results of observations obtained in the framework of the WEBT program (http://www.to.astro/blazars/webt/). The color variability of the blazar was studied in eight time intervals with a sufficient number of multicolor optical observations; JHK data are available for only one of these. The spectral energy distribution (SED) of the variable component remained constant within each interval, but varied strongly from one interval to another. After correction for dust absorption, the SED can be represented by a power law in all cases, providing evidence for a synchrotron nature of the variable component. We show that the variability at both optical and IR wavelengths is associated with the same variable source.Comment: 11 pages, 9 figures, 4 tables, accepted for publication in Astronomy Report

Stochastic model of optical variability of BL Lacertae

We use optical photometric and polarimetric data of BL Lacertae that cover a period of 22 years to study the variability of the source. The long-term observations are employed for establishing parameters of a stochastic model consisting of the radiation from a steady polarized source and a number of variable components with different polarization parameters, proposed by Hagen-Thorn et al. earlier. We infer parameters of the model from the observations using numerical simulations based on a Monte Carlo method, with values of each model parameter selected from a Gaussian distribution. We determine the best set of model parameters by comparing model distributions to the observational ones using the chi-square criterion. We show that the observed photometric and polarimetric variability can be explained within a model with a steady source of high polarization, ~40%, and with direction of polarization parallel to the parsec scale jet, along with 10+-5 sources of variable polarization.Comment: 4 pages, 10 figures, published by Astronomy and Astrophysics; v2: typos correcte

Optical Phonon Lineshapes and Transport in Metallic Carbon Nanotubes under High Bias Voltage

We calculate the current-voltage characteristic of metallic nanotubes at high bias voltage showing that a bottleneck exists for short nanotubes in contrast to large ones. We attribute this to a redistribution of lower-lying acoustic phonons caused by phonon-phonon scattering with hot optical phonons. The current-voltage characteristic and the electron and phonon distribution functions are derived analytically, and serve to obtain in a self-contained way the frequency shift and line broadening of the zone center optical phonons due to the electron-phonon coupling at high bias. We obtain a positive frequency shift from the zero bias shift and no broadening of the optical phonon mode at very high voltages, in agreement with recent experiments.Comment: 16 pages, 8 figures, minor changes, pusblished in PR

The Stellar Content of the Polar Rings in the Galaxies NGC 2685 and NGC 4650A

We present the results of stellar photometry of polar-ring galaxies NGC 2685 and NGC 4650A, using the archival data obtained with the Hubble Space Telescope's Wide Field Planetary Camera 2. Polar rings of these galaxies were resolved into ~800 and ~430 stellar objects in the B, V and Ic bands, considerable part of which are blue supergiants located in the young stellar complexes. The stellar features in the CM-diagrams are best represented by isochrones with metallicity Z = 0.008. The process of star formation in the polar rings of both galaxies was continuous and the age of the youngest detected stars is about 9 Myr for NGC 2685 and 6.5 Myr for NGC 4650A.Comment: 21 pages, 9 figures, AJ 2004 February, accepte

Path integral and stochastic differential equations

The statistics of rare events, the so-called black-swan events, is governed by non-Gaussian distributions with heavy power-like tails. We calculate the Green functions of the associated Fokker-Planck equations and solve the related stochastic differential equations. We also discuss the subject in the framework of path integration

Gravitational waves in general relativity: XIV. Bondi expansions and the ``polyhomogeneity'' of \Scri

The structure of polyhomogeneous space-times (i.e., space-times with metrics which admit an expansion in terms of $r^{-j}\log^i r$) constructed by a Bondi--Sachs type method is analysed. The occurrence of some log terms in an asymptotic expansion of the metric is related to the non--vanishing of the Weyl tensor at Scri. Various quantities of interest, including the Bondi mass loss formula, the peeling--off of the Riemann tensor and the Newman--Penrose constants of motion are re-examined in this context.Comment: LaTeX, 28pp, CMA-MR14-9

Galilean Lee Model of the Delta Function Potential

The scattering cross section associated with a two dimensional delta function has recently been the object of considerable study. It is shown here that this problem can be put into a field theoretical framework by the construction of an appropriate Galilean covariant theory. The Lee model with a standard Yukawa interaction is shown to provide such a realization. The usual results for delta function scattering are then obtained in the case that a stable particle exists in the scattering channel provided that a certain limit is taken in the relevant parameter space. In the more general case in which no such limit is taken finite corrections to the cross section are obtained which (unlike the pure delta function case) depend on the coupling constant of the model.Comment: 7 pages, latex, no figure