Are Quasar Jets Matter or Poynting Flux Dominated?

If quasar jets are accelerated by magnetic fields but terminate as matter dominated, where and how does the transition occur between the Poynting-dominated and matter-dominated regimes? To address this question, we study constraints which are imposed on the jet structure by observations at different spatial scales. We demonstrate that observational data are consistent with a scenario where the acceleration of a jet occurs within 10^{3-4} R_g. In this picture, the non-thermal flares -- important defining attributes of the blazar phenomenon - are produced by strong shocks formed in the region where the jet inertia becomes dominated by matter. Such shocks may be formed due to collisions between the portions of a jet accelerated to different velocities, and the acceleration differentiation is very likely to be related to global MHD instabilities.Comment: to appear in "Astrophysical Sources of High Energy Particles and Radiation", AIP Proceedings Series, eds. T. Bulik, G. Madejski, and B. Rudak (20-24 June 2005, Torun, Poland

Stochastic perturbation of dynamical systems: The weak convergence of measures

AbstractIn this paper we consider the asymptotic behaviour of randomly perturbed discrete dynamical systems. We treat this problem by examining the evolution of the corresponding sequences of distributions. We show that an average contractive property is sufficient to ensure the weak convergence of the sequence of distributions to a unique stationary measure

User-driven design of robot costume for child-robot interactions among children with cognitive impairment

The involvement of arts and psychology elements in robotics research for children with cognitive impairment is still limited. However, the combination of robots, arts, psychology and education in the development of robots could significantly contribute to the improvement of social interaction skills among children with cognitive impairment. In this article, we would like to share our work on building and innovating the costume of LUCA's robot, which incorporating the positive psychological perspectives and arts values for children with cognitive impairment. Our goals are (1) to educate arts students in secondary arts school on the importance of social robot appearance for children with cognitive impairment, and (2) to select the best costume for future child-robot interaction study with children with cognitive impairments

Robustness of entanglement-based discrete- and continuous-variable quantum key distribution against channel noise

Discrete-variable (DV) and continuous-variable (CV) schemes constitute the two major families of quantum key distribution (QKD) protocols. Unfortunately, since the setup elements required by these schemes are quite different, making a fair comparison of their potential performance in particular applications is often troublesome, limiting the experimenters' capability to choose an optimal solution. In this work we perform a general comparison of the major entanglement-based DV and CV QKD protocols in terms of their resistance to the channel noise, with the otherwise perfect setup, showing the definite superiority of the DV family. We analytically derive fundamental bounds on the tolerable channel noise and attenuation for entanglement-based CV QKD protocols. We also investigate the influence of DV QKD setup imperfections on the obtained results in order to determine benchmarks for the parameters of realistic photon sources and detectors, allowing the realistic DV protocols to outperform even the ideal CV QKD analogs. Our results indicate the realistic advantage of DV EPR-based schemes over their CV counterparts and suggests the practical efforts for maximizing this advantage.Comment: 24 pages, 9 figure

XMM-Newton observations of two black hole X-ray transients in quiescence

We report on XMM-Newton observations of GRO J1655-40 and GRS 1009-45, which are two black hole X-ray transients currently in their quiescent phase. GRO J1655-40 was detected with a 0.5 - 10 keV luminosity of 5.9 10^{31} erg/s. This luminosity is comparable to a previous Chandra measurement, but ten times lower than the 1996 ASCA value, most likely obtained when the source was not yet in a true quiescent state. Unfortunately, XMM-Newton failed to detect GRS 1009-45. A stringent upper limit of 8.9 10^{30} erg/s was derived by combining data from the EPIC-MOS and PN cameras. The X-ray spectrum of GRO J1655-40 is very hard as it can be fitted with a power law model of photon index ~ 1.3 +/- 0.4. Similarly hard spectra have been observed from other systems; these rule out coronal emission from the secondary or disk flares as the origin of the observed X-rays. On the other hand, our observations are consistent with the predictions of the disc instability model in the case that the accretion flow forms an advection dominated accretion flow (ADAF) at distances less than a fraction ~ 0.1 - 0.3) of the circularization radius. This distance corresponds to the greatest extent of the ADAF that is thought to be possible.Comment: 6 pages, 4 figures. Submitted to Astronomy and Astrophysic

Deterministic Brownian motion generated from differential delay equations

This paper addresses the question of how Brownian-like motion can arise from the solution of a deterministic differential delay equation. To study this we analytically study the bifurcation properties of an apparently simple differential delay equation and then numerically investigate the probabilistic properties of chaotic solutions of the same equation. Our results show that solutions of the deterministic equation with randomly selected initial conditions display a Gaussian-like density for long time, but the densities are supported on an interval of finite measure. Using these chaotic solutions as velocities, we are able to produce Brownian-like motions, which show statistical properties akin to those of a classical Brownian motion over both short and long time scales. Several conjectures are formulated for the probabilistic properties of the solution of the differential delay equation. Numerical studies suggest that these conjectures could be "universal" for similar types of "chaotic" dynamics, but we have been unable to prove this.Comment: 15 pages, 13 figure

Expanding Semiflows on Branched Surfaces and One-Parameter Semigroups of Operators

We consider expanding semiflows on branched surfaces. The family of transfer operators associated to the semiflow is a one-parameter semigroup of operators. The transfer operators may also be viewed as an operator-valued function of time and so, in the appropriate norm, we may consider the vector-valued Laplace transform of this function. We obtain a spectral result on these operators and relate this to the spectrum of the generator of this semigroup. Issues of strong continuity of the semigroup are avoided. The main result is the improvement to the machinery associated with studying semiflows as one-parameter semigroups of operators and the study of the smoothness properties of semiflows defined on branched manifolds, without encoding as a suspension semiflow

Three Additional Quiescent Low-Mass X-ray Binary Candidates in 47 Tucanae

We identify through their X-ray spectra one certain (W37) and two probable (W17 and X4) quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars in a long Chandra X-ray exposure of the globular cluster 47 Tucanae, in addition to the two previously known qLMXBs. W37's spectrum is dominated by a blackbody-like component consistent with radiation from the hydrogen atmosphere of a 10 km neutron star. W37's lightcurve shows strong X-ray variability which we attribute to variations in its absorbing column depth, and eclipses with a probable 3.087 hour period. For most of our exposures, W37's blackbody-like emission (assumed to be from the neutron star surface) is almost completely obscured, yet some soft X-rays (of uncertain origin) remain. Two additional candidates, W17 and X4, present X-ray spectra dominated by a harder component, fit by a power-law of photon index ~1.6-3. An additional soft component is required for both W17 and X4, which can be fit with a 10 km hydrogen-atmosphere neutron star model. X4 shows significant variability, which may arise from either its power-law or hydrogen-atmosphere spectral component. Both W17 and X4 show rather low X-ray luminosities, Lx(0.5-10 keV)~5*10^{31} ergs/s. All three candidate qLMXBs would be difficult to identify in other globular clusters, suggesting an additional reservoir of fainter qLMXBs in globular clusters that may be of similar numbers as the group of previously identified objects. The number of millisecond pulsars inferred to exist in 47 Tuc is less than 10 times larger than the number of qLMXBs in 47 Tuc, indicating that for typical inferred lifetimes of 10 and 1 Gyr respectively, their birthrates are comparable.Comment: Accepted for publication in ApJ. 13 pages, 7 figures (2 color