On three-dimensional reconstruction of optically thin solar emission sources

Calculations are given for constructing the three dimensional distribution of optically thin EUV emission sources associated with solar active regions, from two dimensional observations (projections) recorded by the spectroheliograph on the OSO 7 satellite. The relation of the method to other image reconstruction methods is briefly discussed as well as the special requirements imposed in the solar case such as a knowledge of the true solar rotation function. A useful correlation criterion for establishing the physical validity of solutions is given

Relaxation timescales and decay of correlations in a long-range interacting quantum simulator

We study the time evolution of correlation functions in long-range interacting quantum Ising models. For a large class of initial conditions, exact analytic results are obtained in arbitrary lattice dimension, both for ferromagnetic and antiferromagnetic coupling, and hence also in the presence of geometric frustration. In contrast to the nearest-neighbour case, we find that correlations decay like stretched or compressed exponentials in time. Provided the long-range character of the interactions is sufficiently strong, pronounced prethermalization plateaus are observed and relaxation timescales are widely separated. Specializing to a triangular lattice in two spatial dimensions, we propose to utilize these results for benchmarking of a recently developed ion-trap based quantum simulator.Comment: 19 pages, 6 figures; v2: one section removed, appendices added; v3: upper bound corrected + minor corrections; v4: as publishe

Analysis of Pregnancy and Other Factors on Detection of Human Papilloma Virus (HPV) Infection Using Weighted Estimating Equations for Follow-Up Data

Generalised estimating equations have been well established to draw inference for the marginal mean from follow-up data. Many studies suffer from missing data that may result in biased parameter estimates if the data are not missing completely at random. Robins and coworkers proposed to use weighted estimating equations (WEE) in estimating the mean structure if drop-out occurs missing at random. We illustrate the differences between the WEE and the commonly applied available case analysis in a simulation study. We apply the WEE and re-analyse data on pregnancy and HPV infection. We estimate the response probabilities and demonstrate that the data are not missing completely at random. Upon use of the WEE, we are able to show that pregnant women have an increased odds for an HPV infection compared with study subjects after delivery (p = 0.027). We conclude that the WEE are useful in analysing follow-up data with drop-outs

Speeding Up Computer Simulations: The Transition Observable Method

A method is presented which allows for a tremendous speed-up of computer simulations of statistical systems by orders of magnitude. This speed-up is achieved by means of a new observable, while the algorithm of the simulation remains unchanged.Comment: 20 pages, 6 figures Submitted to Phys.Rev.E (August 1999) Replacement due to some minor change

Flicker-light induced visual phenomena: Frequency dependence and specificity of whole percepts and percept features

Flickering light induces visual hallucinations in human observers. Despite a long history of the phenomenon, little is known about the dependence of flicker-induced subjective impressions on the flicker frequency. We investigate this question using Ganzfeld stimulation and an experimental paradigm combining a continuous frequency scan (1–50 Hz) with a focus on re-occurring, whole percepts. On the single-subject level, we find a high degree of frequency stability of percepts. To generalize across subjects, we apply two rating systems, (1) a set of complex percept classes derived from subjects’ reports and (2) an enumeration of elementary percept features, and determine distributions of occurrences over flicker frequency. We observe a stronger frequency specificity for complex percept classes than elementary percept features. Comparing the similarity relations among percept categories to those among frequency profiles, we observe that though percepts are preferentially induced by particular frequencies, the frequency does not unambiguously determine the experienced percept

AND Protocols Using Only Uniform Shuffles

Secure multi-party computation using a deck of playing cards has been a subject of research since the "five-card trick" introduced by den Boer in 1989. One of the main problems in card-based cryptography is to design committed-format protocols to compute a Boolean AND operation subject to different runtime and shuffle restrictions by using as few cards as possible. In this paper, we introduce two AND protocols that use only uniform shuffles. The first one requires four cards and is a restart-free Las Vegas protocol with finite expected runtime. The second one requires five cards and always terminates in finite time.Comment: This paper has appeared at CSR 201

Effect of Quantum Confinement on Electron Tunneling through a Quantum Dot

Employing the Anderson impurity model, we study tunneling properties through an ideal quantum dot near the conductance minima. Considering the Coulomb blockade and the quantum confinement on an equal footing, we have obtained current contributions from various types of tunneling processes; inelastic cotunneling, elastic cotunneling, and resonant tunneling of thermally activated electrons. We have found that the inelastic cotunneling is suppressed in the quantum confinement limit, and thus the conductance near its minima is determined by the elastic cotunneling at low temperature ($k_BT \ll \Gamma$, $\Gamma$: dot-reservoir coupling constant), or by the resonant tunneling of single electrons at high temperature ($k_BT \gg \Gamma$).Comment: 11 pages Revtex, 2 Postscript figures, To appear in Phys.Rev.

Phase transitions and configuration space topology

Equilibrium phase transitions may be defined as nonanalytic points of thermodynamic functions, e.g., of the canonical free energy. Given a certain physical system, it is of interest to understand which properties of the system account for the presence of a phase transition, and an understanding of these properties may lead to a deeper understanding of the physical phenomenon. One possible approach of this issue, reviewed and discussed in the present paper, is the study of topology changes in configuration space which, remarkably, are found to be related to equilibrium phase transitions in classical statistical mechanical systems. For the study of configuration space topology, one considers the subsets M_v, consisting of all points from configuration space with a potential energy per particle equal to or less than a given v. For finite systems, topology changes of M_v are intimately related to nonanalytic points of the microcanonical entropy (which, as a surprise to many, do exist). In the thermodynamic limit, a more complex relation between nonanalytic points of thermodynamic functions (i.e., phase transitions) and topology changes is observed. For some class of short-range systems, a topology change of the M_v at v=v_t was proved to be necessary for a phase transition to take place at a potential energy v_t. In contrast, phase transitions in systems with long-range interactions or in systems with non-confining potentials need not be accompanied by such a topology change. Instead, for such systems the nonanalytic point in a thermodynamic function is found to have some maximization procedure at its origin. These results may foster insight into the mechanisms which lead to the occurrence of a phase transition, and thus may help to explore the origin of this physical phenomenon.Comment: 22 pages, 6 figure

The close T Tauri binary system V4046 Sgr: Rotationally modulated X-ray emission from accretion shocks

We report initial results from a quasi-simultaneous X-ray/optical observing campaign targeting V4046 Sgr, a close, synchronous-rotating classical T Tauri star (CTTS) binary in which both components are actively accreting. V4046 Sgr is a strong X-ray source, with the X-rays mainly arising from high-density (n_e ~ 10^(11-12) cm^(-3)) plasma at temperatures of 3-4 MK. Our multiwavelength campaign aims to simultaneously constrain the properties of this X-ray emitting plasma, the large scale magnetic field, and the accretion geometry. In this paper, we present key results obtained via time-resolved X-ray grating spectra, gathered in a 360 ks XMM-Newton observation that covered 2.2 system rotations. We find that the emission lines produced by this high-density plasma display periodic flux variations with a measured period, 1.22+/-0.01 d, that is precisely half that of the binary star system (2.42 d). The observed rotational modulation can be explained assuming that the high-density plasma occupies small portions of the stellar surfaces, corotating with the stars, and that the high-density plasma is not azimuthally symmetrically distributed with respect to the rotational axis of each star. These results strongly support models in which high-density, X-ray-emitting CTTS plasma is material heated in accretion shocks, located at the base of accretion flows tied to the system by magnetic field lines.Comment: paper accepted by Ap