UV and X-Ray Monitoring of AG Draconis During the 1994/1995 Outbursts

The recent 1994-1995 active phase of AG Draconis has given us for the first time the opportunity to follow the full X-ray behaviour of a symbiotic star during two successive outbursts and to compare with its quiescence X-ray emission. With \ros observations we have discovered a remarkable decrease of the X-ray flux during both optical maxima, followed by a gradual recovering to the pre-outburst flux. In the UV the events were characterized by a large increase of the emission line and continuum fluxes, comparable to the behaviour of AG Dra during the 1980-81 active phase. The anticorrelation of X-ray/UV flux and optical brightness evolution is shown to very likely be due to a temperature decrease of the hot component. Such a temperature decrease could be produced by an increased mass transfer to the burning compact object, causing it to slowly expand to about twice its original size.Comment: 12 pages postscript incl. figures, Proc. of Workshop on Supersoft X-Ray Sources, to appear in Lecture Notes in Physics vol. 472 (1996

Swift Observations of Hard X-ray Emitting White Dwarfs in Symbiotic Stars

The X-ray emission from most accreting white dwarfs (WDs) in symbiotic binary stars is quite soft. Several symbiotic WDs, however, produce strong X-ray emission at energies greater than ~20 keV. The Swift BAT instrument has detected hard X-ray emission from 4 such accreting WDs in symbiotic stars: RT Cru, T CrB, CD -57 3057, and CH Cyg. In one case (RT Cru), Swift detected X-rays out to greater than 50 keV at a > 5 sigma confidence level. Combining data from the XRT and BAT detectors, we find that the 0.3-150 keV spectra of RT Cru, T CrB, and CD -57 3057 are well described by emission from a single-temperature, optically thin thermal plasma, plus an unresolved 6.4-6.9 keV Fe line complex. The X-ray spectrum of CH Cyg contains an additional bright soft component. For all 4 systems, the spectra suffer high levels of absorption from material that both fully and partially covers the source of hard X-rays. The XRT data did not show any of the rapid, periodic variations that one would expect if the X-ray emission were due to accretion onto a rotating, highly magnetized WD. The X-rays were thus more likely from the accretion-disk boundary layer around a massive, non-magnetic WD in each binary. The X-ray emission from RT Cru varied on timescales of a few days. This variability is consistent with being due to changes in the absorber that partially covers the source, suggesting localized absorption from a clumpy medium moving into the line of sight. The X-ray emission from CD -57 3057 and T CrB also varied during the 9 months of Swift observations, in a manner that was also consistent with variable absorption.Comment: Accepted for publication in ApJ. 9 pages, 6 figure

A shadowing problem in the detection of overlapping communities: lifting the resolution limit through a cascading procedure

Community detection is the process of assigning nodes and links in significant communities (e.g. clusters, function modules) and its development has led to a better understanding of complex networks. When applied to sizable networks, we argue that most detection algorithms correctly identify prominent communities, but fail to do so across multiple scales. As a result, a significant fraction of the network is left uncharted. We show that this problem stems from larger or denser communities overshadowing smaller or sparser ones, and that this effect accounts for most of the undetected communities and unassigned links. We propose a generic cascading approach to community detection that circumvents the problem. Using real and artificial network datasets with three widely used community detection algorithms, we show how a simple cascading procedure allows for the detection of the missing communities. This work highlights a new detection limit of community structure, and we hope that our approach can inspire better community detection algorithms.Comment: 14 pages, 12 figures + supporting information (5 pages, 6 tables, 3 figures

Router-level community structure of the Internet Autonomous Systems

The Internet is composed of routing devices connected between them and organized into independent administrative entities: the Autonomous Systems. The existence of different types of Autonomous Systems (like large connectivity providers, Internet Service Providers or universities) together with geographical and economical constraints, turns the Internet into a complex modular and hierarchical network. This organization is reflected in many properties of the Internet topology, like its high degree of clustering and its robustness. In this work, we study the modular structure of the Internet router-level graph in order to assess to what extent the Autonomous Systems satisfy some of the known notions of community structure. We show that the modular structure of the Internet is much richer than what can be captured by the current community detection methods, which are severely affected by resolution limits and by the heterogeneity of the Autonomous Systems. Here we overcome this issue by using a multiresolution detection algorithm combined with a small sample of nodes. We also discuss recent work on community structure in the light of our results

Contemporaneous Observations of Direct and Raman Scattered O VI in Symbiotic Stars

Symbiotic stars are binary systems consisting of a hot star, typically a white dwarf, and a cool giant companion. The wind from the cool star is ionized by the radiation from the hot star, resulting in the characteristic combination of sharp nebular emission lines and stellar molecular absorption lines in the optical spectrum. Most of the emission lines are readily identifiable with common ions. However, two strong, broad emission lines at $\lambda\lambda$ 6825, 7082 defied identification with known atoms and ions. In 1989 Schmid made the case that these long unidentified emission lines resulted from the Raman scattering of the O VI resonance photons at $\lambda\lambda$ 1032, 1038 by neutral hydrogen. We present contemporaneous far-UV and optical observations of direct and Raman scattered O VI lines for nine symbiotic stars obtained with the Hopkins Ultraviolet Telescope (Astro-2) and various ground-based telescopes. The O VI emission lines are present in every instance in which the $\lambda\lambda$ 6825, 7082 lines are present in support of the Schmid Raman scattering model. We calculate scattering efficiencies and interpret the results in terms of the Raman models. Additionally, we measure the flux of the Fe II fluorescence line at $\lambda$1776, which is excited by the O VI line at $\lambda$1032, and calculate the first estimates of the conversion efficiencies of this process.Comment: 48 pages, 5 figure

Condition Monitoring of Kaplan Turbine Bearings Using Vibro-diagnostics

Vibration diagnostics is an indispensable method to evaluate the technical condition of machinery. If the conditions of machines are regularly monitored, the problems can be corrected even before they arise. The most often overloaded components of the turbine are bearings that similarly like other machinery are subject to degradation wear. The article deals with the bearings´ condition of the Kaplan turbine. Within the research, the frequencies at which maximum accelerations were achieved and their impact on plant operation were observed. Time records were converted to the frequency domain by means of Fast Fourier Transformation and the records were processed by means of filters. Acceleration peaks have determined the frequencies responsible for the outer and inner ring damages, but based on an evaluation of the operating state of the bearing using Root Mean Square (RMS) values could be concluded that that damage to any part of the bearing is not demonstrable and the turbine is operability

"Inter-Longitude Astronomy" (ILA) project : current highlights and perspectives : I. Magnetic vs. non-magneticinteracting binary stars

We present a review of highlights of our photometric and photo-polarimetric monitoring and mathematical modeling of interacting binary stars of different types classical, asynchronous, intermedi ate polars with 25 timescales corresponding to differ ent physical mechanisms and their combinations (part "Polar"); negative and positive superhumpers in nova-like and dwarf novae stars ("Superhumper"); symbiotic ("Symbiosis"); eclipsing variables with and without ev idence for a current mass transfer ("Eclipser") with a special emphasis on systems with a direct impact of the stream into the gainor star's atmosphere, which we propose to call "Impactors", or V361 Lyr-type stars. Other parts of the ILA project are "Stellar Bell" (pul sating variables of different types and periods - M, SR, RV Tau, RR Lyr, Delta Sct) and "New Variable".

Photometry of the dust nova V705 Cassiopeiae

We present the results of our UBV photoelectric photometry of V705 Cas (Nova Cassiopeiae 1993) obtained at the Skalnaté Pleso, Stará Lesná, Hlohovec and Brno Observatories. The observations cover the light curve development of this nova over its stages of dust formation and disappearance of the envelope. The results of the light curve analysis are presented. We have also used the visual brightness estimates of V705 Cas by the members of the VSNET network as an auxiliary data set in order to determine the nova's brightness behaviour as accurately as possible. The basic parameters of the brightness decline of the nova $t_{2,B}= 40$ d, $t_{2,V} =$ 33 d, $t_{3,B} =$ 63 d, $t_{3,V} =$ 61 d, with the accuracy not worse than one day, were obtained on the basis of our light curves. The relevant value of $M_{B}= (-7.27 \pm{0.3})$ mag and the value $M_{V}= (-7.32\pm{0.22})$ mag in the maximum of brightness were derived. The corresponding value of the mass of the white dwarf component of the presumable underlying nova binary is $M_{\rm WD}= (0.79\pm{0.06})$ $M_{\odot}$. The colour excess at brightness maximum is $E_{B{-}V}= 0.56$ mag and the corresponding value of the distance $r= (1.67\pm{0.34})$ kpc. This value of the distance is very probably underestimated due to the strong extinction by the dust envelope of the nova. The general problem of the use of statistical calibrations as well as that of the distance determination for dust novae is discussed