Pulsar reflections within the Crab Nebula

Between 1997 August and October, the radio pulses from the Crab pulsar were followed by discrete moving echoes, which appear to be reflections from part of an ionized shell in the outer part of the Crab Nebula, crossing the line of sight to pulsar. Similar events have now been recognized in recordings from the past 30 yr, and it seems that the Nebula must contain a large number of ionized shell-like surfaces on a much finer scale than recognized hitherto.     

A unified model of dwarf nova outbursts based on the disk instability

A unified model for outbursts of dwarf novae is proposed based on the disk instability model in cataclysmic variable stars. In this model, two different intrinsic instabilities (i.e., the thermal instability and the tidal instability) within accretion disks are considered in non-magnetic cataclysmic variable stars. It is suggested that all of three sub-classes of dwarf novae (i.e., U Gem-type, Z Cam-type and SU UMa-type dwarf novae) may be explained in terms of two model parameters of the orbital period of the binary and of the mass transfer rate within the framework of the disk instability model.     

On the mathematical description of fine structure under the conditions of weak variability

The idea of the weak variability of the fields is introduced, which means that when they are recorded by some measuring instrument (MI), the output signal obtains increments of the order of one/several quanta of sensitivity at the discreteness interval. At small scales (microstructure), this field is described in terms of random discrete values which reflect the properties of both the field measured and the MI. Theoretical relationships which define the probabilities of increments at the compound intervals in terms of the probabilities of increments at the sampling intervals, are derived for the case of independent random increments at the sampling intervals of discreteness. The validity of this model is illustrated by respective computations using data of microstructure measurements of the temperature profile in the main thermocline in the Sargasso Sea.Translated by Mikhail M. Trufanov.     

The interannual leading modes of the extratropical variability in the Southern Hemisphere simulated by the ECHAM-4 atmospheric model

 An ensemble of twenty-three 14-year experiments conducted with the ECHAM-4 GCM has been examined to test the model's capability to simulate the principal modes of interannual variability. The integrations were performed under specified monthly SST between 1979–1993. The analysis was focused on the Southern Hemisphere (SH) extratropics. Empirical orthogonal functions analysis (EOF) using seasonal anomaly fields has been performed to isolate the principal modes that dominate the southern extratropical variability at the interannual time scale. Leading patterns of 500 hPa geopotential height (z500) have been compared with those estimated from the ECMWF re-analysis dataset. The model is able to adequately reproduce the spatial pattern of the annular mode, but it represents the temporal variations of the oscillation less satisfactorily. The model simulation of the Pacific South American (PSA) pattern is better, both in the shape of the pattern and in the temporal evolution. To verify if the capability of the model to adequately simulate the temporal oscillation of the propagating patterns is related to the increased influence of the tropical external forcing, covarying SST-atmospheric modes have been identified by singular value decomposition (SVD). In winter (July-August-September, JAS) the tropical SST variability is highly correlated with the ENSO mode. In summer (January-February-March, JFM) the strength of the teleconnections is related to strong westerly anomalies, disrupted by a meridional out of phase relation near to South America. The large size of the ensemble was exploited by comparing the time-varying model spread and degrees of freedom of the simulated extratropical circulation. Results show that when the extratropical circulation has a few degrees of freedom, the reproducibility is relatively low and the ensemble is governed by a fairly robust zonally symmetric structure of dispersion. Received: 9 May 2000 / Accepted: 30 January 2001     

Introduction

A historical sketch of the nonlinear theory of variable stars is outlined briefly. The main break-through came from the hydrodynamic study of stellar pulsation. From a theoretical point of view, coupling needs to be discussed more carefully. The impact of new opacities on the astrophysical problems is also discussed.     

Long-term seismicity of Istanbul and of the Marmara Sea region

We examine the long-term seismicity of the Marmara Sea region in Turkey over a period of twenty centuries to attempt to answer two questions: (1) how well recorded in history is the earthquake activity of this region? and (2) does seismicity over this long period of time differ from that over the present century? We study this densely-populated and fast-developing region because it is one of the most tectonically active regions on the continents that appears today to be relatively quiescent.     

Contribution of chromospheric features to UV irradiance variability from spatially-resolved Ca II K spectroheliograms

We have digitized the Ca ii K spectroheliograms, observed at the National Solar Observatory at Sacramento Peak, for the period 1980 (maximum of solar cycle 21), 1985 (minimum of solar cycle 21), 1987 (beginning of the ascending phase of solar cycle 22), 1988 and 1989 (ascending phase and maximum of solar cycle 22), and 1992 (declining phase of solar cycle 22). A new method for analyzing the K spectroheliograms has been developed and applied to the K images for the time interval of 1992. Using histograms of intensity, we have segregated and measured the cumulative intensity and area of various chromospheric features like the plages, magnetic network and intranetwork elements. Also, the full width at half maximum (FWHM) derived from the histograms has been introduced as a new index for describing the chromospheric activity in the K-line. The full-disk intensity (spatial K index) has been derived from spatially-resolved K images and compared to the spectral K index derived from the line profiles for the full disk. Both the spatial K index and FWHM have been compared to the UV irradiance measured in the Mg ii h and k lines by the NOAA9 satellite and found that they are highly correlated with the Mg ii h and k c/w ratio.NRC Resident Research Associate, on leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

A viable mechanism for the production of energy in active galactic nuclei

It is suggested that a collapsing supermassive object, which acts as an ultra-high energy particle accelerator, is the precursor of an active galactic nucleus and that the gravitational energy released during the collapse of the object is locked in the quark-gluon plasma permeated by leptons into which the entire matter in the core of the object is converted as a result of the collapse. It is also pointed out that the collapse of the object to a space-time singularity is inhibited by Pauli's exclusion principle as well as by Heisenberg's uncertainty principle and that the object explodes, before it could collapse to a singularity, thereby releasing the enormous amount of energy locked in the quark-gluon plasma.     

H II regions and radio visibility of quasar nuclei

The core (injector) and the jet (relativistic plasma outflow) of AGN objects are surrounded by an ionized medium, an H II region observed in emission lines. The synchrotron radiation from the core and the jet is observed through a thin screen that cocoons the structure under consideration. The screen transparency depends on wavelength and distance from the injector. We consider the objects 3C 345 and 1803+784 whose core emission at decimeter wavelengths is absorption by more than 25 dB. The visible bright compact component is the bright nearby portion of the jet that extends outside the dense part of the screen. We explore the possibility of measuring the screen transparency from absorption in Hα recombination lines with different quantum numbers at centimeter wavelengths.     

Structure of Early-Type Galaxies

We outline the results of a two-dimensional (2D) fit to the light distribution of early-type galaxies belonging to a complete volume-limited sample and discuss briefly the significant correlations among the structural parameters. In particular we reconfirm that the lack of structural homology is probably a characteristic of hot stellar systems. This revised version was published online in July 2006 with corrections to the Cover Date.