Zur Interpretation der Messungen der Lichtablenkung am Sonnenrand

The agreement between radiastronomical measurements of the gravitational deflection of light at the solar limb and the theory of relativity is compatible with the exces found by most optical observers. The optical excess is caused by stars so near to the Sun that radioastronomical observations are impossible. Optical and radioastronomical measurements both make it likely that there is an additional term inversely proportional to a power higher than the first of the distance from the Sun. Results found by BOUET indicate that this additional term is of solar origin.     

Pillars of Heaven

Sometimes the most beautiful things are the hardest to understand. Pillars like those of the Eagle Nebula form at the boundary between some of the hottest (10000~K) and coldest (10~K) gas in the Galaxy. Many physical processes come into play in the birth and growth of such gaseous pillars: hydrodynamic instability, photoionization, ablation, recombination, molecular heating and cooling, and probably magnetic fields. High-quality astronomical observations, quantitative numerical simulations, and scaled laser experiments provide a powerful combination for understanding their formation and evolution. We put our most recent hydrodynamic model to the test, by creating simulated observations from it and comparing them directly to the actual radioastronomical observations. Successfully reproducing major characteristics of the observations in this manner is an important step in designing appropriate laser experiments.     

Coherent Emission in Astrophysics: A Critique

The applications of 'coherent emission' to radioastronomical sources is discussed critically. The applications considered are electron cyclotron maser emission applied to certain solar and stellar radio emissions, pulsar radio emission, and suggested coherent emission from active galactic nuclei. This revised version was published online in July 2006 with corrections to the Cover Date.     

Polarization spectra of synchrotron radiation and the plasma composition of relativistic jets

We investigate the problem of determining the plasma composition of relativistic jets in blazars and microquasars from the polarization frequency spectra of their synchrotron radiation. The effect of plasma composition on this radiation is attributable to a change in the structure of the ordinary and extraordinary waves in plasma, depending on the presence of a nonrelativistic electron-proton component in it and on the type of relativistic particles (electrons, positrons). The structure of the normal waves determines the properties of the observed radiation and primarily the shape of the polarization frequency spectrum. Our analytic calculations of the polarization spectra for simple models of jets with a uniform magnetic field and with a magnetic-field shear revealed characteristic features in the polarization spectra. These features allow us to differentiate between the synchrotron radiation from an admixture of relativistic particles in a cold plasma and the radiation from a relativistic plasma. However, definitive conclusions regarding the relativistic plasma composition (electrons or electron-positron pairs) can be reached only by a detailed analysis of the polarization frequency spectra that will be obtained in future radioastronomical studies with high angular and frequency resolutions.     

Correlation Between the Sunspot Number,the Total Solar Irradiance,and the Terrestrial Insolation

Our study deals with the correlations between the solar activity on the one hand and the solar irradiance above the Earth’s atmosphere and at ground level on the other. We analyzed the combined ACRIM I+II time series of the total solar irradiance (TSI), the Mauna Loa time series of terrestrial insolation data, and data of terrestrial cosmic ray fluxes. We find that the correlation between the TSI and the sunspot number is strongly non-linear. We interpret this as the net balance between brightening by faculae and darkening by sunspots where faculae dominate at low activity and sunspots dominate at high activity. Such a behavior is hitherto known from stellar analogs of the Sun in a statistical manner. We perform the same analysis for the Mauna Loa data of terrestrial insolation. Here we find that the linear relation between sunspot number and insolation shows more than 1% rise in insolation by sunspot number variations which is much stronger than for the TSI. Our conclusion is that the Earth atmosphere acts as an amplifier between space and ground, and that the amplification is probably controlled by solar activity. We suspect the cosmic rays intensity as the link between solar activity and atmospheric transparency. A Fourier analysis of the time series of insolation shows three dominant peaks: 10.5, 20.4, and 14.0 years. As a matter of fact, the cosmic rays data show the same pattern of significant peaks: 10.7, 22.4, and 14.9 years. This analogy supports our idea that the cosmic rays variation has influence on the transparency of the Earth atmosphere.     

Cosmological models of gamma-ray bursts

We review models of cosmological gamma-ray bursts (GRBs). The statistical and -ray transparency issues are summarized. Neutron-star and black-hole merger scenarios are described and estimates of merger rates are summarized. We review the simple fireball models for GRBs and the recent work on non-simple fireballs. Alternative cosmological models, including models where GRBs are analogs of active galactic nuclei and where they are produced by high-field, short period pulsars, are also mentioned. The value of neutrino astronomy to solve the GRB puzzle is briefly reviewed.     

利用大视场测光系统评估观测天气质量

介绍一种利用大视场测光系统对观测天气质量进行评估的统计方法．此方法可以 给出大气透明度、背景天光、视宁度和观测极限星等等多种与天文观测有关的天气质 量参数．通过检验证明,此方法可提供一个天气状况的实时监视方法,对北京天文台 施密特ＣＣＤ测光系统以及制定观测计划和评估观测质量有重要意义．     

Plans for a new radioheliograph in Belgium

The existence of spikes with very short time scales (1 to 10 ms) in the decimetric radio spectrum emphasizes the necessity of building radio instruments with very high time resolution. Therefore, we are planning to build a multi-frequency decimetric radioheliograph at the radioastronomical station of the Observatoire Royal de Belgique (Humain, Belgium). The existing array primarily devoted to observe at 408 MHz will constitute the starting structure of the project.Paper presented at the 4^th CESRA workshop in Ouranopolis (Greece) 1991     

A simple flux integration photometer for day time site testing at Oukaimeden

We describe here a simple irradiance photometer built at Astrophysical and Geophysical Laboratory (LAG) for the integrated photometry of sun light at Oukaimeden, a 2700m summit in the Morrocan High Atlas. The solar irradiance measurement is performed simultaneously at three wavelengths to sample different levels of the sky transparency. The total of 1182 days in the data base shows that the number of clear hours is about 3086 per year which represents 65.32% of clear sky. The extinction coefficient is computed to evaluate the quality of the sky transparency at the site. We find that there is a seasonal effect for the sky transparency, and the monthly average of the extinction coefficient has a minimum value of about 0.05 in winter (oct-nov-dec) and a maximum of about 0.15 in summer (jun-jul) at 700 nm.     

Radio observations of the solar neutron flare of 3 June, 1982

In this paper the solar neutron and white-light flare is studied on the basis of radioastronomical observations. It is shown that the 3 June, 1982 flare had an impulsive character. A strong shock wave (M _A 2.9) was observed unusually soon after the impulsive phase of the flare. The radio spectrum of this event shows that the primary acceleration process probably occurred in the region with an electron density of n _e = 4.4 × 10¹⁵ m^–3. The pulsations of the type IV radio burst, observed 15 min after the mass ejection process, manifest the reconnection process in the post-flare stage.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

Penetration of coronal magnetic fields into solar-wind streams

The formation of solar-wind stream structure is investigated. Characteristic features of the solar and coronal magnetic-field structure, morphological features of the white-light corona, and radio maps of the solar-wind transition (transonic) region are compared. The solar-wind stream structure is detected and studied by using radio maps of the transition region, the raggedness of its boundaries, and their deviation from spherical symmetry. The radio maps have been constructed from radioastronomical observations in 1995–1997. It is shown that the structural changes in the transition region largely follow the changes occurring in regions closer to the Sun, in the circumsolar magnetic-field structure, and in the solar-corona structure. The correlations between the magnetic-field strength in the solar corona and the location of the inner (nearest the Sun) boundary of the transition region are analyzed. The distinct anticorrelation between the coronal magnetic-field strength and the distance of the transition region from the Sun is a crucial argument for the penetration of solar magnetic fields into plasma streams far from the Sun.     

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.     

A simple problem of radiative transfer by multilevel atoms

We consider the problem of determining the radiation fields reflected and transmitted by a slab containing multilevel hydrogen atoms and illuminated on one side by a given radiation field. We treat the extreme non-LTE situation in which the populations of the different levels are determined by the radiative processes. We take into account the population and the transfer effects in a self-consistent way by solving the transfer equations in all the lines and continua together with the equations of statistical equilibrium for all levels. We limit ourselves to the idealistic case of rectangular profiles in the lines and continua and to a model of atoms with 4 levels and a continuum. Under conditions close to thermodynamic equilibrium we empirically derive a Schuster-like law for the continua with transmitted radiation fields varying as the inverse of the optical thickness. Turning to out-of-equilibrium conditions we emphasize the crucial role of the loss probability of the Ly photons. Owing to the rapid decrease of the excitation/ionization degree in the medium and contrary to the conservative case the optical thicknesses of the subordinate transitions now remain finite even when the population of the fundamental level along the line-of-sight becomes infinite. As a result of this relative transparency the strong emission lines formed by recombination mechanisms can escape from the medium. Although the present problem remains largely academic because of the number of simplifications introduced we suggest some possible applications and developments.Published in Astrofizika, Vol. 37, No. 2, pp. 313–338, April–June, 1994.     

Impact of aerosols and adverse atmospheric conditions on the data quality for spectral analysis of the H.E.S.S. telescopes

The Earth’s atmosphere is an integral part of the detector in ground-based imaging atmospheric Cherenkov telescope (IACT) experiments and has to be taken into account in the calibration. Atmospheric and hardware-related deviations from simulated conditions can result in the mis-reconstruction of primary particle energies and therefore of source spectra. During the eight years of observations with the High Energy Stereoscopic System (H.E.S.S.) in Namibia, the overall yield in Cherenkov photons has varied strongly with time due to gradual hardware aging, together with adjustments of the hardware components, and natural, as well as anthropogenic, variations of the atmospheric transparency. Here we present robust data selection criteria that minimize these effects over the full data set of the H.E.S.S. experiment and introduce the Cherenkov transparency coefficient as a new atmospheric monitoring quantity. The influence of atmospheric transparency, as quantified by this coefficient, on energy reconstruction and spectral parameters is examined and its correlation with the aerosol optical depth (AOD) of independent MISR satellite measurements and local measurements of atmospheric clarity is investigated.     

The global oscillation network group site survey

The Global Oscillation Network Group (GONG) Project is planning to place a set of instruments around the world to observe solar oscillations as continuously as possible for at last three years. The Project has now chosen the sites that will comprise the network. This paper describes the methods of data collection and analysis that were used to make this decision.Solar irradiance data were collected with a one-minute cadence at fifteen sites around the world and analyzed to produce statistics of cloud cover, atmospheric extinction, and transparency power spectra at the individual sites. Nearly 200 reasonable six-site networks were assembled from the individual stations, and a set of statistical measures of the performance of the networks was analyzed using a principal component analysis. An accompanying paper presents the results of the survey.     

A possibility to useCi infrared absorption lines for determination of the background radiation temperature at earlier cosmological epochs

A method for the determination of the background radiation temperature in earlier cosmological epochs at redshiftsz>2 is discussed in detail. The method is based on the fact that in the clouds situated at cosmological distances at redshiftz the background radiation temperature must be (1+z) times more than in the modern epoch (z=0). This shall affect the level populations of the atoms, ions and molecules and, consequently, the parameters of the absorption lines observed in quasar absorption spectra. It is proposed to use the transition³ P ₀–³ P ₁ ofCi (=610 ) for the measurement of the background radiation temperature. Atz>2 this absorption line shifts to the millimeter radio region and can be observed by radioastronomical methods.     

Decrease in the clear air transmission at Helwan Observatory site

Clear air atmospheric transparency of eight nights at Helwan Observatory site during autumn period of 1987 have been determined using the yellow filter GG5. The prevailing atmospheric transmission extends to 58%. A comparison with previous values obtained with the same filter and site during autumn period of 1963 shows a considerable decrease at the present years in the clear air transmission of the site of Helwan. This can be attributed to the aerosol pollution of the site caused mainly by the increase of the industrial centers at Helwan Zone. Clear air atmospheric transparency values at the beginning of winter season and at spring season are also given.     

The X-Ray Facility of the Physics Department of the Ferrara University

We will report on the equipment and performance of the X-ray facility of the University of Ferrara. Initially developed to test the PDS (Phoswich Detection System) instrument aboard the BeppoSAX satellite and to perform reflectivity measurements of mosaic crystal samples of HOPG (Highly Oriented Pyrolytic Graphite), with time the facility has been improved and its applications extended. Now these applications include test and calibration of hard X-ray (> 10 keV) detectors, reflectivity measurements of hard X-ray mirrors, reflectivity tests of crystals and X-ray transparency measurements. The facility is being further improved in order to determine the optical axis mosaic crystals in Laue configuration within a project devoted to develop a hard X-ray (> 60 keV) focusing optics (Pisa, A. et al.: in press, Feasibility study of a Laue lens for hard X-rays for space astronomy, SPIE Proc., 5536).     

Atmospheric transparency over Mount Shatdzhatmaz in the optical and near-infrared ranges

We present the results of a three-year-long monitoring of atmospheric extinction over Mount Shatdzhatmaz (2112 m) in Northern Caucasus in a photometric band with λ _eff = 480 nm and the results of measurements of precipitable water vapor (PWV), which characterizes the atmospheric transparency in the near infrared. The yearly mean fraction of photometric weather is estimated to be 50% of the clear night time. The yearly median extinction is 0ṃ21; themedian PWV on clear nights is 7.7 mm.