Coordinated observation of the massive system 4U1700+24=HD154791

Summary We report results from multifrequency observations of the X-ray source 4U1700+24/HD154791. This source was observed simultaneously with the EXOSAT and IUE satellites on March 25, 1985, and it is continuously monitored in the optical band at the Bologna Astronomical Observatory since July 1984. Our observations show source features which are different from those measured during previous observations. The X-ray spectrum is significantly softer than in previous measurements, and much more absorbed; high excitation lines in UV spectra (C IV and N V) have disappeared; the optical spectrum, despite a fairly good photometric stability of the M giant HD154791 in V and B bands also shows differences with respect to previous observations. We discuss our results in the framework of both a binary model and a coronal model. We point out that our observations strengthen the hypothesis that the X-ray emission is due to mass accretion onto a compact object orbiting around the M giant star HD154791. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Interstellar-dust properties as deduced from FIR and millimetric observations

Summary We have considered some millimetric observations of our Galaxy and of the Large Magellanic Cloud (LMC) taken at the Italian Base in Antarctica. Using other submm data and fitting all the observations with a modified Planck curve, we have obtained useful information about the temperaureT and the spectral index α of the dust grains both for the Galaxy and the LMC. For the galactic spectrum two different couples ofT and α match the detected fluxes:a)T=7K, α=2;b)T=15K, α=1.1. We have tried to identify the nature of the dust grains responsible for the submm and mm emission by comparing these results with the spectral trend of some cosmic analogues studied in laboratory. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

A perspective view to hard-X-ray astronomy

Summary The present observational and instrumental status of hard-X-ray astronomy ((10÷200) keV) is discussed. The relevance of observations in this energy range is stressed and a few examples of unsolved observational problems in galactic and in extragalactic astronomy are discussed. In these examples we focus on the possibility to solve the problems with observations using detectors of the current generation. In this framework, the performances of the most sensitive hard-X-ray detectors are discussed with particular emphasis on the control of systematic errors. Quite simple but unavoidable considerations on limits of the present generation of hard-X-ray detectors (supported by results of simple simulations) lead to the conclusion that a decisive breakthrough can be achieved only using optics with a sufficiently good concentration power. In particular we discuss the feasibility of hard-X-ray telescopes (with a concentration power ≫1), using either grazing incidence or Bragg diffraction. The use of concentrators in this energy band can also make feasible polarimetric measures of a substantial sample of X-ray sources, up to now severely limited by the very low detection efficiency of the devices used in polarimetry. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

X-ray signatures of cold accretion disks in AGNs

Summary We have calculated, by means of Monte Carlo simulations, the properties of the radiation emerging from an X-ray illuminated cold accretion disk. These calculations can apply to AGNs, for which there is evidence of cold, optically thick matter near the central black hole. The resulting spectra show an intense iron fluorescent line and a high-energy bump above about 10 keV, where the scattering becomes important with respect to the photoabsorption; these features have been observed by GINGA in some bright Seyfert 1 galaxies. Our detailed results are indicative of the type of future observations which would lead to estimates of the geometrical parameters of the disk, its iron abundance and the mass of the black hole. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Estimate of the black-hole mass in NGC 6814 from a relativistic accretion disk scenario

Summary By using a fully relativistic model for the spectral line produced by a Keplerian disk orbiting a Schwarzschild black hole, we study the temporal behaviour of the line intensity in response to a continuum variation at an extended central source. We compare our results with the observed properties of the Seyfert galaxy NGC 6814, whose X-ray flux has been observed to decrease by a factor of two in ≈ 50 s, while the iron line intensity variations lag continuum variations by 250 s, at the most. Taking the stationary values of the iron line centroid energy and width, and assuming that the line comes from high-ionisation stages of iron (as several indications suggest) we derive that the inner radius of the line-emitting region is between 6 and 30r _g (r _g =GM/c ²), the inclination is ≤40°, while the mass of the central object is constrained to 8·10⁴ L ₄₃<M<3.9·10⁶ M _⊙(L ₄₃ is the accretion luminosity in units of 10⁴³ erg s⁻¹). Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992. Affiliated to ICRA.     

Preliminary investigation to test extragalactic hypothesis for gamma-ray bursts

Summary Recent observations of CGRO satellite challenged the almost universally accepted galactic origin of gamma-ray bursts and have revived the extragalactic, cosmological models. This hypothesis should imply some variations of the morphological and energetic characteristics of the weaker events to which we think the farthest ones might belong. The existence of such variations due to the cosmological red-shift should be tested. In this paper we examine the possible correlations between temporal morphology of the gamma-ray bursts, peak intensity and photon energy spectra when the red-shift effects do not yet occur. We examine the peak intensities and the energy spectral-hardness distributionversus rise times for events observed by previous space missions, with a logN-logS shape free of cosmological effects. The results of this analysis suggest that a wide range of values of the temporal and spectral characteristic parameters is an intrinsic property of the sources. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

Status of the development of the imaging proportional counters for the Stellar X-Ray Polarimeter

Summary The Stellar X-Ray Polarimeter SXRP will be flown at the focus of the SODART X-ray telescope aboard the Russian satellite SPECTRUM-X-Gamma by the end of 1995. Four imaging proportional counters will detect photons diffracted by a graphite crystal (2.6 and 5.2 keV) and scattered from a lithium rod enclosed in a beryllium case (from 5 to 20 KeV). The counters are position-sensitive by the Wedge and Strip (WS) readout method. The gas mixture is 80% xenon, 10% argon and 10% methane. In this contribution we resume the results of the measurements performed on the engineering models of the counters. Position resolution of about 1.5 mm is attained with an energy resolution of about 25%. The pulse shape discrimination offers more than 98%⁶⁰Co events rejection, with at least 90% X-ray acceptance. Some improvements are foreseen for the flight units in terms of sensitivity and uniformity of response.     

High-energy spectral changes in three Seyfert galaxies observed with GINGA

Summary We have analysed observations of the three Seyfert Galaxies IC4329A, MCG-6-30-15 and NGC 4051 extracted from the GINGA archive. The variations in the (4⋎18) keV spectrum of the three sources can be interpreted in the framework of a model with two components: a ?direct? power law with constant spectral index (α=1.9⋎2.1), and the ?reflection bump? predicted by Guilbert and Rees, and Lightman and White which is produced by reprocessing of the direct power law in optically thick matter in the surroundings of the central source. The differences found in the behaviour of the three sources would arise, in this scenario, from the different size of the ?reprocessing region?. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

The SAX mission for wide-band X-ray astronomy

Summary In the framework of past and future X-ray missions the SAX satellite, to be launched at the end of 1995, stands out for its very wide spectral coverage from 0.1 to 200 keV. The sensitivity of the scientific payload will also allow the exploitation of the full band of SAX for weak sources (1/20 of 3C273), opening new perspectives in the study of spectral shape and variability of several classes of objects. In this paper we briefly describe the main aspects of the mission, the instruments, the scientific objectives and the operations. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

X-ray spectral signatures of very thick cold matter in the Spectra of seyfert galaxies

Summary Over the last few years, GINGA observations of several bright Seyfert galaxies have revealed the existence of a new flat component in the X-ray spectrum of these objects above 8 keV. This component (the ?high-energy bump?) is probably produced by the reprocessing of the intrinsic emission (through Compton scattering and photoelectric absorption) by a very thick and cold medium in the environment of the central source, such as an accretion disk. An alternative explanation is provided by partial covering and reprocessing by thick clouds, whose existence is supported by the latest results on the broad line region obtained by line reverberation measurements in the optical-ultraviolet band. The presence of iron fluorescent lines at 6.4 keV with an equivalent width ≅ (100÷200) eV in all those galaxies supports both scenarios. The implications of the presence of the high-energy bump on the spectral shape of the intrinsic continuum, the level of the soft X-ray excess and the X-ray background are briefly pointed out. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Temporal structure of the gamma-ray bursts GRB881024 and GRB880810

Summary In this paper we analyse two particularly significant cosmic gamma-ray bursts recorded by the French-Soviet experiment APEX on board the space mission PHOBOS. The October 24th 1988 event, lasting more than 16 seconds, is among the strongest events observed up to now. The very high temporal resolution available for its light curve, 1/8 second, allows us to study the power spectrum density (PSD) up to a frequency range more extended than in the past. The August 10th 1988 event, lasting about 1 second, recorded with a temporal resolution 1/128 second, higher than in the precedent one, permits us to reach still higher frequencies. We discuss the results of our analysis, comparing them with the results obtained for other events recorded by precedent space missions, and giving interpretative suggestions. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Variability of GX3+1 observed with EXOSAT

Summary We report results from an observation of the X-ray source GX3+1 carried out with the EXOSAT satellite on 1983, October 23. The source showed a strong X-ray variability in excess of the counting statistics noise. This variability increased toward low frequencies (?red noise?) and was different in different energy bands, with the lower energy X-rays showing a more pronounced increase of the variability at low frequencies. The variation function of the source is also shown to depend on energy, with the highest values associated with the highest energies. A comparison of the time variability properties of GX3+1 with those of other bright low mass X-ray binaries is presented. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

The origin of the continuum of Active Galactic Nuclei

Summary The high-energy continuum of active galactic nuclei can be interpreted by nonthermal models in which electron-positron pairs play a crucial role. The produced primary γ-rays interact with softer photons in the inner, high photon density regions of the source. The created pairs are relativistic, and contribute to the emission, especially in the X-ray band. When the photon density is large enough, therefore for large values of the luminosity-to size ratio, the spectrum produced by the reprocessing due to pairs is predicted to have a characteristic spectral index of one. This power law continuum partly impinges on cold matter, possibly in an accretion disk. The resulting Compton reflected radiation contributes to the X-ray emission, and the predicted overall spectrum is in agreement with the observed shape of the high-energy continuum. The proposed model is easily testable: observations in the hard—X-ray band above (30÷50) keV are crucial to discriminate these nonthermal models against thermal ones. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

An analysis of the red noise in the power spectrum of X-ray binaries as a shot noise process

Summary We have simulated, using a Monte Carlo method, time series representing the aperiodic variability typically observed in some X-ray sources. We have supposed that such red-noise features are generated by a shot noise process. The resulting time series have been analyzed by fast Fourier transform techniques. Using different sets of parameters we have been able to generate power spectra of different shapes. Some preliminary results of these simulations are discussed. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

The Use of Radioisotopes and Radiation in USSR,Great Britain,France, and Czechoslovakia

A short survey on the radioisotope applications in the four European countries is given. This survey is based on the detailed report prepared by the group of 15 experts during their 70 day study tour organized by the International Atomic Energy Agency in Vienna, at the end of 1966. The purpose of the tour was the acquaintance with the application of radioisotopes and radiation in industry and research.Organization of industrial radioisotope work and basic forms of radioisotope applications are described.     

A solid-state noise source for millimetre wave spectrometry

A novel wide-band noise source for millimetre-wave spectrometry is described. It uses power combined Schottky diodes, reverse biased to avalanche breakdown, mounted in a wide-band tapered slot antenna. Power has been produced from 15 to 200 GHz with an equivalent temperature of 28200 K at 40 GHz.     

Eine graphische Methode zur experimentellen Bestimmung des „over-all correction factor‟ kernphysikalischer Zählpparaturen

Es wird eine graphische Methode zur experimentellen Bestimmung des ?over-all correction factor? kernphysikalischer Zählapparaturen, insbesondere mit Geiger-Müller-Zählrohren, beschrieben, Hauptvoraussetzungen für die Anwendung dieser Methode sind a) mehrere ?ähnliche? Kernstrahlungsquellen, die sich ausschlieβlich in ihrer Aktivität jeweils um etwa den Faktor 5 unterscheiden, b) eine konstante und durch das Auswechseln der Strahlungsquellen unbeeinfluβte Versuchsanordnung, Die beschriebene Methode erweist sich besonders aus praktischen Gründen den bisher bekannten Methoden zur unmittebaren experimentellen Bestimmung des ?over-all correction factor? g als überlegen (g = z_w/z mit z_w = wahre Zählrate, z = registrierte Zählrate).     

A widely tunable hybrid brillouin-erbium fiber laser (BEFL) system

This paper presents a tunable hybrid brillouin-erbium fiber laser system with a maximum tuning range of more than 36.7 nm. The system was configured in a ring set-up with another fiber laser used as the brillouin pump at about 8 mW maximum launched power. Stable Stokes signals at an average of about 0.08 nm wavelength shift were observed across the tuning range. The dependency of the Stokes signal tuning range on the laser's spectral gain is also elaborated.