Search

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%Co 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 distribution versus rise times for events observed by previous space missions, with a log N-log S 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. [ABSTRACT FROM AUTHOR]

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 30 r ( r = GM/c ), the inclination is ≤40°, while the mass of the central object is constrained to 8·10 L < M<3.9·10 M ( L is the accretion luminosity in units of 10 erg s). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We briefly discuss the use of existing astronomical data, mainly deep optical observations performed in order to obtain galaxy and star counts, for the identification of gamma-ray burst sources in particular and of newly discovered X- and gamma-ray sources in general. Use of such data cannot only reduce the amount of required telescope time but also, in comparison with present-day observations, allow the search for variability and proper motion of candidate optical counterparts over a time span of several years. [ABSTRACT FROM AUTHOR]

The Phoswich Detection System (PDS) is one of the four narrow-field instruments on board the SAX satellite. The experiment will be dedicated to temporal and spectral studies of celestial X-ray sources in the 15÷300 keV energy band. The PDS detector is composed of 4 actively shielded NaI(TI)/CsI(Na) phoswich scintillators and has a field of view of 1.4 (FWHM). The total geometric area is 800 cm. The instrument is designed with the goal to achieve an energy resolution better than 17% at 60 keV and a 3σ flux sensitivity for an observation time of 10 s of about 1×10 photons cm s in an energy band 40 keV wide centred at 100 keV. This sensitivity corresponds to about 2 milliCrab flux units in the same energy range. The expected 3σ sensitivity of the PDS to cyclotron emission lines is about 3×10 photons cm s at 60 keV in 10 s. The CsI(Na) lateral shields of the PDS will also be used as a gamma-ray burst monitor. In this report we give a detailed description of the experiment design and we discuss the expected in-flight performances of the PDS. [ABSTRACT FROM AUTHOR]

In this paper we report the development status of a research programme devoted to study the possibility of utilizing the Bragg diffraction technique to concentrate hard X-rays. We present the results of our theoretical studies. We discuss the expected performances of a hard-X-ray (>10 keV) concentrator which will be made of nested paraboloidal mirrors of graphite (002) mosaic crystals in a reflection configuration. We also report the differences in performance between a perfect mirror geometry and an imperfect geometry. We give a short description of the X-ray test facility which has been set up to perform reflectivity measurements of flat samples of mosaic crystals. We also discuss perspectives of this programme in the context of the space astrophysics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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». [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The FIGARO II (French Italian Gamma-Ray Observatory) experiment has been launched successfully three times: in July 1986 from Milo (Trapani), in November 1988 from Charleville (Australia) and in July 1990 again from Milo. In the first flight the observational program was limited to the Crab pulsar PSR0531+21 only because of a telemetry failure: the high sensitivity of FIGARO II allowed an accurate study of the pulse shape as well as a phase-resolved spectroscopy. It was also possible to evaluate the dispersion measure of the Crab pulsar at the flight date from the time delay between gamma-ray and radio pulses. The major results of the second flight were a stringent upper limit to the low-energy gamma-ray flux from PSR 0833-45 (Vela pulsar)-well below the detection claimed by the UCR group -and the observation of a strong emission in the 0.511 MeV annihilation line from the inner region of our Galaxy. The data acquired in the third flight are still under analysis, but preliminary results suggest changes in the shape of the pulse profile. [ABSTRACT FROM AUTHOR]

Copyright of Il Nuovo Cimento: C is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Il Nuovo Cimento: C is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)