Engineering topics
Publications and source records attributed to Avni, Y..
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Methods are derived for applying the V/Vmax variable when several complete samples are analyzed simultaneously. A new, related, variable, V(e)/V(a), which is superior to V/Vmax when sufficient observational data are available, is introduced. It is shown how statistically independent complete samples can be generated from partially overlapping samples such that each observed object appears only once in the analysis. These methods make it possible to combine the information from different complete samples, and to make an efficient use of all available data. Illustrative examples for quasars are given. It is shown how the generalized V/Vmax and V(e)/V(a) can be used to find the amount of cosmological evolution of quasars and its associated range of uncertainty.
Results of an investigation of the X-ray properties of quasars conducted using the Einstein Observatory (HEAO 2) are reported. The positions, fluxes and luminosities of 35 known quasars were observed by the Einstein high-resolution imaging detector and the imaging proportional counter. Assuming optical redshifts as valid distance indicators, 0.5-4.5 keV X-ray luminosities ranging from 10 to the 43rd to 10 to the 47 ergs/sec are obtained, with evidence of very little cold gas absorption. Flux variability on a time scale of less than 10,000 sec is observed for the quasar OX 169, which implies a mass between 8 x 10 to the 5th and 2 x 10 to the 8th solar masses for the black hole assumed to be responsible for the emission. Preliminary results of the quasar survey also indicate that quasars contribute significantly to the diffuse X-ray background.
A point summation technique has been used to analyze systematically the Uhuru data for X-ray emission from the 88 Seyfert galaxies listed by Weedman (1977), plus MCG 8-11-11 reported by the Ariel 5 group. In addition to measuring the average X-ray intensity for 15 sources reported in the 4U and 2A catalogs, three new candidate sources are found. X-ray variability has previously been reported for NGC 4151, 3C 390.3, and MCG 8-11-11; Mrk 279 is now also found to vary. Furthermore, significant flaring activity from NGC 4151 was observed with as much as a factor of 10 increase in intensity on a time possibly as short as 730 seconds. The local X-ray volume emissivity of Seyfert galaxies is measured, and it is found, with standard assumptions, that from 6% to 25% of the diffuse 2-10-keV X-ray background can be attributed to emission from Seyfert galaxies. The data show that the luminosity function for X-ray Seyferts is rather steep.
We discuss the existence of pulsing X-ray sources that consist of neutron stars in highly compact binary systems (orbital periods not more than Od3), undergoing accretion from low-mass late-type dwarf or degenerate-dwarf companions. An appropriate mass transfer rate can be driven by the decay of the orbit due to gravitational radiation, a self-excited wind, and/or the evolution of the companion. Such a system may result from the evolution of a cataclysmic variable, wherein a degenerate dwarf collapses to form a neutron star after accreting sufficient mass to exceed the Chandrasekhar limit. We apply this model to the 7s7 X-ray pulsar 3U 1626-67, and demonstrate that it can explain the apparent lack of Doppler shifts in the X-ray pulsations from this source. The model may also account for other observed properties of the source, including (1) the apparent faintness and large ultraviolet excess of the optical counterpart, (2) the lack of X-ray eclipses, and (3) an approximately 1000 s quasi-periodic oscillation in the source intensity that was recently observed with the SAS 3 satellite.
A procedure for estimating the ranges of parameters that describe the spectra of X-rays from clusters of galaxies is presented. The applicability of the method is proved by statistical simulations of cluster spectra; such a proof is necessary because of the nonlinearity of the spectral functions. Implications for the spectra of the Perseus, Coma, and Virgo clusters are discussed. The procedure can be applied in more general problems of parameter estimation.
By comparing observed and simulated rich clusters of galaxies, it is shown that the observed clusters actually possess physical cores. The accuracy with which core radii can be determined is found. It is also shown that the observations of density profiles of galaxies in the clusters give no significant evidence for a dynamical reason as the cause of the anomalously close resemblance found previously between such density profiles and the isothermal gas distribution.
Estimates of system parameters and component masses are reviewed. The mass for the X-ray source in 3U0900-40 is found. The importance of determining the X-ray eclipse duration in binary systems is explained. A list of important observations for testing and utilizing the standard picture is presented.