Engineering topics

Branduardi, G.

Publications and source records attributed to Branduardi, G..

8 records

NASA NTRS1980

Evidence for the degenerate dwarf nature of Cygnus X-2

Observations of Cygnus X-2 using the Copernicus satellite have revealed a novel relationship between the spectral hardness and the intensity of this source in the 3-8 keV band. This behavior is very similar to that predicted for a spherically accreting nonmagnetic degenerate dwarf. The data, interpreted in terms of this model, determine solar mass, luminosity, and the distance for Cyg X-2; these parameters are at variance with the values derived from optical observations of the system. The implications of this discrepancy for the understanding of the source are examined.

Branduardi, G.↗

NASA NTRS1979

A high-sensitivity X-ray survey using the Einstein Observatory and the discrete source contribution to the extragalactic X-ray background

Surveys of two fields have been carried out utilizing the X-ray imaging instruments of the Einstein Observatory to a limiting flux of 1.3 x 10 to the -14th ergs/sq cm s in the 1-3 keV band, a several hundred-fold increase in sensitivity with respect to previous surveys. Forty-three objects are observed with positional accuracies varying from 5 to 60 arcsec. Optical and radio studies of candidate counterparts are described. The number-intensity relationship for extragalactic sources is evaluated, and the consequences of the findings are discussed with particular emphasis on the discrete source contribution to the extragalactic X-ray background. It is concluded that a large fraction of the background is due to previously unresolved discrete sources.

Giacconi, R.↗

NASA NTRS1979

X-ray studies of quasars with the Einstein Observatory

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.

Tananbaum, H.↗

NASA NTRS1979

Detection of X-ray emission from distant clusters of galaxies

The paper reports the first extensive detection of X-ray emission from clusters of galaxies at cosmological distances. The properties of these objects are similar to those observed in objects at low redshifts. The 0.5-4.5 keV luminosities are in the range of less than 1 x 10 to the 43rd to 2 x 10 to the 45th ergs/s; the core radii are on the order of 0.5 Mpc; and Bautz-Morgan type I clusters are more luminous than types II or III. The observations are consistent with models assuming an evolving cluster potential and moderately efficient galaxy formation, but do not require them when observational selection is considered. X-ray observations of the 3C 295 cluster indicate that there is sufficient intergalactic medium to cause stripping of the cluster spirals, but the colors of these galaxies imply that they have not been stripped. A possible explanation of this discrepancy is discussed.

Henry, J. P.↗

NASA NTRS1979

Discovery of an X-ray star association in VI Cygni /Cyg OB2/

A group of six X-ray sources located within 0.4 deg of Cygnus X-3 has been discovered with the Einstein Observatory. These sources have been positively identified and five of them correspond to stars in the heavily obscured OB association VI Cygni. The optical counterparts include four of the most luminous O stars within the field of view and a B5 supergiant. These sources are found to have typical X-ray luminosities of 5 x 10 to the 33rd ergs/s, with temperatures of 10 to the 6.8th K and hydrogen column densities of 10 to the 22nd/sq cm, and therefore comprise a new class of low-luminosity galactic X-ray sources associated with early-type stars.

Harnden, F. R., Jr.↗

NASA NTRS1979

Search for short time-scale periodicity in the X-ray flux of 4U1700-37

The existence of a short-term periodicity in the X-ray light curve of the binary system 4U1700-37 was investigated by recent Copernicus observations. 4U1700-37 was observed for two complete binary cycles in July, 1978, with a sampling interval of 86.5 sec which allowed for the detection of periodicities down to 3 min. No evidence of the periodicity which had been previously reported in the ranges 20 to 30 min or 90 to 100 min was found, nor was there any in other ranges down to 3 min, even when X-ray noise caused by intense flares on 4U1700-37 was taken into account. Possible explanations for the recent absence of periodicity are proposed.

Branduardi, G.↗

NASA NTRS1979

The Einstein /HEAO 2/ X-ray Observatory

The Einstein (HEAO 2) X-ray Observatory, launched in 1978, includes a fully imaging focusing X-ray telescope with an angular resolution of a few arc sec, a field of view of up to one deg, and a sensitivity several hundred times greater than previously available in any X-ray astronomy experiment. A high-resolution imager, an imaging proportional counter, a focal plane crystal spectrometer, and a monitor proportional counter are among the principal instruments on board the Einstein X-ray Observatory. About 20% of the total effective observing time in the first year of the X-ray astronomy experiment has been reserved for guest observers.

Giacconi, R.↗

NASA NTRS1976

X-ray observation of 3U 1700-37

X-ray observations with Copernicus reveal three categories of flux variability in 3U 1700-37. High amplitude hourly variations are energy independent in the 3-11 keV range while a change in the low energy absorbing column causes variations in flux level on an orbital time scale. This absorption is most severe prior to eclipse ingress, suggesting that the distribution of absorbing material around the X-ray source is asymmetrical with respect to the line of centers of the binary system. The absorbing material may be identical with a high density region inferred from optical observations of HD 153919. In the third category, the maximum source intensity per binary cycle is variable by at least a factor of two between observations. Measurement of the eclipse duration on three occasions indicate that it is significantly less than when observed by Uhuru.

Mason, K. O.↗