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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Ultraviolet observations of weak-helium stars

Ultraviolet filter photometry aboard OAO-2 has been carried out for the weak helium stars 3 Cen A, 3 Sco, HD 144334, HD 21699, HD 144844, alpha Scl, HR 8535, HR 8770 and HD 144661. The flux distribution is compared with that of B3 to B8 main sequence and giant stars in terms of color-color diagrams. When allowance is made for line blocking shortward of 2800 A that most of the weak helium stars have normal fluxes in good agreement with their ground-based colors. The star alpha Scl is a light and spectrum variable. The strengthening of Ti II and Sr II lines around 2400 A appears to redistribute flux longward of the Balmer discontinuity. The flux distribution indicates an effective temperature about 1500 K higher than previously determined.

Bernacca, P. L.↗
NASA NTRS1972

The X-ray spectrum of NGC 5128.

Evaluation of the X-ray spectrum of NGC 5128 (Cen A) obtained by sounding rocket and balloon observations in the range from 1 to 180 keV. NGC 5128 may be fitted by a power law with a photon index -2 less than or equal to n less than or equal to -1.45. Inverse Compton models are presented, and constraints on their parameters are discussed. In all such models, the electrons and magnetic field have not yet equilibrated. Alternatively, the X rays may originate in a compact central core source.

Lampton, M.↗
NASA NTRS1972

Evidence for the binary nature of Centaurus X-3 from Uhuru X-ray observations.

Analysis of data spanning a year of observations of the pulsating X-ray source Cen X-3 from Uhuru has revealed the existence of periodic variations in intensity of the source and correlated sinusoidal variations in the period of the 4.8-sec pulsations. We interpret this effect as due to an occulting binary system. The changes in intensity are then due to occultation of the X-ray source by a large massive companion, and the sinusoidal variations in the period of the 4.8-sec pulsations are due to Doppler effect. Physical parameters for the system are derived, and evidence for the existence and nature of an extended atmosphere surrounding the massive occulting object is discussed.

Schreier, E.↗
NASA NTRS1972

On the optical identification of Centaurus X-3.

It is suggested that the previously photographed emission-line star WRA 795 is the optical counterpart of Cen X-3, the X-ray pulsar which is a member of an occulting binary system. The star falls in the X-ray error box, which measures only 0.01 square degrees, and shows strong emission variability on a timescale of days, similar to HDE 226868, the suggested Cyg X-1 counterpart.

Margon, B.↗
NASA NTRS1972

Discovery of a periodic pulsating binary X-ray source in Hercules from Uhuru.

We have discovered a new pulsating X-ray source with a 1.24-sec period in the constellation Hercules. Analysis of 5 months of data has shown the existence of periodic variations in the intensity of the source and correlated sinusoidal variations in the period of the 1.24-sec pulsations. As in the case of the pulsating X-ray source Cen X-3, we interpret this effect as due to an occulting binary system, with the intensity changes due to occultation of the X-ray source by its companion and with the sinusoidal variations in the period of the 1.24-sec pulsations due to the Doppler effect. In addition, we have observed a longer-time scale cycle in which the source is bright and pulsing for approximately 9 days during which we can observe the 1.7-day occulting, followed by approximately 27 days during which the source is not detected above background on individual 20-sec scans.

Tananbaum, H.↗
NASA NTRS1973

Neutron-star accretion in a stellar wind - Model for a pulsed X-ray source.

Description of a relatively simple stellar-wind-driven accretion model that appears to satisfactorily account for many of the observed features of the pulsing X-ray sources Cen X-3 and Her X-1. A rotating magnetized neutron star orbits a more massive slightly evolved star and accretes mass from the stellar wind emanating from that star. Some of the implications of the model are discussed.

Davidson, K.↗
NASA NTRS1973

Hard cosmic X-ray sources.

A review of the observational status of X-ray sources detected in the 20 to 500 keV range is presented. Of the approximately 115 sources listed in the March 1972 edition of the UHURU 2-6 keV sky survey catalog, about 15 sources have been studied in hard X rays. Most of the data have been obtained from balloons, although the OSO-3, and more recently the OSO-7, have contributed. With the exception of CEN A, the SMC, and possibly M-87, all the sources detected at higher energies are galactic and heavily concentrated in the galactic plane. The Crab Nebula has been measured to about 500 keV in continuous emission and a component at the 33-msec pulsar period comprising about 20% of the total emission has been detected to 10 MeV. Objects such as SCO-1 and CYG-2 are characterized by an exponential spectrum, which varies over a 10-min time scale about a factor of two, and a flatter spectrum extending to above 40 keV which exhibits independent variability.

Peterson, L. E.↗
NASA NTRS1973

Accretion at a magnetic pole of a neutron star

As accreted material falls to the surface of a magnetized neutron star like Cen X-3 or Her X-1, it is arrested by radiation pressure in such a manner that a hot, dense mound of nearly stationary gas protrudes above each magnetic pole. Energy released above the mound diffuses out as moderately hard X rays; but that released within the mound emerges as soft X rays from the whole surface of the neutron star.

Davidson, K.↗
NASA NTRS1974

Oscillator strengths for ionized iron and manganese

The observed strengths of interstellar absorption lines of Fe II and Mn II in the spectra of alpha Vir, beta Cen, pi Sco, and zeta Oph along with laboratory f values of some of these lines between 2343 and 2606 A have been used to determine curves of growth for these ions and the f-values of ten lines of Fe II and three lines of Mn II between 1055 and 1261 A. The Fe and Mn abundances are derived.

De Boer, K. S.↗
NASA NTRS1974

Extended observations of higher than 7-keV X-rays from Centaurus X-3 by the OSO-7 satellite

The UCSD X-ray telescope on board OSO-7 provided 43 days of continuous coverage of the variable X-ray source Cen X-3 at energies above 7 keV during December 1971 and January 1972. We detected the 4.8-sec pulsation period, the 2.087-day eclipse cycle, and an apparently nonperiodic, low-intensity state lasting more than 12 days. Spectra obtained over the 7-30 keV range during noneclipsed high-intensity states are steeper than those previously reported. Large changes, which may be characterized by a number spectral index alpha varying between 3.0 plus or minus 0.2 and 2.0 plus or minus 0.3, or by exponential spectra with kT varying from 6 plus or minus 2 to 13 plus or minus 3 keV, occur at different high-intensity states.

Baity, W. A.↗
NASA NTRS1974

OAO-2 observations of the helium spectrum variable a Centauri

Ultraviolet photometry and spectrometer data are presented for a Cen, which exhibits large variations in the strength of its He I lines. The photometry indicates that the star becomes brighter at the helium-weak phase by 0.05 mag throughout the ultraviolet and visual region. A model atmosphere analysis indicates that the photometric variations are not due to flux redistribution from back-warming effects, but can be interpreted as due to the rotation of a distorted stellar disk in the shape of a prolate spheroid.

Molnar, M. R.↗
NASA NTRS1975

Time variations of UV emission features of Be stars

The UV spectra of three Be stars (gamma Cas, sigma Tau, eta Cen) were studied. Of the six Be stars observed in the first four lines of the Balmer series, three stars showed at least one of the Balmer lines to be variable in the equivalent width amounting to a few percent with time scales of 3 to 30 minutes. Photoelectric spectrum scans of five southern Wolf-Rayet stars showed night-to-night variations. A simple model is proposed to account for the behavior of these emission lines. Scans of gamma square Vel showed rapid variations of emission strengths of He II 4686 and C III - IV 4650. These variations have time scales of 1 minute and longer. Night-to-night variations were also found. Scans of four Be stars in H alpha showed a definite variation of 3 to 4 percent, with time scales of 1 minute and longer in sigma Tau. In 48 Per and kappa Dra the variations are not as well established. No variation of any significance was found for nu Gem.

Bahng, J. D. R.↗
NASA NTRS1975

Research study of Beta Cephei variable stars using data from OAO-2

Photometric data from the Wisconsin Experiment package on OAO-2 were obtained for six Beta Cephei variable stars. The data were reduced in accordance with the OAO 2/Wisconsin Experiment Package Photometer Users Guide. For delta Cet and gamma Peg, there were enough data points to form reliable composite light curves at seven or eight ultraviolet wavelengths. The light curves are well represented by sine waves in phase with the blue light curve for each star. The amplitude of the light variation increases with shorter wavelengths. For theta Oph, epsilon Cen, eta Sco, and Lambda Sco, mean magnitudes and light ranges were obtained at several ultraviolet wavelengths. No significant differences were found between the mean, de-reddened ultraviolet colors of the observed stars, and the mean values for standard stars computed by Bottemiller. An attempt to derive a temperature scale from a comparison of the observed ultraviolet colors with Kurucz models was unsuccessful.

Lesh, J. R.↗
NASA NTRS1975

Hot vibrating white dwarf models of pulsating X-ray sources

The hypothesis that pulsating X-ray sources could be vibrating white dwarfs is investigated. The masses are calculated for several white-dwarf models having various modes of vibration which have the observed eigenfrequencies of the Centaurus X-3 and the Hercules X-1 sources. If the hypothesis is correct, Cen X-3 is vibrating radially in the fundamental mode or not higher than the first two overtones, and its mass is in the range 0.7-1.2 solar masses. The Her X-1 source would be vibrating in a quadrupole P mode, and would have a mass in the range 1.1-1.25 solar masses.

Cameron, A. G. W.↗
NASA NTRS1975

Physical parameters of the Centaurus X-3 system

Photographic spectra of Cen X-3 show that the primary star has a spectral type near O6.5 with weak, variable emission at 4640 and 4686 A. No orbital motion of the emission or absorption lines is detected; for the latter the upper limit is about plus or minus 50 km/sec. Analysis of the available data indicates that the primary is a factor of 2-3 less massive than expected from normal evolutionary models while the X-ray source has a mass near 1.5 times the solar mass.

Osmer, P. S.↗
NASA NTRS1975

Australian rocket results

Cen X-3 was observed twice. A pulse profile and spectrum was obtained in the 2- to 10-keV band, but no soft flux was detected. Vela X-1 was observed to have a rapid flickering characteristic on time scales down to 0.25 sec and a hard spectrum. The source 3U 0614+09 was observed and found to have a softer spectrum (kT of about 4 keV) and an intensity comparable to that of earlier observations. The source eta Car was not detected at a level of 0.03 ph/sq cm/sec in the band from 0.2 to 1.5 keV, and the Large Magellanic Cloud was observed in the band from 0.5 to 1.5 keV at a level of about 0.1 ph/sq cm/sec. Two soft extended regions were observed in Gem-Mon and Eridanus.

Garmire, G.↗
NASA NTRS1975

On the origin of 4640-4650 A emission in X-ray stars

The origin is investigated for a prominent and rapidly variable emission feature near the 4640-A line which has been observed in the optical counterparts of Sco X-1, Cyg X-2, Her X-1, Cen X-3, 3U 0900-40, and 3U 1700-37. It is suggested that the emission is generated in the vicinity of a compact X-ray source by the interaction of X-rays with nearby gaseous matter and that three likely sources of the emission are the ions C III, N III, and O II. Results are presented for a detailed mathematical model of an X-ray source surrounded by a uniform distribution of matter and are used to evaluate possible emission mechanisms. It is concluded that selective emission of N III (4634 to 4641 A) in the Bowen fluorescence process may be the responsible mechanism.

Mcclintock, J. E.↗
NASA NTRS1975

Period changes in Centaurus X-3

A number of mechanisms which can change the orbital period are applied to various models of Cen X-3. Only four models which give rise to feasible mechanisms are found. Possible observations which could distinguish between these models are suggested.

Sparks, W. M.↗