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At least 217 records · Page 12
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NASA NTRS1976

Results from the Ariel 5 All-Sky Monitor

Results are summarized for the first year of operation of the Ariel 5 All-Sky Monitor. The monitor is described in detail, and representative observations of the transient sources A1524-62, Cen X-mas, A0535+26, and A0620-00 are discussed. It is shown that the transients are largely confined to the galactic plane and that about 100 such sources per year can be expected in the Galaxy. Long-term observations of Sco X-1, Cyg X-3, and Cyg X-1 are reported which illustrate that the large intensity variations in Sco X-1 appear to correlate on time scales greater than one Ariel 5 orbit, that Cyg X-3 has a widely varying intensity, and that the predominant feature of the intensity variation in Cyg X-1 is a minimum near superior conjunction.

Holt, S. S.↗
NASA NTRS1977

Observations of celestial X-ray sources above 20 keV with the high-energy scintillation spectrometer on board OSO 8

High-energy X-ray spectra of the Crab Nebula, Cyg- XR-1, and Cen A were determined from observations with the scintillation spectrometer on board the OSO-8 satellite, launched in June, 1975. Each of these sources was observed over two periods of 8 days or more, enabling a search for day-to-day and year to year variations in the spectral and temporal characteristics of the X-ray emission. No variation in the light curve of the Crab pulsar was found from observations which span a 15-day period in March 1976, with demonstrable phase stability. Transitions associated with the binary phase of Cyg XR-1 and a large change in the emission from Con A are reported.

Crannell, C. J.↗
NASA NTRS1976

Stellar occultation measurements of atmospheric ozone and chlorine from OAO3

The Princeton University telescope on the Orbiting Astronomical Observatory 3 was used for occultation measurements of upper-atmospheric species. The target star, Beta Cen, and observing date were selected so that the line of sight grazed earth's limb at local midnight. Observations at 2580, 2825, 2997, 3100, and 3428 A produced ozone absorption data of high statistical accuracy between 48-km and 114-km tangent altitude. Near-exponential density profiles are obtained up to 85 km. Near 100 km, a significant density excess above the extrapolated low-altitude profile is observed. Near-equatorial observations at altitudes up to approximately 75 km agree well with theoretical calculations. Ozone densities measured at 6.5 to 12.5 deg S latitude above approximately 75 km exceed the theoretical calculations and are not understood at the present time. A search for the 1188-A chlorine line in the 1187-A O2 absorption feature was carried out at tangent altitudes of 96-116 km. An upper limit of 3 ppb to the atomic chlorine mixing ratio at 106 km is found. This upper limit exceeds the expected value for atomic chlorine by a factor of approximately 2.5.

Riegler, G. R.↗
NASA NTRS1976

Observational constraints on the masses of neutron stars

The present state of empirical knowledge about neutron-star masses is reviewed. It is shown how the mass function of a pulsar-containing binary system can be inferred from measurements of the pulsation period and the projected semimajor axis of the pulsar orbit plus independent information concerning the inclination of the orbital plane, the mass of the companion star, or both. Relevant observational properties, the type of information used to constrain the pulsar mass, and the range of allowable pulsar masses are summarized for the binary systems 3U 0900-40, Cen X-3, SMC X-1, Her X-1, and PSR 1913+16. It is found that as long as the general theory of relativity is correct, neutron-star masses should range from about 1.4 to 1.9 solar masses if the companion is a normal white dwarf or should be less than about 1.9 solar masses if the companion is some other object. It is concluded that these mass estimates are entirely consistent with the predictions of nuclear physics theory.

Joss, P. C.↗
NASA NTRS1976

Stellar model chromospheres. V - Alpha Centauri A /G2 V/ and Alpha Centauri B /K1 V/

Models for the upper photospheres and lower chromospheres of Alpha Centauri A and B are derived from high-dispersion spectrograms of the Ca II K-line emission cores and damping wings. Effective temperatures, surface gravities, and ages consistent with the measured broadband colors, metallicities, and absolute magnitudes are estimated for the two stars. The spectrograms are calibrated by fitting the far-wing K profiles with synthetic fluxes based on radiative-equilibrium models, and the model atmospheres are obtained through a partial-redistribution analysis of the line cores and damping wings. These models are compared with those previously proposed for Procyon, Arcturus, and the sun. Many features in all the models are found to be quite similar, and some evidence suggests that Alpha Cen A may be significantly older and more evolved than the sun, even though both stars exhibit similar chromospheric properties.

Ayres, T. R.↗
NASA NTRS1977

X-ray sources in stellar winds

A model of a binary X-ray source immersed in a stellar wind from its companion star is considered. The geometry of the constant ionization and temperature contours in the model is discussed, and X-ray radiative transfer through the wind is described. The results are applied to make predictions about the presence and variability of iron K-shell fluorescence lines, to explain the light curve of Cen X-3 during a 'low' to 'high' state transition in July 1972, and to predict variability with orbital phase of resonance ultraviolet lines such as N V at 1238 and 1242 A from the binary system.

Hatchett, S.↗
NASA NTRS1977

Binary X-ray pulsars

Recent progress in the study of the pulse profiles of the nine known binary X-ray pulsars is discussed. The sources considered include SMC X-1, Her X-1, Cen X-3, A0535+26, GX1+4, 3U 0900-40, A1118-61, GX301-2, and 3U 0352+30. Sample X-ray profiles for these sources are provided, and general features exhibited by the profiles are identified. It is noted that the apparent existence of a gap in the period distribution of X-ray pulsars, in the range from about 5 to 100 sec, seems to be genuine within obvious statistical limitations and is probably not an observational effect. Two explanations for the production of X-ray pulsations are examined: one requiring a temporal modulation of mass accretion onto the surface of a neutron star and another requiring the funnelling of matter along magnetic field lines to the neutron star's surface.

Rappaport, S.↗
NASA NTRS1977

On the temperature and the interstellar nature of coronal gas observed by Copernicus

More detailed scans of ions expected in interstellar absorption at temperatures of 100,000 to 1 million K have been made with Copernicus in five stars: Alpha Vir, Beta Cen, Lambda Sco, Mu Col, and HD 28497. Preliminary data show that the absorption due to O VI is stationary compared with the velocities of stellar absorption in the spectrum of the spectroscopic binary Lambda Sco, thus demonstrating its nonstellar nature. Ionization temperatures interpreted using steady-state or time-dependent assumptions yield values of 280,000 to 700,000 K. There is some evidence that a range of temperatures consistent with this spread may in fact exist in Alpha Vir. For this limited sample of data, there is no clear evidence for a generic association of low-column-density H2 and the O VI absorption. It is concluded that the O VI absorption refers to regions with temperatures exceeding 250,000 K. The only UV diagnostic for the gas appears to be the O VI features, though C IV may be useful in some cases.

York, D. G.↗
NASA NTRS1977

UV stellar occultation measurements of nighttime equatorial ozone

The ultraviolet spectrometer-telescope on Copernicus was used for stellar occultation measurements of atmospheric ozone. Two sets of observations of the target star Beta-Cen were carried out on 26 July 1975 and 13-14 June 1976 at wavelengths from 2550 A to 3100 A. After unfolding of the data, ozone density profiles near the equator within 3 hours of local midnight were obtained at altitudes from 47 to 114 km. A secondary maximum at 97 km has been observed in both sets of data. The ozone density between 47 and 75 km is a factor of 2 to 3 times as large as current models predict. At the lower boundary, about half the ozone destruction should be caused by NOx and ClOx. Above 55 km, virtually all loss is due to HOx. These results suggest an overestimate of HOx and ClOx loss processes or a serious underestimate of the Ox production rate.

Riegler, G. R.↗
NASA NTRS1977

Gamma-ray astrophysics

The experimental development of gamma-ray astronomy and detectors for gamma-ray observations is reviewed together with recent results obtained by the SAS-2, COS-B, and other satellites. Diffuse gamma-ray emission from the galactic plane is discussed, various localized sources are examined, and the primary production processes for celestial gamma-rays are treated in relation to the difficulty involved in detecting this radiation. Several different types of gamma-ray detectors are described in detail, and the production of high-energy gamma-rays by cosmic-ray interactions with interstellar matter is investigated. Observations of gamma-rays from the Crab Nebula and pulsar, the Vela pulsar, Cen A, and other point or localized sources are summarized with observations of nuclear gamma-ray lines during solar flares. Possible interpretations are considered for the diffuse gamma-radiation detected in regions away from the galactic plane.

Fichtel, C. E.↗
NASA NTRS1977

Linear radial and nonradial modes of oscillation of hot white dwarfs

Low-order linear radial and nonradial quadrupole modes of oscillation of hot white dwarfs with hydrogen envelopes are evaluated in the quasi-adiabatic approximation. Luminosities range from 10 to the 36th to 10 to the 38th power. Masses range between 0.6 and 1.4 solar masses. Models are found with unstable radial modes as high as the third harmonic. The first nonradial quadrupole gravity mode is always unstable. The results may be relevant to pulsating X-ray sources in general and are specifically applied to Cen X-3, Her X-1, and SMC X-1.

Degregoria, A. J.↗
NASA NTRS1978

Gamma ray spectroscopy in astrophysics: An observational overview (50 keV-20 MeV)

The observational status of gamma ray astronomy based on results reported prior to April 1978 is discussed. Specific line observations are reviewed from the Sun, the galactic plane, the Galactic Center region, the active Galaxy NGC 5128 (Cen A), the Crab Nebula region, and a transient source in the general direction of the anti-Galactic Center region. A statistical method is described which determines the relative probability that a given, reported observation is due to an external source as compared to a random fluctuation in the experimental background counting rate. Only a few of the many reported observations can be considered likely extraterrestrial observations of an extraterrestrial source made with a confidence level of 99%.

Chupp, E. L.↗
NASA NTRS1978

The X-ray structure of NGC 5128

During an observation of NGC 5128 (Cen A) by the rotating modulation collimator on board the SAS-3 X-ray observatory in early June 1975, data were obtained which suggest the presence of an extended source with about 3 arcmin FWHM surrounding an extremely compact object located at the radio/IR nucleus. The point component exhibited a 25% increase in intensity over a period of 2 to 5 hours.

Delvaille, J. P.↗
NASA NTRS1978

X-ray and gamma-ray line production by nonthermal ions

X-ray production at about 6.8 keV by the 2p to 1s transition in fast hydrogen- and helium-like iron ions is calculated, following both electron capture to excited levels and collisional excitation. A refinement of the Oppenheimer-Brinkman-Kramers (1930) approximation is used to obtain an improved charge-exchange cross-section. This, and the corresponding ionization cross section, were used to determine equilibrium charge fractions for iron ions as functions of their energy. The effective X-ray line production cross section was found to be sharply peaked in energy at about 8 to 12 MeV per amu. Since fast ions of similar energies can also excite nuclear levels, the ratio of selected strong gamma-rays line emissivities to the X-ray line emissivity is calculated. These calculations are employed to set limits on the intensity of gamma-rays line emission from the galactic center and the radio galaxy Cen A, and it is found that these limits are generally lower than those reported in the literature.

Bussard, R. W.↗
NASA NTRS1978

The X-ray emitting galaxy Centaurus A

OSO-8 X-ray observations of Cen A in 1975 and 1976 are reported. The source spectrum can be well fitted in both years by a power law of number index 1.66 and absorption due to 1.3 by 10 to the 23rd power atoms/sq cm. The total flux varied by a factor of 2 between 1975 and 1976. In 1976 there were flux variations of approximately 40% on a time scale of days. Measurements of the 6.4-keV Fe fluorescent line and the 7.1-keV absorption edge imply Fe/H of approximately 0.000016. Simultaneous radio measurements show variation in phase with X-ray variability. Consideration of radio, millimeter, infrared, and X-ray data shows that all the data can be accounted for by a model in which the X-rays are due to a synchrotron self-Compton source embedded in a cold H2 cloud.

Mushotzky, R. F.↗
NASA NTRS1978

Position and pulse profile of the X-ray transient 4U 0115 + 63

The celestial position of the 3.6-s X-ray pulsar, 4U 0115 + 63, has been measured to a precision of 12 arcsec in one dimension with the scanning modulation collimator experiment on HEAO 1. This result, together with a position obtained with SAS 3, has led to the identification of an early-type star as the optical counterpart. An X-ray luminosity of approximately 3 by 10 to the 37th power erg/s (3-13 keV) is inferred from the observed flux of about 30% that of the Crab and an estimated distance of 5 kpc. The pulse profile over the energy range 3-13 keV is similar to that of Cen X-3.

Johnston, M.↗
NASA NTRS1978

Extreme-ultraviolet observations of nearby B stars - Constraints on hot circumstellar plasma

The extreme-ultraviolet telescope on the Apollo-Soyuz Test Project Mission observed several nearby stars of spectral type earlier than B3 in search of thermal emission from possible hot circumstellar plasma. The stars studied in the 45-185 and 114-185 A passbands of the instrument included Lambda Sco, Nu Sco, Beta CMa, Beta Cen, Sigma Sgr, and Alpha Pav. Diffuse emission around these stars in excess of about 1 billionth erg/sq cm/sec per sr/eV in the 45-185 A band and about 1 hundred-millionth erg/sq cm/sec per sr/eV in the 114-185 A band was not detected. The experimental data have been used to place upper limits on the emission measure of the plasma and the stellar-wind luminosity, using standard models. The data rule out the existence of sizable bubbles of hot plasma around any of the stars surveyed. In all cases the electron density in these regions, if they exist, may not exceed about 0.2 electron/cu cm.

Paresce, F.↗
NASA NTRS1978

Period variations in pulsating X-ray sources. I - Accretion flow parameters and neutron star structure from timing observations

Torque fluctuations which can lead to variations in the periods of pulsating X-ray sources are examined. A description of torque variations in terms of noise processes is developed, and the resulting noise models are applied to observations of several pulsating X-ray sources. It is shown that fluctuations in accretion torque could account for the observed period variations and spindown episodes in Her X-1 and Cen X-3. The values of the torque noise strengths inferred from either a nonresonant response or, in the case of Her X-1, a Tkachenko-mode interpretation of the data are found to be consistent with those expected from processes at the magnetospheric boundary of an accreting neutron star. Ways to distinguish among the various interpretations of the period variations are considered. It is noted that fluctuating mass-flow rates may be responsible for other phenomena observed in compact X-ray sources, such as wobble with zero initial amplitude and binary period variations in close binary systems experiencing mass transfer.

Lamb, F. K.↗