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At least 1,171 records · Page 65
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NASA NTRS1993

ROSAT x ray survey observations of active chromospheric binary systems and other selected sources

The connection between processes that produce optical chromospheric activity indicators and those that produce x-rays in RS CVn binary systems by taking advantage of the ROSAT All-Sky Survey (RASS) results and our unique ground-based data set was investigated. In RS CVn systems, excess emission in the Ca 2 resonance (K & H) and infrared triplet (IRT) lines and in the Balmer lines of hydrogen is generally cited as evidence for chromospheric activity, which is usually modeled as scaled up solar-type activity. X-ray emission in RS CVn systems is believed to arise from coronal loop structures. Results from spectra data obtained from RASS observations are discussed and presented.

Ramsey, Lawrence W.↗
NASA NTRS1992

X-ray observations of planetary nebulae with binary nuclei

Einstein and EXOSAT satellite observations of X-ray emission from the planetary nebulae A63 and LoTr 5 are reviewed. Both of these systems contain binary central stars. No flux was detected from A63 (central star UU Sge). LoTr 5 (central star IN Com) is a previously unreported X-ray emitter; it showed no statistically significant X-ray variability. Three models for the source of the X-ray emission in such systems are considered in the light of these and previous results.

Apparao, K. M. V.↗
NASA NTRS1992

Tidal heating and mass loss in neutron star binaries - Implications for gamma-ray burst models

A neutron star in a close binary orbit around another neutron star (or stellar-mass black hole) spirals inward owing to gravitational radiation. We discuss the effects of tidal dissipation during this process. Tidal energy dissipated in the neutron star's core escapes mainly as neutrinos, but heating of the crust, and outward diffusion of photons, blows off the outer layers of the star. This photon-driven mass loss precedes the final coalescence. The presence of this eject material impedes the escape of gamma-rays created via neutrino interactions. If an e(+) - e(-) fireball, created in the late stages of coalescence, were loaded with (or surrounded by) material with the mean column density of the ejecta, it could not be an efficient source of gamma-rays. Models for cosmologically distant gamma-rays burst that involve neutron stars must therefore be anisotropic, so that the fireball expands preferentially in directions where the column density of previously blown-off material is far below the spherically averaged value which we have calculated. Some possible 'scenarios' along these lines are briefly discussed.

Meszaros, P.↗
NASA NTRS1992

Optical observations of low-mass X-ray binaries and millisecond pulsars

We review recent results from optical observations of low-mass X-ray binaries (LMXBs) and millisecond pulsars. We discuss the optical and X-ray properties of those LMXBs which have been suspected of harboring black holes, reviewing in particular recent work on GX 339-4. We also present preliminary results from new optical observations of the ablating millisecond pulsar system PSR 1957 + 20. The orbital modulation is smooth and symmetrical, and is at least four magnitudes deep in R. However, the overall shape of the light curve can be reproduced by a simple model assuming blackbody reradiation from the secondary.

Callanan, Paul J.↗
NASA NTRS1992

Ginga observations of the dipping low-mass X-ray binaries XB 1916-053 and EXO 1748-676

Ginga observations of XB 1916-053 and EXO 0748-676, low-mass X-ray binaries with pronounced dips of variable depth and duration in their X-ray light curves, are reported. Periodicity studies of XB-1916-053 show results consistent with those from previous Ginga observations, specifically, that the derived X-ray period is shorter than the optical period by about 1 percent. This problem is discussed, and a more refined model for the system is described. Three bursts were detected during the observation of XB 1916-053, the second of which occurred during a dip. Spectral analysis suggests that the second burst caused an almost instantaneous ionization of the absorbing medium in the line of sight. This material then returned to its equilibrium state on the same time scale as the burst decay. Loose limits are placed upon the possible clumpiness of the material responsible for the accretion disk structure in XB 1916-053 and EXO 0748-676.

Smale, Alan P.↗
NASA NTRS1992

Identification of the nebula G70.7 + 1.2 as a bow shock powered by a pulsar/Be-star binary

New data are presented here that firmly establish the nonthermal nature of the radio emission from the enigmatic radio and optical nebula G70.7 + 1.2. H-alpha and forbidden O I Fabry-Perot observations are used to argue that the extended optical emission from the nebula arises from a bow shock powered by a mass-losing luminous star moving supersonically through dense gas. The strong nonthermal radio emission from the object is then explained as the shocked relativistic wind from a pulsar, which is proposed here as a companion to the Be star. The coincidence of the optical and radio emission requires the pulsar and stellar winds to be mixed together. The system has a large overall velocity of about 60 km/s which is inexplicable in all other models but which is typical of binary pulsars.

Kulkarni, S. R.↗
NASA NTRS1992

Pycnonuclear reaction rates for binary ionic mixtures

Through a combination of compositional scaling arguments and examinations of Monte Carlo simulation results for the interparticle separations in binary-ionic mixture (BIM) solids, we have derived parameterized expressions for the BIM pycnonuclear rates as generalizations of those in one-component solids obtained previously by Salpeter and Van Horn and by Ogata et al. We have thereby discovered a catalyzing effect of the heavier elements, which enhances the rates of reactions among the lighter elements when the charge ratio exceeds a critical value of approximately 2.3.

Ichimaru, S.↗
NASA NTRS1992

The susceptibility critical exponent for a nonaqueous ionic binary mixture near a consolute point

We report turbidity measurements of a nonaqueous ionic solution of triethyl n-hexylammonium triethyl n-hexylboride in diphenyl ether. A classical susceptibility critical exponent gamma = 1.01 +/- 0.01 is obtained over the reduced temperature range t between values of 0.1 and 0.0001. The best fits of the sample transmission had a standard deviation of 0.39 percent over this range. Ising and spherical model critical exponents are firmly excluded. The correlation length amplitude xi sub 0 from fitting is 1.0 +/- 0.2 nm which is much larger than values found in neutral fluids and some aqueous binary mixtures.

Zhang, Kai C.↗
NASA NTRS1993

The magnetic field configurations of AM Herculis binaries

We have considered equilibrium configurations of a white dwarf and a red dwarf companion in AM Herculis type binary system where the red dwarf is assumed to have an intrinsic centered dipole field, and the white dwarf an offset dipole or a centered dipole plus a quadrupole field, and the interaction is purely magnetic. The restoring torque, due to the system being perturbed from its stable equilibrium configuration, is compared with the accretion torque. The results are used to show that the observed alignment of the main cyclotron emission region relative to the red star can be explained quite simply by these models without having to invoke gravitational torques. We also provide a possible explanation of the observation that the more strongly accreting pole has the weaker field strength in the few systems where fields have been determined at both poles.

Wu, Kinwah↗
NASA NTRS1993

A spectroscopic search for colliding stellar winds in O-type close binary systems. IV - Iota Orionis

We present H-alpha and He I 6678 A line profiles for the eccentric orbit binary Iota Ori. We have applied a tomography algorithm which uses the established orbital velocity curves and intensity ratio to reconstruct the spectral line profiles for each star. The He I profiles appear as pure photospheric lines, and H-alpha shows variable emission in the line core throughout the orbit (which is typical of O giants) and in the blue wing near periastron passage. We show that the blue wing emission is consistent with an origin between the stars which probably results from a dramatic focusing of the primary's stellar wind at periastron. We also present IUE archival spectra of the UV wind lines N V 1240 A and C IV 1550 A.

Gies, Douglas R.↗
NASA NTRS1993

Effects of inclination angle on the spectra of X-ray binaries

Iron emission and absorption features from nine X-ray binaries are compared with a model that predicts characteristics of iron K-shell features corresponding to different viewing angles for the accretion disk. In this model, emission and absorption features arise from an accretion disk corona whose structure is computed under the assumptions of ionization, thermal, and hydrostatic balance of gas illuminated by the central continuum source. Hence the equivalent widths of the emission and absorption features are predicted without the introduction of any free parameters to control the physical conditions in the disk corona. Our data/model comparison suggests an explanation for the equivalent widths of the iron K-alpha emission and the depth of the absorption edge in terms of (1) the angle of inclination between the observer's line of sight and the plane of the accretion disk and (2) the shape of the incident source spectrum.

Vrtilek, S. D.↗
NASA NTRS1993

Dynamics of the young binary LMC cluster NGC 1850

BV CCD images and echelle spectra of 52 supergiants are used to examine the age and internal dynamics of the young binary LMC cluster NGC 1850. A color-magnitude diagram was constructed and found to contain three distinct populations of stars. Using star-subtracted, median filtered BV images, attempts were made to determine ellipticity parameters, BV luminosity profiles were constructed to project radii greater than 40. A power law model was applied to test for the presence of a tidal cutoff in the luminosity profile. Of the 52 stars with echelle spectra, a subset of 36 were used to study the cluster dynamics. A rotational signal in the radial velocities has been detected at the 93-percent confidence level, implying a rotation axis at a position angle of 100 +/- 40 deg. Values for the slope of the mass function were determined using mass luminosity relationships and different forms for the IMF. The current cluster age is similar to its central relaxation time.

Fischer, Philippe↗
NASA NTRS1993

Spectroscopic observations of the detached binary PG 1413 + 015

We present improved estimates of the stellar parameters of the eclipsing, precataclysmic binary system PG 1413 + 015 (GH Vir), which has an orbital period of only 8h16m. Model atmosphere fits a Balmer line profiles yield T(eff) = 48,800 +/- 1200 K and log g = 7.70 +/- 0.11 for the DAO white dwarf primary star, from which a mass of 0.51 +/- 0.04 solar mass is inferred using evolutionary models. An ultraviolet spectrum obtained with the IUE Observatory has a slope consistent with this temperature and the assumption of no interstellar extinction. A red CCD spectrum of the secondary star during the 12-minute total eclipse indicates a spectral type of M3 V-M5 V. Reanalysis of the eclipse light curve leads to an inferred radius of 0.15 solar radius and a mass of 0.10 solar mass for the secondary, the latter being marginally consistent with the spectral type. Reprocessing on the facing side of the secondary produces phase-dependent Balmer line emission and detectable variations in the continuum from 6500-9000 A. The observed levels of reprocessing are consistent with expectations based on the above stellar parameters.

Fulbright, Michael S.↗
NASA NTRS1993

HW Virginis - A short period eclipsing binary containing an sdB star

Simultaneous photometry of the binary star HW Vir, a detached system with an orbital period of 2 h 48 min is described. The new UBVR light curves obtained using the Wilson-Devinney code are found to constrain the orbital inclination of the system within the narrow range of 80.6 +/-0.2 deg. The temperature of the primary star is between 29,000 and 36,000 K, and the temperature of the secondary star is near 3,700 K. The possible masses and radii of the two stars are calculated using the published amplitude of the radial velocity curve of the primary star, 87.9 +/-4.8 km/s. It is found that, for the primary star log g1 is greater than 4.8 and less than or equal to 5.8. From the temperature and gravity of the primary star, its distance is found to be in the range between 42 and 151 pc. It is suggested that the system will begin mass transfer when the orbital period has decreased enough to bring the secondary star into contact with its Roche lobe.

Wood, Janet H.↗
NASA NTRS1993

On the optimum bit orders with respect to the state complexity of trellis diagrams for binary linear codes

It was shown earlier that for a punctured Reed-Muller (RM) code or a primitive BCH code, which contains a punctured RM code of the same minimum distance as a large subcode, the state complexity of the minimal trellis diagram is much greater than that for an equivalent code obtained by a proper permutation on the bit positions. To find a permutation on the bit positions for a given code that minimizes the state complexity of its minimal trellis diagram is an interesting and challenging problem. This permutation problem is related to the generalized Hamming weight hierarchy of a code, and is shown that for RM codes, the standard binary order of bit positions is optimum at every bit position with respect to the state complexity of a minimal trellis diagram by using a theorem due to Wei. The state complexity of trellis diagram for the extended and permuted (64, 24) BCH code is discussed.

Kasami, Tadao↗
NASA NTRS1993

A spectroscopic search for colliding stellar winds in O-type close binary systems. III - 29 UW Canis Majoris

The orbital-phase variations in the optical emission lines and UV P Cygni lines of the massive O-type binary 29 UW Canis Majoris are investigated in a search for evidence of colliding winds. High SNR spectra of the H-alpha and He I 6678-A emission lines are presented, and radial velocity curves for several features associated with the photosphere of the more luminous primary star are given. The H-alpha features consists of a P Cygni component that shares the motion of the primary, and which probably originates at the base of its wind, and a broad, stationary emission component. It is proposed that the broad emission forms in a plane midway between the stars where the winds collide. A simple geometric model is used to show that this placement of the broad component can explain the lack of orbital velocity shifts, the near-constancy of the emission strength throughout the orbit, the large velocities associated with the H-alpha wings, and the constancy of the velocity range observed.

Wiggs, Michael S.↗
NASA NTRS1993

A Broad-Band X-Ray Telescope spectrum of the massive X-ray binary X Persei

The Broad Band X-Ray Telescope, covering the 0.3-12 keV bandpass with moderate spectral resolution, observed the Be/X-ray binary X Per in 1990 December during the Astro-l mission on the Space Shuttle Columbia. The data obtained are the best to date to search for lines and edges. The data are well fitted by a power-law spectrum with a high-energy cutoff. A low value for the high-energy cutoff is found, implying a slightly weaker magnetic field strength for the X-ray pulsar. No iron line is present at about 6.5 keV with an equivalent width greater than 30-40 eV. The BBXRT observation corresponded to the 'off' state of X Per's recent 'phase change'.

Schlegel, Eric M.↗
NASA NTRS1992

Multiperiodicities in the photometric variability of RS CVn binaries - What we know after 15 years

An overview of the behavior of the RS CVn binaries is presented. It is argued that chromospherically active stars vary in brightness on a variety of different time scales, each involving very different physics. Some of them are literally periodic, some are quasi-periodic or cyclical, and some can be described only in terms of a time scale. Connections between some of the different periods, cycles, and time scales are explained. Attention is given to orbital period, rotation period, migration, spot lifetimes, magnetic cycles and flares.

Hall, Douglas S.↗