Abstract: An eclipsing X-ray binary pulsator consistent with the location of A1540-53 has been observed. The source pulse period was 528.93 + or - 0.10 s. The binary nature is confirmed by a Doppler curve for the pulsation period. The eclipse angle of 30.5 + or - 3 deg and the 4-hour transition to and from eclipse suggest an early-type giant or supergiant primary star.

Abstract: As X-ray astronomy enters the Einstein era when faint, distant, and exotic X-ray sources are being investigated, we thought it might be useful to review some of the progress made over the past decade on a class of relatively nearby objects that have contributed richly to our understanding of astrophysics — the binary X-ray pulsars. We shall touch briefly on three general topics related to X-ray pulsar research: (1) pulse profiles, (2) studies of pulse periods, and (3) orbital determinations and the evaluation of binary system parameters. We shall also discuss three special topics of current interest: (1) the apsidal motion test in X-ray binaries; (2) 4U1626-67 and the nature of highly compact X-ray binary systems; and (3) searches for new X-ray pulsars in the Einstein era. Most of the data that we shall invoke were obtained with the Uhuru, Copernicus, Ariel 5, SAS-3, 0S0-8, and HEAO-1 satellites.

Abstract: In this paper we calculate the probability that existing binaries in various stellar systems have suffered exchange collisions with single stars. After such a collision the former single star is one of the new binary components while the star it replaces is ejected from the binary. In the solar neighborhood, about one visual binary in 10/sup 3/ has a nonprimordial stellar component which it captured from the general field. The two stellar components of such a binary can have very different ages and chemical compositions. Dynamically, such binaries are indistinguishable from primordial binaries except for their high average orbital eccentricities. It is likely that many of the x-ray binaries in the Galactic disk and bulge have captured their relativistic components by exchange collisions. In open clusters most binaries with semimajor axes aapproximately-greater-than10/sup 3/ AU have suffered one or more exchange collisions. In the cores of dense globular clusters, this is true for binaries with aapproximately-greater-than1 AU.

Abstract: Eccentric-orbit binary models for transient X-ray sources are investigated. In these models, a compact star is in an eccentric orbit around a more massive star. As the compact star accretes mass from the stellar wind of the massive star, the accretion rate becomes time-dependent. The accretion rate is determined by Bondi's accretion radius, which depends on both the relative velocity of the stellar wind to the compact star and the sound velocity through the stellar wind. With reasonable sets of the eccentricity, the semi-major axis, the stellar wind velocity and the sound velocity, we obtain the variations of the light curves compatible with observations for the transient X-ray sources. It is likely that many transient X-ray sources are explainable by eccentric-orbit binary models.

Abstract: The evolution of two low-mass close binary systems is followed numerically through the phase of mass exchange, both with and without the inclusion of gravitational radiation losses. In both systems, the primary, mass-losing components are initially homogeneous zero-age main-sequence stars of Populations I and II, respectively, whereas the secondary components are just assumed to be compact (i.e., white dwarfs, neutron stars, or black holes). It is found that gravitational radiation, via orbital angular-momentum losses, markedly influences the evolution of the system: (a) mass loss starts much earlier; and (b) the system spends a much longer time in the mass-exchange phase with, therefore, a smaller mass-transfer rate when compared with the case where gravitational radiation is neglected. The implications for observations are briefly discussed. In particular, these systems with white dwarfs as secondaries would constitute realistic models for cataclysmic variables, whereas if the compact component is a neutron star or black hole, the systems would be producing X-rays (L/sub x/approximately-less-than10/sup 36/ ergs s/sup -1/) for about 10/sup 10/ years and would constitute low-intensity globular cluster X-ray sources.

Abstract: THE recent observation1 of ∼ 100-MeV γ rays from Cygnus X-3, which have a comparable luminosity to the X-ray emission and which vary in phase with the 4.8-h period displayed by the X-ray and infrared emission, has been taken2, 3 as support for the suggestion4 that the X-ray emission is produced by a very young pulsar in a short period binary system. In this letter, we show that the observed γ-ray luminosity can be produced by an accretion disk around a non-magnetic neutron star or low-mass black hole, in a magnetised corona which accelerates relativistic electrons that decay by inverse Compton losses modified through the photon–photon and pair-annihilation processes.

Abstract: We examine various physical mechanisms which may produce tidal friction in close binary stars. We find that the most efficient in stars with convective envelopes is turbulent viscosity retarding the equilibrium tide, and in stars with radiative envelopes the action of radiative damping on the dynamical tide. Theoretical predictions based on these dissipative processes are in good agreement with the rotational velocities and orbital eccentricities observed in close binaries. The results are applied to the X-ray binaries Her X-1 and Cen X-3.

Abstract: Disk accretion onto a magnetized spinning neutron star or a white dwarf will not be possible if the radius of the magnetosphere exceeds the corotation radius A dead disk serving as a reservoir of material will then form around the star Such a disk could retard the spin of the star; it might produce a transient x-ray source of a flare in a dwarf binary In particular, a disk reservoir could persist for several years during turn-off of the Hercules X-1 source

Abstract: I n this talk, 1 will be discussing observations of rotating neutron stars, and in particular, what we can learn about neutron stars from studying various timing effects. I intend to concentrate on the observations and their immediate interpretations; other papers will be concerned with more of the theory. In the spirit of this session, 1 will try not to deal with observations which are primarily concerned with binary companions, third body effects, general relativistic orbit corrections, or anything else which doesn’t directly concern neutron star structure, except where these effects offer alternative explanations. Furthermore, the topic of neutron star masses and particularly limiting masses will be dealt with in the following paper. I n reviewing the radio pulsar literature of the last two or three years, I find little new observational data which directly bear on neutron star structure. The binary pulsar is much more exciting for its implications about mass, evolution, and relativistic effects. However, certain x-ray pulsar data have recently been interpreted in terms of the response of the star to accretion torques with direct application to determinations of magnetic fields and possibly moments of inertia. I will thus concentrate on these data, drawing parallels with the radio pulsars as appropriate.

Abstract: Differential rotation could significantly increase the upper mass limit of neutron stars. By adjusting the angular velocity at each point in the star to balance the local pressure gradient, it may be possible to stabilize massive stars against collapse. However, we shall show that differential rotation damps to a state of rigid body rotation in a time-scale of a few days and, consequently, cannot affect neutron star masses on longer time-scales. We shall consider dissiptative mechanisms in two regimes: the formation of a hot differentially rotating neutron star-the usual supernovae hypothesis; and a less likely alternative, an initial state of a cold differentially rotating neutron star. In both cases, we take the rotational velocity to be of order c, the speed of light, since it is necessary to have large velocities to significantly increase the mass of a star near gravitational collapse.

Abstract: AN examination of the kinematical properties of binary OB stars, binary X-ray sources and pulsars suggests an evolutionary sequence linking an apparent low-velocity class of pulsars to the binary nature of their extreme Population I progenitors.

Abstract: COE et al1 have reported that Cyg X-1 and the transient source A 0620-00 both display anticorrelated spectral variability. That is to say, flux enhancements at low energy (E ≲ 10 keV) tend to be accompanied by a decline at high energy and vice versa. Both sources may be considered possible black holes; Cyg X-1 is well known, while the candidacy of A 0620-00 (≡ Nova Monocerotis 1975, distance □ 2 kpc)2,3 follows if its peak luminosity □ 5 × 1038 erg s−1 is identified with the Eddington limit luminosity 1.3 M/M⊙ × 1038 erg s−1 for mass accretion. Thus the accreting object has a mass □ 4 M⊙, just greater than the accepted limiting mass for a neutron star. It has therefore been suggested1 that anticorrelated spectral variability, perhaps resulting from inverse Compton scattering in the accretion disk4,5 may be common to X-ray emitting binary systems which contain black holes. In this paper, we wish to show that anticorrelated variability is not confined to sources which are black hole candidates.

Abstract: Photometric and spectroscopic observations are presented of both members of the binary star system HD 105902 (ADS 8448) whose primary component is a barium star of spectral class K5 Ba 2. The visual absolute magnitude for the barium star primary is found to be +2.7 and the distance to the system is 190 pc. The mass for the barium star is estimated to be 2.6 solar masses.

Abstract: The spectrum of the peculiar O9 star HD 93521 is studied and compared with those of the standard stars 10 Lac and AE Aur Several possibilities are examined which might explain the high galactic latitude of this star, corresponding toz>750 pc and its slight helium excess It is suggested that HD 93521 is a ‘runaway’ binary system composed of the O9 star plus a neutron star left over from a supernova explosion

Abstract: An eclipsing X-ray binary pulsator consistent with the location of A1540-53 was observed. The source pulse period was 528.93 plus or minus 0.10 seconds. The binary nature is confirmed by a Doppler curve for the pulsation period. The eclipse angle of 30.5 deg plus or minus 3 deg and the 4 h transition to and from eclipse suggest an early type, giant or supergiant, primary star.

Abstract: The article reviews the processes that can be expected to occur in the first phase of mass-cxchange in binary systems, particularly those likely to produce binary x-ray sources. Recent studies, following the pioneering investigation of Benson, ' indicate that the system will evolve into contact followed by a phase o f rapid and substantial loss of mass and angular momentum. Thus, earlier evolutionary schemes proposed f o r x-ray systems will require alteration in the sense of demanding progenitors of higher mass and greater initial periods.

Abstract: In a close binary system with circular orbits in which mass transfer occurs when material from one star fills up and possibly spills over its Roche lobe, the usual mass-radius relation cannot be used as it would lead to unreasonable results in some situations.