Tenma observations of bright binary X-ray sources

Spectroscopic study of bright binary X-ray sources, performed with the gas scintillation proportional counters on board Tenma, is reviewed. Properties of an iron emission line from two classes of bright binary X-ray sources: X-ray pulsars and low-mass binary sources, are first presented. It is shown that a most likely candidate for the line emitting region is an Alfven shell in case of X-ray pulsars, whereas that of low mass binary sources is an outer accretion disk. Next, nature of the continuum emission from low-mass binary sources is consistently interpreted by a picture that an optically thick accretion disk extends down to very near the surface of a weakly magnetized neutron star. Origin of ultrasoft spectra of black hole candidate sources is also discussed.     

Theories of magnetospheres around accreting compact objects

A wide class of galactic X-ray sources are believed to be binary systems where mass is flowing from a normal star to a companion that is a compact object, such as a neutron star. The strong magnetic fields of the compact object create a magnetosphere around it. We review the theoretical models developed to describe the properties of magnetospheres in such accreting binary systems. The size of the magnetosphere can be estimated from pressure balance arguments and is found to be small compared to the over-all size of the accretion region but large compared to the compact object if the latter is a neutron star. In the early models the magnetosphere was assumed to have open funnels in the polar regions, through which accreting plasma could pour in. Later, magnetically closed models were developed, with plasma entry made possible by instabilities at the magnetosphere boundary. The theory of plasma flow inside the magnetosphere has been formulated in analogy to a stellar wind with reversed flow; a complicating factor is the instability of the Alfvén critical point for inflow. In the case of accretion via a well-defined disk, new problems of magnetospheric structure appear, in particular the question to what extent and by what process the magnetic fields from the compact object can penetrate into the accretion disk. Since the X-ray emission is powered by the gravitational energy released in the accretion process, mass transfer into the magnetosphere is of fundamental importance; the various proposed mechanisms are critically examined.Proceedings of the NASA/JPL Workshop on the Physics of Planetary and Astrophysical Magnetospheres.     

The structure of low-mass X-ray binaries

The present knowledge of the structure of low-mass X-ray binary systems is reviewed. We examine the orbital period distribution of these sources and discuss how the orbital periods are measured. There is substantial observational evidence that the accretion disks in low-mass X-ray binaries are thick and structured. In a number of highly inclined systems, the compact X-ray emitting star is hidden from direct view by the disk and X-radiation is observed from these only because photons are scattered into the line of sight by material above and below the disk plane. In such systems the X-ray emission can appear extended with respect to the companion star, which can lead to partial X-ray eclipses. There are substantial variations in the thickness of the disk rim with azimuth. These give rise to the phenomenon of irregular dips in the X-ray flux which recur with the orbital period, or to an overall binary modulation of the X-ray flux if the source is extended. The X-ray spectra of low-mass X-ray binaries can be used to probe the innermost emission regions surrounding the compact star. The spectra of the bright Sco X-1 variables can be fitted with two components which are provisionally identified as originating in the inner disk and the boundary layer between the disk and the neutron star respectively. The characteristic energy dependent flaring of the Sco X-1 sub-class may be a geometric effect triggered by an increase in the thickness of the inner disk or boundary layer. The X-ray spectra of the lower luminosity systems, including the bursters, are less complex, and in many cases can be represented by a single power law with, in some sources, a high energy cut-off. Iron line emission is a characteristic of most low-mass X-ray binaries, irrespective of luminosity.     

A magnetic explanation for the rapid burster

The observations of X-ray Type II bursts from the low-mass X-ray binary MXB 1730-335 can be explained by a particular form of magnetic gating in the presence of steady external accretion. The requirements are a strong magnetic field of the neutron star (7×10¹¹–2×10¹² gauss at the surface), rotational symmetry and alignment of the field axis with the axis of a steadily accreting disk to within 6°.     

Long-term cycles in cosmic X-ray sources

Most of what we know about galactic X-ray binaries comes from their time variation, particularly periodic variations corresponding to neutron star rotation, and binary motion. Longer cycles or quasi-cycles are much harder to observe because of the shortage of instrumentation suitable for long-term monitoring. Nonetheless, cycle with periods up to a few years have been seen in several galactic binaries.Cycles of 30–300 days have been confirmed for four high-mass systems, LMC X-4, Her X-1, SS433, and Cyg X-1, and are suspected in several others. These cycles are observed in both the X-ray and optical bands, and represent cyclic variations in both the inner and outer parts of the accretion disk. Some component of these systems is precessing, but we are not certain which. It could be a misaligned companion star; the outer rim of the accretion disk, driven by radiative feedback; or the neutron star.Several low-mass X-ray binaries have quasi-periodic cycles, with periods ranging from 1/2 to 2 years. The amplitude of modulation ranges between 50 and 100%, i.e., both persistent and transient objects fall into this class. This activity is reminiscent of the superoutburst cycles of the SU UMa cataclysmic variables, and may be caused by similar mass-transfer instabilities.Periodic outbursts in the Be/neutron star systems seem to result from variable mass transfer in a wide, eccentric orbit. The relationship between the orbital cycle and the flux outbursts, however, is not well understood, and even the equivalence of the outburst and binary cycles remains hypothetical for most objects. Most likely, the periodic outbursts result from enhanced mass transfer at periastron.Compared to other aspects of X-ray astronomy, long-term activity has been much less intensively studied by both observers and theoreticians. A simple all-sky monitor in permanent operation could provide for the X-ray sky the same kind of data base provided to optical observers by the Harvard plates.     

EXOSAT observations of the X-ray pulsar 4U1145-619

EXOSAT observations of the X-ray pulsar 4U1145-619 during June and July 1983 and July 1984 confirm that this source shows a regular 187 day outburst cycle in X-rays. The results from pulse timing and X-ray spectroscopy are discussed in terms of a model for 4U1145-619 involving an eccentric binary system in which there is variable accretion from the Be primary star onto a companion neutron star.     

Photometric effects of accretion disks in long-period eclipsing binaries

Summary A multi-year photometric program on long-period eclipsing binaries has begun to uncover some properties of accretion disks in these systems. Emission and transmission properties can sometimes be found from light curve features produced by partial eclipses of the disk by the cool star, and by partial occultations of the cool star by the disk. These disks do not have the classical alpha structure. They are optically thin normal to the orbital plane, but may be geometrically thicker than purely gravitationally-stratified disks. Disk gas may be contaminated by dust particles acquired from the outer layers of the cool loser. In some systems, high states, produced by elevated mass accretion by the hot star, occur, suggesting that the mass distribution in the disk is clumpy. However mass-transfer rates are found, they lie between 10^-7 and 10^-6 solar masses per year.While this binary sample is small at the moment, some of its properties are shared with other systems. The author has five-color observations of about a dozen additional systems, which may fill out this picture more fully.     

HD 190918: Arbiter between models for Wolf-Rayet stars

New orbital elements for the 112.4 day WN4.5+O9.5Ia spectroscopic binary HD 190918 are presented. Solutions consistent with the O star being a main sequence star or a supergiant are obtained wheni=25° or 20°, respectively. We predict the X-ray flux to be expected from this system and note how an observation of the X-ray flux from HD 190918 would help one to choose the most probable value of the inclination.     

Algol, Beta Lyrae, and W Serpentis: Some new results for three well studied eclipsing binaries

The properties of the eclipsing binaries Algol, Beta Lyrae, and W Serpentis are discussed and new results are presented. The physical properties of the components of Algol are now well determined. High resolution spectroscopy of the H-alpha feature by Richards et al. and by Gillet et al. and spectroscopy of the ultraviolet resonance lines with the International Ultraviolet Explorer satellite reveal hot gas around the BBV primary. Gas flows also have been detected apparently originating from the low mass, cooler secondary component and flowing toward the hotter star through the Lagrangian L1 point. Analysis of 6 years of multi-bandpass photoelectric photometry of Beta Lyrae indicates that systematic changes in light curves occur with a characteristic period of -275 ± 25 days. These changes may arise from pulsations of the B8II star or from changes in the geometry of the disk component. Hitherto unpublished u, v, b, y, and H-alpha index light curves of W Ser are presented and discussed. W Ser is a very complex binary system that undergoes complicated, large changes in its light curves. The physical properties of W Ser are only poorly known, but it probably contains one component at its Roche surface, rapidly transfering matter to a component which is embedded in a thick, opaque disk. In several respects, W Ser resembles an upscale version of a cataclysmic variable binary system.     

Intermittent stellar wind accretion in Pop. I binary systems containing an X-ray pulsar

We review the long term variability properties of accretion powered X-ray pulsars in massive Pop. I binary systems and discuss how their characteristics, in particular the large dynamic range in luminosity of the transient pulsars, can be understood in terms of the interaction of the accreting material with the neutron star magnetosphere. We point out that the X-ray pulsar transient activity in general can be due to the transition between direct wind accretion and a regime in which the centrifugal drag exerted by the pulsar magnetosphere inhibits accretion onto the neutron star surface.     

Towards a Unified View of Inhomogeneous Stellar Winds in Isolated Supergiant Stars and Supergiant High Mass X-Ray Binaries

Massive stars, at least \(\sim10\) times more massive than the Sun, have two key properties that make them the main drivers of evolution of star clusters, galaxies, and the Universe as a whole. On the one hand, the outer layers of massive stars are so hot that they produce most of the ionizing ultraviolet radiation of galaxies; in fact, the first massive stars helped to re-ionize the Universe after its Dark Ages. Another important property of massive stars are the strong stellar winds and outflows they produce. This mass loss, and finally the explosion of a massive star as a supernova or a gamma-ray burst, provide a significant input of mechanical and radiative energy into the interstellar space. These two properties together make massive stars one of the most important cosmic engines: they trigger the star formation and enrich the interstellar medium with heavy elements, that ultimately leads to formation of Earth-like rocky planets and the development of complex life. The study of massive star winds is thus a truly multidisciplinary field and has a wide impact on different areas of astronomy.In recent years observational and theoretical evidences have been growing that these winds are not smooth and homogeneous as previously assumed, but rather populated by dense “clumps”. The presence of these structures dramatically affects the mass loss rates derived from the study of stellar winds. Clump properties in isolated stars are nowadays inferred mostly through indirect methods (i.e., spectroscopic observations of line profiles in various wavelength regimes, and their analysis based on tailored, inhomogeneous wind models). The limited characterization of the clump physical properties (mass, size) obtained so far have led to large uncertainties in the mass loss rates from massive stars. Such uncertainties limit our understanding of the role of massive star winds in galactic and cosmic evolution.Supergiant high mass X-ray binaries (SgXBs) are among the brightest X-ray sources in the sky. A large number of them consist of a neutron star accreting from the wind of a massive companion and producing a powerful X-ray source. The characteristics of the stellar wind together with the complex interactions between the compact object and the donor star determine the observed X-ray output from all these systems. Consequently, the use of SgXBs for studies of massive stars is only possible when the physics of the stellar winds, the compact objects, and accretion mechanisms are combined together and confronted with observations.This detailed review summarises the current knowledge on the theory and observations of winds from massive stars, as well as on observations and accretion processes in wind-fed high mass X-ray binaries. The aim is to combine in the near future all available theoretical diagnostics and observational measurements to achieve a unified picture of massive star winds in isolated objects and in binary systems.     

A 0535 + 26/HDE 245770: A typical X-ray/Be system

Among the X-ray/Be systems, A 0535 + 26/HDE 245770 has been noted, since its discovery, for its peculiar features in several respects, in a wide energy range. For this reason and for a series of concomitant favorable causes, this system has been one of the most studied among the massive X-ray binary systems. The most remarkable incident was that its optical identification with an early-type-emission-line star (O9.7IIIe) has led to a deep studies on Be stars and their interactions with neutron stars, which have allowed to discover, without unbiguity, the presence of optical indicators of consequent X-ray flares, as well as that Be stars in X-ray/Be systems behave just as normal Be stars. Overmore, thanks to the multifrequency coordinated observations of this system, the X-ray emissions from binary companion of the Be stars are best explained by assuming the presence of a thick equatorial disk with low expansion velocity and a thin polar region with high expansion velocity. This picture reconciled the strong discrepancy in mass loss rate evaluations coming from IR and from UV measurements, assuming that the observed regions are enterely distinct from each other, one being a high-density, low-velocity region, and the other being a low-density, very hot, rapidly-expanding disk-like zone.Since, this picture seems to be the best up-to-date frame to cuckold all the experimental panorama available on X-ray/Be systems, we would like to paint in this paper the multifrequency behaviour of A 0535 + 26/HDE 245770, which is the best studied among such systems, in order to stimulate future coordinated experimental-theoretical works on this very interesting class of objects.     

Observational Manifestations of Precession of Accretion Disk in the SS 433 Binary System

Basic properties of the unique object SS 433 are described. Observational spectroscopic and photometric manifestations of a precessing accretion disk around a relativistic object in this X-ray binary system are presented.     

Algol,Beta Lyrae,and W Serpentis: Some new results for three well studied eclipsing binaries

The properties of the eclipsing binaries Algol, Beta Lyrae, and W Serpentis are discussed and new results are presented. The physical properties of the components of Algol are now well determined. High resolution spectroscopy of the H-alpha feature by Richards et al. and by Gillet et al. and spectroscopy of the ultraviolet resonance lines with the International Ultraviolet Explorer satellite reveal hot gas around the BBV primary. Gas flows also have been detected apparently originating from the low mass, cooler secondary component and flowing toward the hotter star through the Lagrangian L1 point. Analysis of 6 years of multi-bandpass photoelectric photometry of Beta Lyrae indicates that systematic changes in light curves occur with a characteristic period of -275 ± 25 days. These changes may arise from pulsations of the B8II star or from changes in the geometry of the disk component. Hitherto unpublished u, v, b, y, and H-alpha index light curves of W Ser are presented and discussed. W Ser is a very complex binary system that undergoes complicated, large changes in its light curves. The physical properties of W Ser are only poorly known, but it probably contains one component at its Roche surface, rapidly transfering matter to a component which is embedded in a thick, opaque disk. In several respects, W Ser resembles an upscale version of a cataclysmic variable binary system.     

The mass and helium discrepancy in massive stars: The case Vela X-1

A NLTE-analysis is presented of high S/N spectra of the optical component of the standard massive X-ray binary Vela X-1. In combination with the orbital parameters we conclude that the optical star is highly helium enriched and is significantly overluminous compared to standard evolutionary tracks of massive accretion stars. We then propose a new accretion model able to explain these features.     

The effects of magnetic fields on period changes, mass transfer and evolution of algol binaries

Variations in the magnetic pressure and flux blocking by starspots during the magnetic cycle of the cool semidetached component of an Algol binary may cause cyclic changes in the quadrupole moment and moment of inertia of the star which can cause alternate period changes. Since several different processes and timescales are involved, the orbital period changes may not correlate strongly with the indicators of magnetic activity. The structural changes in the semidetached component can also modulate the mass transfer rate. Sub-Keplerian velocities, supersonic turbulence, and high temperature regions in circumstellar material around the accreting star may all be a consequence of magnetic fields embedded in the flow. Models for the evolution of Algols which include the effects of angular momentum loss (AML) through a magnetized wind may have underestimated the AML rate by basing it on results from main sequence stars. Evolved stars appear to have higher AML rates, and there may be additional AML in a wind from the accretion disk.     

Evolution of massive close binaries

This paper briefly reviews the competition between massive single star and massive close binary evolution the last two decades. The status of the binary evolutionary model is summarized, the assumptions and simplifications are critically discussed. Using all computations performed since 1970, general conclusions are drawn and a comparison with massive single star evolution is presented. Special attention is given at the assumptions behind the commonly accepted model for the mass gainer and a new accretion model is proposed. The binary results in combinarion with single star evolution are compared with observations of massive stars with emphasis on the HR diagram, star number counts, WR stars, SN 1987A, OBN and OBC stars.     

The effects of magnetic fields on period changes,mass transfer and evolution of algol binaries

Variations in the magnetic pressure and flux blocking by starspots during the magnetic cycle of the cool semidetached component of an Algol binary may cause cyclic changes in the quadrupole moment and moment of inertia of the star which can cause alternate period changes. Since several different processes and timescales are involved, the orbital period changes may not correlate strongly with the indicators of magnetic activity. The structural changes in the semidetached component can also modulate the mass transfer rate. Sub-Keplerian velocities, supersonic turbulence, and high temperature regions in circumstellar material around the accreting star may all be a consequence of magnetic fields embedded in the flow. Models for the evolution of Algols which include the effects of angular momentum loss (AML) through a magnetized wind may have underestimated the AML rate by basing it on results from main sequence stars. Evolved stars appear to have higher AML rates, and there may be additional AML in a wind from the accretion disk.     

Mass flow in interacting binaries observed in the ultraviolet

Recent satellite observations of close binary systems show that practically all binaries exhibit evidence of mass flow and that, where the observations are sufficiently detailed, a fraction of the matter flowing out of the mass-losing component is accreted by the companion and the remainder is lost from the binary system. The mass flow is not conservative. During the phase of dynamic mass flow, the companion star becomes immersed in optically-thick plasma and the physical properties of that star elude close scrutiny.     

Mass flow in interacting binaries observed in the ultraviolet

Recent satellite observations of close binary systems show that practically all binaries exhibit evidence of mass flow and that, where the observations are sufficiently detailed, a fraction of the matter flowing out of the mass-losing component is accreted by the companion and the remainder is lost from the binary system. The mass flow is not conservative. During the phase of dynamic mass flow, the companion star becomes immersed in optically-thick plasma and the physical properties of that star elude close scrutiny.