The COS-B X-ray observations of X-ray binaries

The observational information on X-ray binaries that was collected with the 80 cm² auxiliary X-ray detector onboard the COS-B gamma-ray satellite is reviewed. The results illustrate that in the study of X-ray binaries observations of long duration are extremely effective, even when using a small instrument.     

X-ray binaries and stellar evolution

Observational evidence suggests that most — if not all — binary X-ray sources are neutron stars. The evolutionary status and possible formation mechanisms of the type I (massive) and type II (low-mass) X-ray binaries are discussed. The difference between the standard massive X-ray binaries and the Be/X-ray binaries is ascribed to a somewhat different evolutionary history and status, and possible reasons for the existence of short- and long — period X-ray pulsars are discussed. Type II X-ray sources in globular clusters were most probably formed by capture processes; their formation rate inferred from the observations indicates that only a small fraction ( 1 to 10 percent) of the originally formed neutron stars have remained in their clusters. Type II sources in the galactic bulge may also have formed from cataclysmic binaries in which a white dwarf was driven over the Chandrasekhar limit by accretion.     

Formation and evolution of low-mass Algols

We discuss the origin, evolution and fate of low-mass Algols (LMA) that have components with initial masses less than 2.5 M₀. The semi-major axes of orbits of pre-LMA do not exceed 20–25 R₀. The rate of formation of Algol-type stars is 0.01/year. Magnetic stellar winds may be the factor that determines the evolution of LMA. Most LMA end their lives as double helium degenerate dwarfs with M₁/M₂ 0.88 (like L870-2). Some of them even merge through angular momentum loss caused by gravitational waves.     

CCD photometry of low mass X-ray binaries: LMCX-2, 1556-60 and 1957+11

We present preliminary results from V-band CCD photometric observations of several low-mass X-ray binaries with faint optical counterparts obtained at ESO and CFHT from February to September 1984. LMCX-2 exhibits large (0.3–0.5 magnitude) variations consistent with a possible 6.4 hour period. 1556-605 shows 0.2–0.5 magnitude variations suggesting that the orbital period is longer than 7 hours. 1957+115 shows short time scale (1 hour) variations of 0.1 magnitude.Based on observations obtained at the European Southern Observatory, La Silla, Chile and with the Canada-France-Hawaii telescope, Mauna Kea, HI.     

Hard X-ray structure of the Crab Nebula

Two-dimensional maps of the Crab Nebula have been synthesized in 22–64 keV range through the modulation collimator experiment. The effective angular resolution is about 15. The result indicates that the Crab morphology is strictly controlled by the pulsar.     

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.     

NLTE analysis of hot binaries

We present quantitative spectroscopic NLTE analyses of the components of well detached early type binaries (DH Cep, Y Cyg, V453 Cyg, and CW Cep). The position of the stars in the logL-logT _eff diagram is discussed. We find significantly higher temperatures for the components of Y Cygni from spectral analysis by means of unblanketed NLTE model photospheres than those given by the orbit analysis. Therefore the comparison with evolutionary tracks yields larger masses. The spectroscopic temperatures of V453 Cygni and CW Cephei agree with the orbit data, but the evolutionary tracks point to larger masses also. However, if we account for some 2000K lower effective temperatures due to line blanketing, the luminosities, temperatures and masses of all stellar components are in good agreement, except for the case of DH Cep.     

TeV emission from close binaries

It is commonly accepted that candidates for very high energy -ray sources are neutron stars, binary systems, black holes etc. Close binary systems containing a normal hot star and a neutron star (or a black hole) form an important class of very high energy -ray sources. Such systems are variable in any region of the electromagnetic spectrum and they enable us to study various stages of stellar evolution, accretion processes, mechanisms of particle acceleration, etc. Phenomena connected with this class of very high energy -ray sources are discussed. Particular emphasis has been placed on the TeV energy region.     

Doppler tomography of O-type binaries: The physical properties of seven systems

We have analyzed UV photospheric lines of seven O-type binaries, by means of crosscorrelation and Doppler tomographic methods, with the goal of estimating the physical properties of the individual stars. These systems are HD 1337 (AO Cas), HD 47129 (Plaskett's star), HD 57060 (29 UW CMa), HD 37043 (Iota Ori), HD 215835 (DH Cep), HD 152218, and HD 152248. Mass ratios have been obtained primarily from a cross-correlation technique, but also by several other techniques. The tomographic techniques allow us to separate the spectra of the components. We then can estimate the individual spectral types and luminosity classes of the stars (and henceT _eff and logg, respectively), the luminosity ratio, and projected rotational velocities. We discuss the physical properties of these O-type binaries. These are some of the early results of a large scale project involving 36 O-type double-lined binary systems (from the catalog of Battenet al. 1989) which we will study using IUE and complementary ground-based data.     

Evolution into contact of the low-mass close binary systems

We investigated the effect of mass accretion on the secondary components in close binomy systems (M _total ≤ 2.5 M _⊙ M _2,0 ≤ 0.75 M _⊙) exchanging mass in the case A. The evolution of the low-mass close binary systems (M _total ≤ 2.5 M _⊙) exchanging the mass in the case A depends on the three main factors:

-the initial mass ratio (q ₀ = M _2,0/M _1,0), which determines the rate of mass transfer between components;

-the inital mass of the secondary component (M _2,0) and

-the effectiveness of the heating of the photosphere of the secondary component, by infalling matter.

The second factor allows to divide all systems into two essentially different groups:

1. systems in which the secondary component is a star with a radiative envelope, or with a thin convection zone in the uppermost layers;
2. and systems in which secondary component has a thick convective envelope or is fully convective.

The systems from the first group evolve into contact in a characteristic time scale 10⁵ – 10⁷ years, and reach contact after transfering of 0.03 – 0.3 M _⊙. The mass exchange proceeds only in a thermal time scale. For the systems from the group b the effectiveness of the heating of the stellar surface is the most important. In the case when the entropy of the newly accreted matter is the same as the surface entropy of the secondary, a convective star should shrink upon accretion. Then contact binaries are not formed. In the case when the entropy of the infalling matter is greater then that on the surface, the reaction of the secondary is different. The radius of the secondary component grows rapidly in response to accretion, and the systems reaches contact after the 10³ – 3 10⁶ years, and after transfer of 0.002 – 0.2. M _⊙. The reaction of the secondary is determined by the formation of the temperature inversion layer below the stellar surface. Full references in: Sarna, M.J. and Fedorova, A.V. (1988) “Evolutionary status of W UMa-type Binaries — Evolution into contact”, Astron. Astrophys., in press.     

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.     

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.