Abstract: We estimate the frequency of mergers of binary neutron stars and black holes by using a numerical scenario computer program to model the binary star population of the Galaxy. An extrapolation of the estimates shows that, with the gravitational wave detectors now under development, one may hope to detect up to ∼100 neutron star mergers per year at distances ≥200 Mpc. The calculated merger rates based on binary pulsar statistics probably underestimate this detection rate, because of the short lifetimes of most new-born neutron star binaries. The rate of detection of black hole mergers may be comparable to that of neutron star mergers because of the strong dependence of the power of outbursts on the masses of merging objects

Abstract: a companion star. Optical obser­ vations have shown that there are two main classes of such X-ray binaries: the high-mass X-ray binaries, in which the mass donor is an 0 or B star, and the low-mass X-ray binaries, which have donors less massive than the Sun. It has become clear during the past decade that the neutron star in an X-ray binary may switch on as a radio pulsar when the mass transfer stops. Accordingly, two classes of radio pulsars in binaries can be delin­ eated. Those in which the radio pulsar has another neutron star or a relatively massive (�IM 0) white dwarf as a companion probably have evolved from a high-mass X-ray binary. Those in which the neutron star is accompanied by a low-mass white dwarf (;5 0.5M 0) may have evolved from low-mass X-ray binaries. The origin and evolution of these X-ray binaries and binary radio pulsars are the subject of this review. Origin of high-mass and oflow-mass X-ray binaries and their evolution into binary radio pulsars are discussed in Section 3 and in Section 4. In order not to have to interrupt our discussion of the evolutionary scenarios with the technical explanations of the various processes occurring in them, we start with a separate discussion of these

Abstract: We apply the binary black hole model to explain the wiggles in the milliarcsec radio jet of the superluminal quasar 1928+738 (4C 73.18) observed with VLBI at 1.3 cm wavelength by Hummel et al. The period and amplitude of the wiggles can be explained as due to the orbital motion of a binary black hole with mass of order 10 8 M ○. , mass ratio larger than 0.1, and orbital radius ∼10 16 cm. The jet's inclination to the line of sight should be small confirming the standard interpretation of superluminal motion and one-sidedness as due to relativistic motion in a direction close to the line of sight

Abstract: Among the most promising and interesting sources of gravitational waves for interferometric detectors, such as the ground-based Laser Interferometer Gravitational-wave Observatory (LIGO)/VIRGO system and the proposed space-based Laser Gravitational-Wave Observatory in Space (LAGOS), is the last several minutes of inspiral of a compact binary (one made of neutron stars and/or black holes). This paper is the first in a series that will carry out detailed calculations relevant to such binaries, in the case where one body is a small-mass black hole or neutron star and the other is a much more massive black hole, and the orbit is circular (aside from its gradual inspiral)

Abstract: 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.

Abstract: The X-ray source X1957+11 has an unusually soft X-ray spectrum, very similar to the X-ray spectra of Galactic black hole candidates. We report on a recent Ginga observation of X1957+11, the first X-ray observation since HEAO-1, and find significant spectral variability, on time-scales of hours, in the sense that the spectrum becomes harder as the luminosity increases. We model the spectrum with a two-component model: a soft component associated with the disc emission and a hard component which is best described by a power law. The spectral variability is predominantly due to changes in the hard component while the soft component remains relatively stable. Recent results suggest that Galactic black hole candidates might be distinguished from X-ray binary systems in which the compact object is a neutron star by their X-ray spectral signatures alone

Abstract: Neutrino burst accompanying the collapse of a neutron star into a black hole is studied Such a collapse is expected to happen, when the mass of an accreting neutron star exceeds the maximum value allowed by general relativity A collapsing neutron star has a baryon number very close to the maximum value allowed for the assumed equation of state of dense matter Neutrinos are produced mainly via non-equilibrium weak interaction processes in the interior of the collapsing star General relativistic equations, describing hydrodynamics of collapse and the propagation of neutrinos, are derived in the simplest case of stellar matter transparent to neutrinos Thermal effects in collapsing matter are studied Heating of collapsing stellar interior is due mainly to non-equilibrium weak interaction processes

Abstract: When a massive star collapses to a neutron star, rapidly losing over half its mass in a symmetric supernova explosiosn, any planets orbiting the star will be unbound. However, to explain the observed space velocity and binary fraction of radio pulsars, an asymmetric kick must be given to the neutron star of birth.

Abstract: We study the effect of X-ray illumination of the secondary star in a close binary on its internal structure and on the evolution of the system. We take into account the fact that X-ray heating is non-spherical and show that this modifies the results obtained in models which assume spherically symmetric illumination. Our results suggest that the effects of X-ray heating have been largely overestimated in earlier papers. We also show that radiation-induced winds in LMXBs with orbital periods less than 10 hr cannot give mass transfer rates larger than about one tenth of the Eddington limit mass flux, making it unlikely that such self-sustained winds have a marked effect on LMXB evolution, contrary to the suggestion of Tavani (1991)

Abstract: We investigate conditions for a double neutron star system to form through two supernova explosions in the evolution of massive binary systems. We calculate, in particular, the orbital change in the helium (or C+O) star-neutron star system and its center-of-mass velocity due to the explosion of the helium (or C+O) star. For the binary pulsars PSR 1534+12 and PSR 1913+16, the presupernova evolution models of helium stars provide important constraints on the parameters of the binary system. If the progenitor helium star is more massive than ∼5M ○ ., its radius would not exceed the Roche lobe so that its explosion should be highly asymmetric imparting a large kick velocity to the newly born neutron star

Abstract: Basic properties of accretion disk instabilities are summarized. We first explain the standard disk model by Shakura and Sunyaev. In this model, the dominant sources of viscosity are assumed to be chaotic magnetic fields and turbulence in gas flow, and the magnitude of viscosity is prescribed by so-called α model. It is then possible to build a particular disk model. In the framework of the standard model, accretion disks are stationary, but when some of the basic assumptions are relaxed, various kinds of instabilities appear. In particular, we focus on the thermal limit-cycle instability caused by partial ionization of hydrogen (and helium). We demonstrate that the disk instability model well accounts for the basic observed features of outbursts of dwarf novae and X-ray nova. We then introduce other kinds of instabilities based on the α viscosity model. They are suspected to produce time variabilities observed on a wide range of timescales in close binaries and active galactic nuclei.

Abstract: The discovery rate of premain-sequence binary and multiple systems has increased dramatically over the last few years, providing us with our first insights into the nature of such systems at young ages. The observational data derived from spectroscopic, occultation, speckle, visual, and other techniques are reviewed. Where possible, the premain-sequence binary population is compared with main-sequence systems. These results are interpreted in terms of binary formation mechanisms. While wide binaries (P is greater than 100 yr) are ordinarily explained by a fragmentation process during the collapse of a rotating protostar, the origin of some systems by capture is a possibility. The origin of close systems is not well understood: various processes, such as fragmentation during late stages of collapse, gravitational instabilities in disks, or orbital decay from a longer-period system are examined.

Abstract: The optical depth for the high energy γ-ray photons on the anisotropic thermal radiation field, emitted by the accretion disk around a compact object, is computed. The γ-ray photons are assumed to originate in the collisions of relativistic hadrons with matter entrained in the disk as postulated by some models of the γ-ray production in X-ray binary systems. We predict that the γ-ray telescopes designed to work in the energy range between a few GeV and ∼0.1 TeV (solar One, EGRET) may have serious problems with the detection of some X-ray binaries (e.g. Her X-1, Cen X-3), even if they are already positively detected in the TeV energy range. However, in the case of other X-ray binaries, similar to Vela X-1, the discussed process is negligible

Abstract: Globular clusters harbor many millisecond pulsars and 10 bright low-mass X-ray binary systems. It has been generally believed that almost all millisecond pulsars are spun up by accretion from a companion star during a (progenitor) low-mass X-ray binary (LMXB) phase. However, most of the short-period millisecond pulsars are found in the apparently younger globular clusters, which have much lower core densities than those characteristic of the apparently older globulars containing the majority of the LMXBs. The vast majority of the globular LMXBs are X-ray burst sources, and among them there are no bright sources similar to those in the Galactic disk, such as Sco X-1, Cyg X-2, and GX 5-1

Abstract: VLBI observations of the X-ray binary LSI+61°303 have been performed at 6 cm wavelength by using the European VLBI Network (EVN) and the VLA. The observations were carried out near a maximum of the source periodic (26.5 d) radio outburst. The source has been detected with a S/N above 7 on all the baselines, with a total flux density of 200 mJy. The map of the source reveals two components, separated by 0.9 mas. The overall size of the source, at a level of 50% of peak intensity, is about 1.6×1.0 mas. The values derived from the data for the magnetic field and energy content are consistent with an adiabatic expansion model of a synchrotron emitting source with prolonged injection of energetic particles

Abstract: We investigate the effects of dynamical Comptonization on the emergent radiation spectrum produced by near-critical accretion onto a neutron star. The flow dynamics and the transfer of radiation are self-consistently solved in the case of a spherically symmetric, cold, pure scattering flow, including general relativity. A sequence of models, each characterized by the value of the total observed luminosity, was obtained assuming that the spectrum at the star surface is blackbody in shape. It is found that when the luminosity approaches the Eddington limit dynamical effects become important shifting the spectrum to the blue and producing a power-law, high-energy tail

Abstract: Isolated magnetic neutron stars are considered at the end of the phase of coherent pulsar emission when accretion from the interstellar medium becomes important but rotation affects the inflow dynamics. When the centrifugal acceleration at the Alfven radius exceeds the gravitational one, a process of piling up of matter is expected to take place, yielding the compression of the magnetospheric boundary. Prompt matter infall to the neutron star surface occurs when the gravitational energy density has increased with time to overcome the centrifugal barrier. Recurrence times are estimated, and it is suggested that aging neutron star can undergo accretion with intermittent or quasi-cycling behavior when passing from the isolated radio pulsar phase to quiescent steady accretion over the life time of the Galaxy

Abstract: The profiles of lines from a flat, relativistic disk around a Schwarzschild black hole are computed with a rigorous treatment of the energy shifts and photon trajectories. It is found that, unless the disk is optically thin, further structures emerge between the two Doppler horns, at high (> 80°) inclination angles. These structures are due to a geodesics focusing effect on photons emitted from the »far« side of the disk. Their shape and relative intensity are a function of the radial dependence and angular distribution of the line emissivity. Since these structures are due to a purely general relativistic effect, they could provide an unambiguous signature of a relativistic disk in Active Galactic Nuclei and in X-ray binaries

Abstract: We discuss the determination of stellar wind velocity profiles for OB stars by means of modelling the variable X-ray absorbing column seen in massive X-ray binary systems. The orbital variability in the column density profiles in eclipsing systems, such as Vela X-1 or 4U1700-37, provides an excellent opportunity for determining the wind velocity laws of OB stars in general. We consider the types of velocity laws that have been assumed in the previous modelling of such systems, and point out an important potential flaw in much of the previous work which probably precludes any detailed comparison of these results with theoretical wind models

Abstract: We study the evolution of secondaries under the effect of illumination by X-ray fluxes comparable to their intrinsic flux We show that the time scale for a radius increase by a fraction of a scale height is compatible with the illumination model for soft X-ray transients only if the companion star is a subgiant or a main sequence star with a period longer than about 9 hr Similarly, we show that these conditions are also required in order to avoid a second outburst immediately following the first one In short period systems, the illumination model may apply if there are intrinsic fluctuations of the secondary radius in the vicinity of the L 1 region, due for example to magnetic activity; this would have observable consequences on the regularity of outbursts

Abstract: We present a set of optical and infrared photometric measurements which show the lightcurve of the Be/X-ray binary system X Persei over the last 25 yr. During this period, the Be star has undergone two extended faint, non-variable phases, the last of which (1990-present) has been shown to be related to the loss of the circumstellar shell associated with such systems. We show the relation between the presence of such a disk and the variability of the system. Spectroscopic data spanning the recent phase change narrows down the final changeover from emission to absorption to a period of about 6 months, and suggests that the Balmer emission faded gradually over a period in excess of 700 d

Abstract: We investigate the ionization structure of the stellar wind of the Cygnus X-3 binary, exposed to luminous X-ray emission from the compact object. Comparing the simulated X-ray spectra with the observed ones, we find that the companion star is fairly massive (5≤M≤10 M ○. ), and that iron in the stellar wind is somewhat depleted, to 0.1-0.5 times the cosmic abundance. The phase-to-phase change of the X-ray spectrum implies that the wind gas is enriched in any one of C, N or O. These results strongly support the recent infrared observation

Abstract: The unidentified X-ray source 4U 0142 + 614 has an unusually soft spectrum and has shown a periodic modulation at ∼ 25 minutes, which could be interpreted as the orbital period of a Low Mass X-ray Binary. Recently a new X-ray source, probably a Be/neutron star system, has been discovered at an angular distance of only 24 arcmin from 4U 0142 + 614. We consider the possibility that the newly discovered source, RX J0146.9 + 6121, is responsible for the periodicity previously attributed to 4U 0142 + 614. If this interpretation is correct, RX J0146.9 + 6121 would be the most slowly rotating neutron star known. We present also new spectroscopic data on the optical counterpart of RX J0146.9 + 6121, that strengthen the similarity with other Be/neutron star binaries

Abstract: We have performed detailed numerical calculations of sub-Eddington spherical accretion onto neutron stars and computed the emergent spectra. We have found that the supersoft X-ray spectra observed in CAL 83, CAL 87 and RX J0527.8-6954 can be explained as the result of near-Eddington accretion onto neutron stars. Our model is consistent with a recently proposed unified model for the Low-Mass X-ray Binaries (LMXRB). If the luminosity of the source is within 10% of the Eddington value, the crucial parameter that determirees whether the source appears as a canonical LMXRB or as a supersoft X-ray source is the extent of the matter that accretes radially. If the radial flow extends to at most a few hundred neutron star radii, the source exhibits the characteristics of a canonical LMXRB

Abstract: We study the temporal evolution of low mass stars when they are -more or less suddenly- placed into a radiation bath of temperature sufficiently close to the stellar T eff to alter their structure. We adopt the model proposed by Tout et al. (1989) and follow the structural changes of stars of masses 0.8, 0.5, 0.3 and 0.1 M ○ .. The stellar radius increases on a thermal timescale of the star in consideration, even if the radiation bath is not particularly extreme. This type of irradiation can become effective for stars which are secondary components in compact binaries, either if the system is detached and the primary is a Millisecond Pulsar (MSP), or in the first stages of mass transfer in a Low Mass X-ray Binary (LMXB), even if a small fraction of the X-ray luminosity intercepts the secondary

Abstract: We report variations with orbital phase in UV resonance lines of HD77581 (B0.5 Iab), the optical counterpart of the high-mass X-ray binary system Vela X-1 (4U0900-40), occurring at low and intermediate negative velocities. We argue that these variations are caused by ionization effects in the stellar wind by the X-ray source, just like the well-known orbital modulation of the high-velocity part of the P Cygni absorption, as was predicted by Hatchett & McCray (1977). We also present UV observations of the C III subordinate line at 2296 A of HD153919 (O6.5 Iaf + ), the optical counterpart of 4U1700-37, which support our earlier interpretation that the orbital variations observed in the subordinate He II 1640 A and N IV 1718 A lines in this star are the result of variable Raman-scattered emission lines