Abstract: The magnetic cataclysmic variable AE Aquarii has been observed at 1.5, 4.9, 15, and 22.5 GHz. The source shows temporal variations in the flux density, often with a flarelike morphology. The flux variations are at times extreme, with values ranging from 1-12 mJy at 15 GHz; the degree of variability increases with frequency. The spectrum shows a variety of short-term behavior, at times increasing as nu, at other times decreasing with nu, and at still others flat. The long-term average of the spectrum increases as nu exp 0.3-0.4. No significant degree of circular polarization was observed. The source properties can be explained as a superposition of flare events in which electrons are accelerated to relativistic energies which then emit synchrotron radiation in discrete, expanding sources. The origin of the flare events may be connected to the disruption of the accretion disk some ditance from the white dwarf primary. The radio emission of AE Aqr shows similarities to the low-level flares from Cygnus X-3.

Abstract: We report the discovery of an 11-ms pulsar, PSR1620–26, in the closest globular cluster, M4 (NGC6121). It is the fifth millisecond pulsar to be found, and the second in a globular cluster. Unlike the other cluster pulsar, PSR1821 – 24 in M28 (ref. 1), PSR1620 – 26 is in a low-mass binary system. This provides strong support for formation mechanisms in which an old neutron star is spun up to millisecond periods by accretion during an X-ray binary phase. The orbit's small eccentricity is likely to have significant implica-tions for both the age of the millisecond pulsar and its surface magnetic field. We conclude that the pulsar's current companion is, in fact, the star which was responsible for the spin-up.

Abstract: The pulsing X-ray source 4U 1626-67 is an accreting neutron star in a binary system with a very low mass companion. The source was observed by Exosat continuously for 23 hr on March 30-31, 1986 UT. It is found that, if the orbital inclination angle equals 90 deg, the optical companion star has a mass of less than about 0.002 solar mass; however, it is found that a companion star mass of greater than about 0.06 solar mass is required if gravitational radiation is responsible for driving the mass transfer in this system. Also presented are results on flaring activity in the system on time scales of about 1000 s, the energy-dependent pulse profiles, and the pulse period history over the past decade.

Abstract: The emission-line spectra of several AGN have broad peaks which are significantly displaced in velocity with respect to the host galaxy An interpretation of this effect in terms of orbital motion of a binary black hole predicts periods of a few centuries It is pointed out here that recent measurements of the masses and sizes of many low-luminosity AGN imply orbital periods much shorter than this In particular, it is found that the elliptical galaxy Arp 102B is the most likely candidate for observation of radial velocity variations; its period is expected to be about 3 yr The H-alpha line profile of Arp 102B has been measured for 5 yr without detecting any change in velocity, and it is thus found that a rather restrictive observational test of the massive binary black hole hypothesis already exists, albeit for this one object

Abstract: Conditions under which neutron star/cold disk systems may result from the evolution of a close binary consisting of a neutron star and a massive white dwarf are investigated. It is found that the expected gamma-ray burst source distribution is consistent with the observed angular distribution. The predicted source density implies repetition time scales of less that about 6-25 years for gamma-ray bursts. 45 references.

Abstract: The hard X-ray and low energy γ-ray emission from the galactic center region (GCR) has four components1,2: a power-law continuum between 20/50 keV and 200/300 keV with a power-law photon index β in the range ~ 2.5 to ~ 3.1; a harder spectrum with β ~ 1.–1.5 between 200/300 keV and ~ 511 keV; a narrow electron-positron annihilation line at 511 keV, reported to disappear3,4 in less than < 1/2 yr, although the temporal variation is controversial5; and an equally variable continuum emission between 511 keV and several MeV (“MeV bump”). All four have luminosities 1037-1038 erg s-1, if they are located 10 kpc away. We propose non-thermal processes in low mass X-ray binaries (LMXB’s) concentrated in the galactic bulge as the direct source of the three continuum components of the emission, as well as of an escaping electron-positron e ± wind whose positron annihilation relatively far from the star could be the source of the 511 keV line. We consider a model for energetic emission from LMXB’s that reproduces the softer power-law component of the GCR continuum through synchrotron emission of relativistic electrons in the strongly non-uniform (dipolar) magnetic field of the neutron star. We also explain, with less confidence, the variable MeV bump as the result of interaction of harder γ-rays with the power-law photons. The harder power law might be due to Compton scattering of relativistic electrons or photons.

Abstract: We explore the change in the period of axial rotation and in the radius of a magnetized compact star in a binary system, induced by the accretion on it of mass with angular momentum from the surface of its non-compact companion. No specific assumption is made concerning the accretion model, and the primary’s interior is described by the Fermi-Dirac statistics for degenerate matter. The rate of change with time of the period and radius is expressed in terms of the compact primary’s physical parameters and total absolute luminosity. The conditions are fully derived under which the above changes can be positive, negative or even vanish. In the case of the millisecond pulsars in binary X-ray sources the predicted values of the period time derivative, depending on the values of the accretion rate and. the absolute luminosity, can be positive or negative—if not vanishing—and they fall absolutely in the range 10−21 −10−17 ss−1, in good agreement with current observational data. The corresponding rate of change of radius, either positive or negative, fall in the range of 10−3 −10−1 cm y−1. Finally, it is proved that the well-known bursters can be explained by thermonuclear flash due to gravitational instability in the accreted matter, but their explanation as a result of direct contraction could be possible only for quite high accretion rates (>10−7 M ⊙ y−1). This last result indicates that, in contrast to the accretion-induced change in period, which can be of either sign irrespective of the primary’s age, the accretion-induced non-catastrophic contraction is impossible, while according to repent results the contraction in general is possible for young compact objects.

Abstract: The X-ray Be star object A0538-66 in the LMC is a binary with a neutron star emitting X-rays in flares when active. It is shown that the enhanced continuum and line emission in the optical and ultraviolet is from the ionized region formed by the X-rays in the gas ring around the Be star. 12 references.

Abstract: L'observation CCD de cette source RX transitoire dont la contre-partie optique est UY Vol montre une courbe de lumiere a eclipses caracteristique des binaires RX de faible masse comme Her X-1 ou 4U 2129+47. La presence d'un disque d'accretion est discutee

Abstract: LMC X-3 was observed quasi-simultaneously at UV (IUE), optical, and IR (ESO, La Silla) frequencies in January, 1987. The overall energy distribution exhibits a broad peak around 10 to the 15th Hz. The UV flux appears well in excess of that measured in March, 1986, which can be attributed to the noncollapsed star in the system. The excess emission can be interpreted in termms of an accretion disk surrounding the collapsed component. A good spectral fit to the excess is obtained with a blackbody distribution with T about 14,000 K and a projected area A cos i about 3.7 x 10 to the 23rd/sq cm. Supposing that the disk radius equals the Roche lobe of the compact object, an inclination of about 55 deg is found. The results are discussed within the model of the system. 17 references.

Abstract: The low upper limit to the period derivative of the millisecond pulsar PSR 1821–24 occurring in the globular cluster M28 implies that it is not a part of any binary system. Various formation scenarios of such a pulsar, involving tidal capture of two stars into a gravitationally bound system, spin-up of the neutron star and disruption or unbinding of the companion star are considered in this letter. Of particular interest to these scenarios is the effect of tidal dissipation of orbital energy on the evolution of the individual stars and the binary system. We consider a scenario in which a massive white dwarf is captured tidally by a low mass star that has evolved off the main sequence, and later collapses to form a neutron star and eventually disrupts the degenerate helium core of the erstwhile companion due to unstable Roche-lobe overflow.

Abstract: On rapporte l'analyse des vitesses radiales et des largeurs equivalentes d'emission Hβ obtenus a l'Observatoire d'Asiago L'utilisatoin de differentes techniques d'analyses suggerent une forte probabilite pour une periode de 55,7 jours

Abstract: A phenomenological approach is developed for studying star formation in the galactic disk, in starbursts, and in protogalaxies. The evidence is reviewed for bimodal star formation, and physical mechanisms are mentioned. A simple expression for the star formation rate in the disk is derived, and applied to estimate star formation time scales in the disk, in starbursts, and in protogalaxies. A specific model for a protogalaxy is developed, and I discuss the origin of magnetic fields and primordial star formation in the protogalactic setting. Implications include formation of dwarf galaxies, globular star clusters, chemical evolution, and the origin of compact galactic nuclei.

Abstract: Observations par l'IUE a differents niveaux de flux de AM Her. On derive une limite superieure de la temperature de la naine blanche a 20000 K, confirmant les resultats de Schmidt (1981)

Abstract: Presentation de donnees spectrophotometriques CCD de la source RX EXO 2030+375 qui montrent que la contrepartie optique precedemment proposee est une etoile de type Oe/Be dont l'emission en Hα est assez forte. Le rougissement interstellaire est calcule a partir du spectre dans le domaine 5000-7000A et la distance de la source est estimee a 4 kpc environ. On propose de ranger cet objet dans la classe des binaires RX ou l'etoile a neutron accrete de la matiere a partir de l'enveloppe circumstellaire de la composante primaire

Abstract: Coronal X-ray emission from the BY Draconis type dMe flare star G867 A was observed on three successive days in May 1980 with the Imaging Proportional Counter on the Einstein Observatory. A 40-percent decrease in the quiescent state X-ray intensity of the star was detected over a three-day period. It is suggested that this slow quiescent state X-ray intensity variation may be the X-ray analogue of the BY Draconis variability observed in the optical band and may be explained by the star spot model. The energy spectra of the star were obtained for the three days and are best fitted by a two-temperature Raymond-Smith thermal model. There is some indication of spectral variations in the low temperature component. Implications of these results are briefly discussed.

Abstract: The long-term variability of Cen X-3 on time scales ranging from days to years has been examined by combining data obtained by the HEAO 1 A-4 instrument with data from Vela 5B A simple interpretation of the data is made in terms of the standard alpha-disk model of accretion disk structure and dynamics Assuming that the low-frequency variance represents the inherent variability of the mass transfer from the companion, the decline in power at higher frequencies results from the leveling of radial structure in the accretion disk through viscous mixing The shape of the observed power spectrum is shown to be in excellent agreement with a calculation based on a simplified form of this model The observed low-frequency power spectrum of Cen X-3 is consistent with a disk in which viscous mixing occurs about as rapidly as possible and on the largest scale possible

Abstract: On presente les resultats d'une analyse d'une observation de 23 h du systeme G1091-10 faite par EXOSAT en decembre 1984 On decrit les resultats de la recherche du mouvement orbital et on donne de nouvelles limites superieures a a x sin(1) pour des periodes de 500 s a 0,5 jours On montre que le spectre pulse de 1E 2259+586 est plus mou que celui de n'importe quel autre pulsar RX binaire connu

Abstract: UV spectra for four X-ray-selected bright (V less than 10) late-type (F and G) stars have been obtained in both the long- and short-wavelength cameras of the IUE satellite. The stars were selected because they had the largest ratios of X-ray to optical luminosity for their spectral type, with known binaries being eliminated from the sample in order to determine if other mechanisms for extreme X-ray emission are present. Three of the four stars in the sample have since been observed to be binaries with radial-velocity variations consistent with periods of a few days. The fourth star, HD 142361 is a pre-main-sequence star based on the detection of Li I at a wavelength of 6707 A in absorption.

Abstract: In the framework of the standard scenario of the evolution of massive binary stars a study is made of the formation of final binary systems in which at least one of the components is a neutron star. It is found that about every fortieth radio pulsar must be a member of a close binary system. This is confirmed by observations. Radio pulsars are not formed in wide binary systems, possibly because of the very slow rotation of the presupernova stars.