Showing papers by "Ronald E. Taam published in 1990"

Hydrodynamic simulations of stellar wind disruption by a compact X-ray source

Abstract: This paper presents two-dimensional numerical simulations of the gas flow in the orbital plane of a massive X-ray binary system, in which the mass accretion is fueled by a radiation-driven wind from an early-type companion star. These simulations are used to examine the role of the compact object (either a neutron star or a black hole) in disturbing the radiatively accelerating wind of the OB companion, with an emphasis on understanding the origin of the observed soft X-ray photoelectric absorption seen at late orbital phases in these systems. On the basis of these simulations, it is suggested that the phase-dependent photoelectric absorption seen in several of these systems can be explained by dense filaments of compressend gas formed in the nonsteady accreation bow shock and wake of the compact object.

On the formation of red giant binaries by tidal capture

Abstract: The nonradial oscillation spectra of a 0.8 solar mass star in various evolutionary stages along the red giant branch have been studied in the context of tidal capture binary formation in globular clusters. For capture of a 1.4 solar mass neutron star with a relative velocity at infinity of 10 km/s, the critical impact parameter, X(crit), decreases as the star evolves, ranging from 2.42 at the base of the subgiant branch to about 1.45 at the onset of helium burning. For the capture of lighter stars by the giant, or for larger relative velocities, X(crit) decreases. The reduction of X(crit) from the usually adopted value of three leads to a diminished frequency of binary systems containing a red giant companion and increases the likelihood of the formation of ultra-short period compact binary systems and mergers. The existence of the binary millisecond radio pulsar PSR 1620-26 in the globular cluster M4 indicates that disruption can be avoided in some cases and that the effects of tidally enhanced expansion are minimal for red giant-neutron star binary progenitor systems of this type. An evolutionary scenario for PSR 1620-26 is constructed, demonstrating the viability of the tidal capture scenario for red giants and neutronmore » stars. 34 refs.« less

Mass transport in a neutron star magnetosphere

Abstract: The interaction between a thin Keplerian accretion disk and a magnetosphere surrounding a central object is investigated within the framework of an analytical description for the magnetic field configuration. The commonly held assumption that all accreting plasma flows from the magnetospheric boundary to the stellar surface is shown to be overly restrictive. If the magnetospheric boundary is defined as the distance where the rotation starts deviating significantly from the Kepler rate, it is found that there is an extensive region inside this boundary where gas, nearly corotating with the star, drifts inward across the field by an interchange instability. The linear analysis of this instability is presented. It is also found that gas tied to field lines can be in equilibrium at positions off the midplane, and that gas can plausibly flow from the midplane to these positions, in certain circumstances. The observational consequences of such a picture are briefly discussed.

The Spectra of Weakly Magnetized Neutron Stars

Abstract: The continuum spectrum of a rotating equatorial emitting region on the surface of an accreting neutron star is investigated within the Schwarzschild spacetime. It is found that the dominant general relativistic effect is the increase in apparent emission area from the neutron star surface due to gravitational light bending. For typical neutron star parameters, the apparent area of the accretion belt is remarkably independent of the viewing angle and, hence, the contribution for a boundary layer to the total X-ray luminosity is found to depend only weakly on the inclination angle of the binary system. For a blackbody spectrum in the local rest frame of the emitting surface, the distortion of the spectral shape by longitudinal and transverse Doppler shifts is minimal for neutron stars rotating at periods greater than about 2 ms. It is shown that the failure to detect a blackbody component from the weakly magnetized neutron star surface in some X-ray burst sources during their quiescent state may be related to the presence of temperature variations along the vertical extent of the boundary layer.

The spectra of relativistic accretion disks - Application to A0620-00

Abstract: The X-ray flux emitted from a geometrically thin, relativistic accretion disk in the steady state approximation is investigated in order to place limits on the quiescent state mass flow rate in the soft X-ray transient black hole candidate source A0620-00. Specific attention is focused on the effects associated with gravitational redshifts, Doppler shifts, and on the enhancement of the apparent accretion disk area due to gravitational light bending on the continuum spectrum. It is found that the upper limit to the mass flow rate within the inner regions of the disk, constrained by the lack of soft X-rays in the quiescent state, is about 2.8 x 10 to the -11th solar mass/yr for black hole masses greater than about 5.4 solar mass. The optical data are consistent with these upper limits provided that the inclination angle of the binary system is less than about 65 deg. The upper limits and the lack of a hard X-ray flux, together, suggest that the soft X-ray transient model based upon a mass transfer instability situated in the stellar envelope of the companion is inapplicable to A0620-00.