Engineering topics

Taam, Ronald E.

Publications and source records attributed to Taam, Ronald E..

29 records · Page 2

NASA NTRS1989

Pressure gradient torque in highly supersonic nonaxisymmetric accretion

The contribution of a pressure gradient torque to the angular momentum transfer rate in highly supersonic nonaxisymmetric accretion flows is considered. This study takes into account the contribution due to the pressure variation in the postaccretion-shock region which is significant for high Mach number accretion. For the case of accretion flow with Mach (infinity) of not less than 5, the overall accretion torque is shown to approach a constant value.

Ho, Cheng↗

NASA NTRS1989

Numerical studies of asymmetric adiabatic accretion flow - The effect of velocity gradients

A numerical study of the time variation of the angular momentum and mass capture rates for a central object accreting from a uniform medium with a velocity gradient transverse to the direction of the mean flow is presented, covering a range of velocity asymmetries and Mach numbers in the incident flow. It is found that the mass accretion rate in a given evolutionary sequence varies in an irregular manner, with the matter accreting onto the central object from either a continuously moving accretion wake or from an accretion disk. The implications of the results from the study of short-term fluctuations observed in the pulse period and luminosity of X-ray pulsars are discussed.

Taam, Ronald E.↗

NASA NTRS1988

Numerical simulation of nonaxisymmetric adiabatic accretion flow

The hydrodynamics of gas flow past a finite-sized gravitating central object is studied in the adiabatic approximation in two spatial dimensions. The flow morphology is shown to depend on the ratio (epsilon) of the axisymmetric accretion radius to the length scale of the density variation. For small to intermediate epsilon values the flows are highly time dependent, and for large epsilon values a quasi-steady state is obtained.

Fryxell, B. A.↗

NASA NTRS1988

A model for the recurrent flares in EXO 2030 + 375

It is shown that nonsteady hydrodynamical flows associated with mass and angular momentum capture by a neutron star during a mass ejection phase from a Be star can produce flares with remarkable resemblance to those observed during an outburst from the X-ray transient pulsar EXO 2030 + 375. To reproduce the recurrent time scale of the flares, the velocity of the outflowing matter is estimated to be about 550 km/s. Since the theoretical model requires that a transient disk circulating in one direction is followed by a transient disk circulating in the opposite direction, the time derivative of the pulse period is expected to change sign after each flare event.

Taam, Ronald E.↗

NASA NTRS1988

On nonsteady accretion in stellar wind-fed X-ray sources

The paper reports on the results of hydrodynamical simulations of nonaxisymmetric gas flow past a finite-sized gravitating object. The asymmetry in the flow is assumed to be due to a transverse density gradient. The solutions are found to be highly time-dependent and exhibit phases in which a disk forms. The direction of circulation of the disk alternates between the direct and counter sense on time scales of approximately 1 hr. While the disk is present, the specific angular momentum is high, and the mass accretion rate, low. The angular momentum capture oscillates about a mean which secularly increases. Such temporal behavior may be relevant to those X-ray pulsing systems (e.g., Vela X-1) which exhibit fluctuations in the X-ray luminosity and pulse period on time scales much shorter than the orbital period of the binary system.

Taam, Ronald E.↗

NASA NTRS1987

The spectra of accretion disks and their application to low-mass X-ray binaries

The influence of external radiation from the boundary layer and/or from a neutron star surface on the emitted spectrum of an accretion disk is studied. The surface properties of the accretion disk are reviewed in the context of the alpha disk model, and the spectral regimes associated with each are discussed. It is found that the spectrum differs significantly from that obtained in the blackbody assumption, especially in cases where the disk is irradiated at fluxes near the Eddington limit. Generally, the spectrum is found to peak at lower energies than the blackbody assumption would indicate and is considerably flatter for energies between about 0.1 and 1 keV. For an irradiated disk where the color temperature characterizing the external radiation is greater than about 1 keV, the spectrum emitted from the disk is harder than that of multiple blackbodies. The emitted spectrum from the disk is sensitive to the inclination angle of the system and the location of the inner disk edge.

Taam, Ronald E.↗

NASA NTRS1987

Nonradial g-mode oscillations in X-ray bursting neutron stars

The oscillation spectrum of nonradial g-modes in X-ray bursting neutron stars has been studied. The pulsation periods are found to be sensitive to the envelope temperature and range from about 15 ms to about 50 ms for the l = 1 g(1) mode during the X-ray burst. From a quasi-adiabatic stability analysis it is likely that a spectrum of l-pole g-modes is unstable due to the epsilon-mechanism associated with rapid alpha captures. As the thermal structure of the envelope of the neutron star changes on time scales less than 0.2 s during the rise of the X-ray burst, the oscillations are expected to be quasi-coherent during this phase. The calculated period derivatives are large during the burst and are about 1 ms/s. The pulsations are short-lived and are most likely to be seen in the immediate vicinity of the burst peak. Finally, the possible relevance of nonradial g-mode pulsations to the recently discovered quasi-periodic oscillations observed in a number of X-ray sources is discussed.

Mcdermott, P. N.↗

NASA NTRS1987

Possible models for the high-energy transient GB790107

The properties of the high-energy transient GB790107 place severe constraints on the viability of models proposed for typical gamma-ray burst events as applied to this soft gamma-ray repeater. Here, the various models proposed for gamma-ray bursts are reviewed. It is shown that a model involving a comet cloud around a neutron star is consistent with the observational data.

Livio, Mario↗

NASA NTRS1987

Numerical simulations of adiabatic axisymmetric accretion flow. I - A new mechanism for the formation of jets

Numerical simulations of the uniform axisymmetric flow past a gravitating sphere have been studied. It is found that the structure of the flow is extremely sensitive to the boundary condition at the surface of the gravitating object. For the case in which the boundary is totally absorbing, a steady state flow is reached. However, for a boundary which is not totally absorbing, steady state flows are not obtained. The morphology of the flow is also sensitive to the Mach number at infinity and to the ratio of the free-fall velocity at the surface of the gravitating object to the flow velocity at inifinity. A new mechanism for the formation of jets is identified in which a fraction of the accretion energy is tapped to drive an anisotropic supersonic outflow with collimation provided by a combination of the inertia of matter which surrounds the beam and the development of multiple shock structures.

Fryxell, B. A.↗

NASA NTRS1987

Theoretical investigations of X-ray bursts

Current theoretical understanding of the X-ray burst phenomenon is reviewed, providing a framework in which the burst radiation can be used as a diagnostic of the fundamental properties of the underlying neutron star. The typical Type I X-ray burst is detected as a rapid increase in emission to a level about a factor of 10 above that seen during the quiescent state and recurs on time scales which range from several hours to several days. The thermonuclear flash model has successfully reproduced the basic features of the X-ray burst phenomenon and thereby provided strong theoretical evidence that neutron stars are involved. Topics covered include: theory of the emission spectrum; oscillation modes and prospects for diagnosing the thermal state of neutron stars through experiments on board the X-Ray Timing Explorer or the Advanced X-Ray Astrophysics Facility; applications to the mass and radius of a neutron star.

Taam, Ronald E.↗

NASA NTRS1987

Energy sources in gamma-ray burst models

The current status of energy sources in models of gamma-ray bursts is examined. Special emphasis is placed on the thermonuclear flash model which has been the most developed model to date. Although there is no generally accepted model, if the site for the gamma-ray burst is on a strongly magnetized neutron star, the thermonuclear model can qualitatively explain the energetics of some, but probably not all burst events. The critical issues that may differentiate between the possible sources of energy for gamma-ray bursts are listed and briefly discussed.

Taam, Ronald E.↗