Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “ACCRETION DISKS”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Ultraviolet observations of accretion disk in LMC X-3

We report information obtained from a series of International Ultraviolet Explorer (IUE) low-resolution spectra and two HST UV spectra of LMC X-3. The HST spectra are used to identify disk emission lines and interstellar absorptions as well as to fit continuum models, which indicate the presence of a hot disk component. The IUE observations, mainly taken over approximately 1.5 years, were intended to study how the observed characteristics of the accretion disk change through the precessional cycle. It is shown that although the emission line strengths and short-wavelength ultraviolet flux are well correlated, both optical and UV data show little long-term periodic modulation was present during our observations, indicating LMC X-3 had dropped into a 'low' state.

Cowley, A. P.↗
NASA NTRS2013

X-ray Reflected Spectra from Accretion Disk Models. III. A Complete Grid of Ionized Reflection Calculations

We present a new and complete library of synthetic spectra for modeling the component of emission that is reflected from an illuminated accretion disk. The spectra were computed using an updated version of our code xillver that incorporates new routines and a richer atomic data base. We offer in the form of a table model an extensive grid of reflection models that cover a wide range of parameters. Each individual model is characterized by the photon index Gamma of the illuminating radiation, the ionization parameter zeta at the surface of the disk (i.e., the ratio of the X-ray flux to the gas density), and the iron abundance A(sub Fe) relative to the solar value. The ranges of the parameters covered are: 1.2 <= Gamma <= 3.4, 1 <= zeta <= 104, and 0.5 <= A(sub Fe) <= 10. These ranges capture the physical conditions typically inferred from observations of active galactic nuclei, and also stellar-mass black holes in the hard state. This library is intended for use when the thermal disk flux is faint compared to the incident power-law flux. The models are expected to provide an accurate description of the Fe K emission line, which is the crucial spectral feature used to measure black hole spin. A total of 720 reflection spectra are provided in a single FITS file suitable for the analysis of X-ray observations via the atable model in xspec. Detailed comparisons with previous reflection models illustrate the improvements incorporated in this version of xillver.

X-ray↗
NASA NTRS1982

Meridional circulation in optically thick accretion disks

Thermal imbalances in stars due to rotation are known to drive mass motions in the meridional plane. A preliminary analytic investigation has been made of a similar effect in optically thick accretion disks using conventional thin-disk approximations. It is found that estimated circulation times can be as short as thermal timescales, resulting in rapid transport of heat and angular momentum. This indicates that the simple approximations commonly used are incomplete with regard to detailed, two-dimensional disk structure.

Cabot, W.↗
NASA NTRS1989

Ultraviolet spectroscopy of pre-main-sequence accretion disks

Low-resolution, ultraviolet spectra of the pre-main-sequence objects Z Canis Majoris, V1057 Cygni, and FU Orionis are presented. Ultraviolet absorption features indicate spectral types of A5 I for FU Ori and F5 I for Z CMa. These results are in reasonable agreement with predictions of simple accretion disk models. The lack of significant veiling of ultraviolet absorption features implies that a hot boundary layer at the inner edge of the disk contributes less than 20 percent of the radiation at 2600-3200 A, which is significantly smaller than expected. It is suggested that accretion has significantly modified the structure of the underlying pre-main-sequence stars in these systems (perhaps by expanding the photosphere or by spinning up a radiative photosphere close to breakup velocity, or both), or that the boundary layer region is much more extended above the disk midplane than standard thin disk theories predict.

Kenyon, S. J.↗
NASA NTRS1992

A new mechanism for viscosity in cloudy accretion disks - Anomalous (turbulent) viscosity

We analyze the dynamics of a rotating cloudy gaseous medium relative to 2D perturbations having wavelengths shorter than the disk thickness. The viscosity due to cloud-cloud collisions is confronted with the derived critical viscosity, below which a collective mode of perturbations in the disk is able to grow until going into the saturation regime where fully developed turbulence is established. It is shown that for the assumed parameters of AGN accretion disks consistent with the observational data, the viscosity due to collective modes usually dominates over that due to cloud-cloud collisions.

Fridman, Aleksei↗
NASA NTRS1992

Spectral evolution of accretion disks of dwarf novae. III - Outburst cycle of SS Cygni

A modified disk atmosphere has been used to produce absorption profiles for H-beta, H-gamma, and He I 4471 A lines to fit those at the maximum light during the October 1981 outbursts of SS Cyg and those during the decline. The coexistence of strong emission cores and absorption wings at the maximum light shows that the mass transfer rate at SS Cyg in the outburst was about 5 x 10 exp 17 g/s. The viscosity parameter in the disk during quiescence is between 0.006 and 0.6. The weak emission and shallow absorption during rise indicate that the outburst is initiated from the inner region of the accretion disk during the outburst. This result is compatible with the disk thermal instability hypothesis for CV outbursts, but incompatible with the mass transfer instability scenario for CV outbursts.

Cheng, F. H.↗
NASA NTRS1988

A test of synthetic accretion disk spectra using ultraviolet flux distributions of novalike variables

Ultraviolet (UV) fluxes and other data for a sample of nine novalike cataclysmic variables are assembled from the literature. The UV fluxes and colors are compared to spectra of steady state model accretion disks, constructed using either Planck functions or model stellar atmosphere spectra as elementary radiators. Deficiencies are found with both sets of models. The restriction to steady state radial temperature profiles is relaxed for the 'stellar atmosphere' disks, but these generalized models still fail to account simultaneously for the absolute UV flux and the UV color of the observed spectra. The conclusion is that stellar atmosphere models do not reflect the physics of disks. More appropriate models probably can and definitely should be made.

Wade, Richard A.↗
NASA NTRS1995

The Production of Jets From Magnetized Accretion Disks: Simulation of the Blandford-Payne Mechanism

We have used massively parallel supercomputer simulations to perform an extensive study of a plausible mechanism for producing the jets seen in extragalactic radio sources - acceleration and collimation by coronal magnetic fields in an accretion disk orbiting a central black hole. We find that such disks can propel jets for a wide range of coronal conditions. The terminal jet velocity is a strong function of the magnetic field equatorial component. Acceleration and collimation are produced by a tight azimuthal field coil generated in the corona, rather than by a stiff poloidal field extending to large distances. The jets are pressure-confined when the external medium pressure is high, but magnetically- confined when it is low.

Blandford-Payne Mechanism↗
NASA NTRS1992

Two-dimensional viscous accretion disk models. I - On meridional circulations in radiative regions

It has been suggested that thin Keplerian disks, like rotating stars, cannot attain hydrostatic and thermal equilibrium simultaneously if the angular velocity is a function of radius only. To address the problem of vertical structure and stability of thin accretion disks, we performed 2D axisymmetric calculations by solving the set of fully nonlinear hydrodynamic equations, including radiation transport. The results indicate that the variation of the rotational velocity with height is too small to sustain hydrostatic equilibrium. Instead, the variation of the radial (inflow) velocity with height suffices to establish a stationary state. In particular, we find that for low values of the viscosity parameter alpha, there is mass outflow in the central parts of the disk close to the equatorial plane, and inflow only near the surface. Only for modestly high values of alpha, the flow throughout the disk is directed inward. Thus, the flow within the disk is the result of viscous radial inflow plus a circulatory flow directed outward in the midplane and inward near the surface.

Kley, W.↗
NASA NTRS1991

Microlensing and the structure of active galactic nucleus accretion disks

Rapid variability has been reported in two of the four gravitationally lensed images of Q2237 + 0305, and this is attributed to microlensing caused by the intervening stars. The associated constraints on the source size and properties are studied and compared with a variety of stationary accretion disk models. The reported microlensing variation in Q2237 + 0305 requires the disk size to be over 3 times smaller than a blackbody disk of similar luminosity, implying that the optical emission is either nonthermal or optically tin. An exploration of nonstationary disk models including orbiting, transient hot spots leads to a similar conclusion. Implications for models of active galactic nucleus optical continua are briefly discussed.

Rauch, Kevin P.↗
NASA NTRS1992

6 Hz quasiperiodic oscillations from low-mass X-ray binaries - The sound of an accretion disk?

Quasiperiodic oscillations (QPOs) at frequencies of 5-8 Hz are a common feature of low-mass X-ray binaries (LMXBs) during their normal branch (NB) spectral states. We propose that this frequency band is essentially a band of rotation frequencies in a thick accretion disk. The high luminosities in the NB state give rise to the thick disk and slow down the dynamical timescales. Density and optical-thickness perturbations in a subsonic region of the disk have the frequency spectrum of sound in a rotating medium. X-rays scattered through the subsonic region are modulated by long wavelength optical-thickness oscillations whose frequencies, determined by the rotation rate and vorticity, are observed as the NB QPOs.

Alpar, M. A.↗
NASA NTRS1977

X-ray bursts from magnetized accretion disks

A model for the X-ray bursts of MXB 1730-335 and 3U 1820-30 (identified with the globular cluster NGC 6624) is presented in which a burst is due to the flarelike release of magnetic energy generated and stored as a toroidal field in an accretion disk. To fit the observed parameters, this model requires compact objects of the order of 10 solar masses, presumably black holes.

Wheeler, J. C.↗
NASA NTRS1988

Line formation in the hot spot region of cataclysmic variable accretion disks

The paper presents a theoretical analysis of the emission lines observed in the cataclysmic variable A0 Psc (=H2252-035), including detailed modeling of the hydrogen Balmer line emission. The analysis makes it possible to deduce the physical conditions in the so called 'hot spot', or 'bulge' region where the accretion column hits the rim of the accretion disk. It is concluded that the bulge is optically thick to the ionizing disk radiation. Consequently, its disk illuminated face is fully ionized whereas the side facing away from the disk is neutral, resulting in modulation of the observed emission lines with the orbital period. The density in the hot spot is about 5 x 10 to the 12th to 10 to the 13th/cu cm.

Elitzur, Moshe↗
NASA NTRS2007

Monitoring the Violent Activity from the Inner Accretion Disk of the Seyfert 1.9 Galaxy NGC 2992 with RXTE

We present the results of a one year monitoring campaign of the Seyfert 1.9 galaxy NGC 2992 with RXTE. Historically, the source has been shown to vary dramatically in 2-10 keV flux over timescales of years and was thought to be slowly transitioning between periods of quiescence and active accretion. Our results show that in one year the source continuum flux covered almost the entire historical range, making it unlikely that the low-luminosity states correspond to the accretion mechanism switching off. During flaring episodes we found that a highly redshifted Fe K line appears, implying that the violent activity is occurring in the inner accretion disk, within 100 gravitational radii of the central black hole. We also found that the Compton y parameter for the X-ray continuum remained approximately constant during the large amplitude variability. These observations make NGC 2992 well-suited for future multi-waveband monitoring, as a test-bed for constraining accretion models.

Mruphy, Kendrah D.↗
NASA NTRS1995

Astro-1 and ground-based observations of Markarian 335: Evidence for an accretion disk

Simultaneous UV and optical observations of the Seyfert galaxy Markarian 335 (z = 0.026) during the Astro-1 mission yield a spectrum spanning the wavelength range of 912-8410 A. In the sub-Ly alpha region a prominent blended emission feature of O VI lambda lambda 1032, 1038, and Ly beta is nearly as strong as C IV wavelength 1549. The continuum flux extends beyond the redshifted Luman limit without a noticeable discontinuity, but a siginificant change in slope exists near the redhsifted Lyman edge. We suggest that such a change may be the signature of a Lyman edge in an accretion disk seen at a small inclination angle. Using a disk model including such an edge, we fit the spectrum with a central black hole mass of 5 x 10(exp 7) solar mass, an accretion rate of 0.07 solar mass/yr, and an optical depth at the Lyman edge of 0.4. To account for the strong O VI emission as well as the soft X-ray excess, we consider the effects of Comptonization on the disk spectrum, which can produce a high-energy tail for the UV bump and also smooth the Lyman edge feature.

Zheng, W.↗
NASA NTRS1987

Multipeaked X-ray bursts from 4U/MXB 1636-53 - Evidence for burst-induced accretion disk coronae

The specific features of multipeaked X-ray bursts from 4U/MXB 1636-53, which are relatively weak and show no correlated photospheric radius expansion or contraction near the burst maximum, are explained using a model in which the observed luminosity is the sum of the direct component from the central source attenuated through an accretion disk corona and a component due to diffuse scattering by the corona. According to this model, the central peak in the triple-peaked burst results from an enhanced scattering emissivity of the direct component. At large inclination angles, the direct component may be unobservable; a burst detected via the scattered component only would be relatively short, weak, and would have similar rise and decay times. The direct and scattered components can be distinguished by X-ray polarimetry.

Melia, Fulvio↗
NASA NTRS2002

Tracking the moving optical photocenters of active galaxies: binary black holes, accretion disks and relativistic jets

We will use the astrometric capabilities of the Space Interferometry Mission to answer three key questions about active galactic nuclei: 1) Does the separation of the radio core and optical photocenter of quasars change on the same timescale as their photometric variability? 2) Does the most compact optical emission from an active galactic nucleus come from an accretion disk or from a relativistic jet? and 3) Do the cores of galaxies harbor binary supermassive black holes remaining from galaxy mergers?

astrometry↗