301 research outputs found
Surface Structure in an Accretion Disk Annulus with Comparable Radiation and Gas Pressure
We have employed a 3-d energy-conserving radiation MHD code to simulate the
vertical structure and thermodynamics of a shearing box whose parameters were
chosen so that the radiation and gas pressures would be comparable. The upper
layers of this disk segment are magnetically-dominated, creating conditions
appropriate for both photon bubble and Parker instabilities. We find little
evidence for photon bubbles, even though the simulation has enough spatial
resolution to see them and their predicted growth rates are high. On the other
hand, there is strong evidence for Parker instabilities, and they appear to
dominate the evolution of the magnetically supported surface layers. The disk
photosphere is complex, with large density inhomogeneities at both the
scattering and effective (thermalization) photospheres of the evolving
horizontally-averaged structure. Both the dominant magnetic support and the
inhomogeneities are likely to have strong effects on the spectrum and
polarization of thermal photons emerging from the disk atmosphere. The
inhomogeneities are also large enough to affect models of reflection spectra
from the atmospheres of accretion disks.Comment: ApJ, in pres
Local Dynamical Instabilities in Magnetized, Radiation Pressure Supported Accretion Disks
We present a general linear dispersion relation which describes the coupled
behavior of magnetorotational, photon bubble, and convective instabilities in
weakly magnetized, differentially rotating accretion disks. We presume the
accretion disks to be geometrically thin and supported vertically by radiation
pressure. We fully incorporate the effects of a nonzero radiative diffusion
length on the linear modes. In an equilibrium with purely vertical magnetic
field, the vertical magnetorotational modes are completely unaffected by
compressibility, stratification, and radiative diffusion. However, in the
presence of azimuthal fields, which are expected in differentially rotating
flows, the growth rate of all magnetorotational modes can be reduced
substantially below the orbital frequency. This occurs if diffusion destroys
radiation sound waves on the length scale of the instability, and the magnetic
energy density of the azimuthal component exceeds the non-radiative thermal
energy density. While sluggish in this case, the magnetorotational instability
still persists and will still tap the free energy of the differential rotation.
Photon bubble instabilities are generically present in radiation pressure
dominated flows where diffusion is present. We show that their growth rates are
limited to a maximum value which is reached at short wavelengths where the
modes may be viewed as unstable slow magnetosonic waves. We also find that
vertical radiation pressure destabilizes upward propagating fast waves, and
that Alfv\'en waves can be unstable. These instabilities typically have smaller
growth rates than the photon bubble/slow modes. We discuss how all these modes
behave in various regimes of interest, and speculate how they may affect the
dynamics of real accretion disk flows.Comment: 30 pages, 5 figures, Submitted to the Astrophysical Journa
The UV Continuum of Quasars: Models and SDSS Spectral Slopes
We measure long (2200-4000 ang) and short (1450-2200 ang) wavelength spectral
slopes \alpha (F_\nu proportional to \nu^\alpha) for quasar spectra from the
Sloan Digital Sky Survey. The long and short wavelength slopes are computed
from 3646 and 2706 quasars with redshifts in the z=0.76-1.26 and z=1.67-2.07
ranges, respectively. We calculate mean slopes after binning the data by
monochromatic luminosity at 2200 ang and virial mass estimates based on
measurements of the MgII line width and 3000 ang continuum luminosity. We find
little evidence for mass dependent variations in the mean slopes, but a
significant luminosity dependent trend in the near UV spectral slopes is
observed with larger (bluer) slopes at higher luminosities. The far UV slopes
show no clear variation with luminosity and are generally lower (redder) than
the near UV slopes at comparable luminosities, suggesting a slightly concave
quasar continuum shape. We compare these results with Monte Carlo distributions
of slopes computed from models of thin accretion disks, accounting for
uncertainties in the mass estimates. The model slopes produce mass dependent
trends which are larger than observed, though this conclusion is sensitive to
the assumed uncertainties in the mass estimates. The model slopes are also
generally bluer than observed, and we argue that reddening by dust intrinsic to
the source or host galaxy may account for much of the discrepancy.Comment: To be published in ApJ, 18 pages, 10 figure
The accretion disc in the quasar SDSS J0924+0219
We present single-epoch multi-wavelength optical-NIR observations of the
"anomalous" lensed quasar SDSS J0924+0219, made using the Magellan 6.5-metre
Baade telescope at Las Campanas Observatory, Chile. The data clearly resolve
the anomalous bright image pair in the lensed system, and exhibit a strong
decrease in the anomalous flux ratio with decreasing wavelength. This is
interpreted as a result of microlensing of a source of decreasing size in the
core of the lensed quasar. We model the radius of the continuum emission
region, sigma, as a power-law in wavelength, sigma lambda^zeta. We place an
upper limit on the Gaussian radius of the u'-band emission region of 3.04E16
h70^{-1/2} (/M_sun)^{1/2} cm, and constrain the size-wavelength power-law
index to zeta<1.34 at 95% confidence. These observations rule out an alpha-disc
prescription for the accretion disc in SDSS J0924+0219 with 94% confidence.Comment: 8 pages, 5 figures. Accepted for publication in MNRA
Broad-band continuum and variability of NGC 5548
We analyze a composite broad-band optical/UV/Xgamma-ray spectrum of the
Seyfert 1 galaxy NGC 5548. The spectrum consists of an average of simultaneous
optical/IUE/Ginga observations accompanied by ROSAT and GRO/OSSE data from
non-simultaneous observations. We show that the broad-band continuum is
inconsistent with simple disk models extending to the soft X-rays. Instead, the
soft-excess is well described by optically thick, low temperature, thermal
Comptonization which may dominate the entire big blue bump. This might explain
the observed tight UV/soft X-ray variability correlation and absence of a Lyman
edge in this object. However, the plasma parameters inferred by the spectrum
need stratification in optical depth and/or temperature to prevent physical
inconsistency. The optical/UV/soft X-ray component contributes about half of
the total source flux. The spectral variations of the soft-excess are
consistent with that of the UV and argue that the components are closely
related. The overall pattern of spectral variability suggests variations of the
source geometry, and shows the optical/UV/soft X-ray component to be harder
when brighter, while the hard X-ray component is softer when brighter.Comment: 5 pages, 3 figures, LaTex, using AIP macro, to appear in Proceedings
of "4th Compton Symposium", April 27-30, 1997, Williamsburg, Virginia, US
Non-LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei. III. Integrated Spectra for Hydrogen-Helium Disks
We have constructed a grid of non-LTE disk models for a wide range of black
hole mass and mass accretion rate, for several values of viscosity parameter
alpha, and for two extreme values of the black hole spin: the maximum-rotation
Kerr black hole, and the Schwarzschild (non-rotating) black hole. Our procedure
calculates self-consistently the vertical structure of all disk annuli together
with the radiation field, without any approximations imposed on the optical
thickness of the disk, and without any ad hoc approximations to the behavior of
the radiation intensity. The total spectrum of a disk is computed by summing
the spectra of the individual annuli, taking into account the general
relativistic transfer function. The grid covers nine values of the black hole
mass between M = 1/8 and 32 billion solar masses with a two-fold increase of
mass for each subsequent value; and eleven values of the mass accretion rate,
each a power of 2 times 1 solar mass/year. The highest value of the accretion
rate corresponds to 0.3 Eddington. We show the vertical structure of individual
annuli within the set of accretion disk models, along with their local emergent
flux, and discuss the internal physical self-consistency of the models. We then
present the full disk-integrated spectra, and discuss a number of
observationally interesting properties of the models, such as
optical/ultraviolet colors, the behavior of the hydrogen Lyman limit region,
polarization, and number of ionizing photons. Our calculations are far from
definitive in terms of the input physics, but generally we find that our models
exhibit rather red optical/UV colors. Flux discontinuities in the region of the
hydrogen Lyman limit are only present in cool, low luminosity models, while
hotter models exhibit blueshifted changes in spectral slope.Comment: 20 pages, 31 figures, ApJ in press, spectral models are available for
downloading at http://www.physics.ucsb.edu/~blaes/habk
Product assurance technology for procuring reliable, radiation-hard, custom LSI/VLSI electronics
Advanced measurement methods using microelectronic test chips are described. These chips are intended to be used in acquiring the data needed to qualify Application Specific Integrated Circuits (ASIC's) for space use. Efforts were focused on developing the technology for obtaining custom IC's from CMOS/bulk silicon foundries. A series of test chips were developed: a parametric test strip, a fault chip, a set of reliability chips, and the CRRES (Combined Release and Radiation Effects Satellite) chip, a test circuit for monitoring space radiation effects. The technical accomplishments of the effort include: (1) development of a fault chip that contains a set of test structures used to evaluate the density of various process-induced defects; (2) development of new test structures and testing techniques for measuring gate-oxide capacitance, gate-overlap capacitance, and propagation delay; (3) development of a set of reliability chips that are used to evaluate failure mechanisms in CMOS/bulk: interconnect and contact electromigration and time-dependent dielectric breakdown; (4) development of MOSFET parameter extraction procedures for evaluating subthreshold characteristics; (5) evaluation of test chips and test strips on the second CRRES wafer run; (6) two dedicated fabrication runs for the CRRES chip flight parts; and (7) publication of two papers: one on the split-cross bridge resistor and another on asymmetrical SRAM (static random access memory) cells for single-event upset analysis
Product assurance technology for custom LSI/VLSI electronics
The technology for obtaining custom integrated circuits from CMOS-bulk silicon foundries using a universal set of layout rules is presented. The technical efforts were guided by the requirement to develop a 3 micron CMOS test chip for the Combined Release and Radiation Effects Satellite (CRRES). This chip contains both analog and digital circuits. The development employed all the elements required to obtain custom circuits from silicon foundries, including circuit design, foundry interfacing, circuit test, and circuit qualification
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