349 research outputs found
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
Gravitational Radiation Instability in Hot Young Neutron Stars
We show that gravitational radiation drives an instability in hot young
rapidly rotating neutron stars. This instability occurs primarily in the l=2
r-mode and will carry away most of the angular momentum of a rapidly rotating
star by gravitational radiation. On the timescale needed to cool a young
neutron star to about T=10^9 K (about one year) this instability can reduce the
rotation rate of a rapidly rotating star to about 0.076\Omega_K, where \Omega_K
is the Keplerian angular velocity where mass shedding occurs. In older colder
neutron stars this instability is suppressed by viscous effects, allowing older
stars to be spun up by accretion to larger angular velocities.Comment: 4 Pages, 2 Figure
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
Resistive double-diffusive instability in the dead-zones of protostellar disks
We outline a novel linear instability that may arise in the dead-zones of
protostellar disks, and possibly the fluid interiors of planets and
protoplanets. In essence it is an axisymmetric buoyancy instability, but one
that would not be present in a purely hydrodynamical gas. The necessary
ingredients for growth include a negative radial entropy gradient (of any
magnitude), weak magnetic fields, and efficient resistive diffusion (in
comparison with thermal diffusion). The character of the instability is local,
axisymmetric, and double-diffusive, and it attacks lengths much shorter than
the resistive scale. Like the axisymmetric convective instability, it draws its
energy from the negative radial entropy gradient; but by utilising the
diffusing magnetic field, it can negate the stabilising influence of rotation.
Its nonlinear saturated state, while not transporting appreciable angular
momentum, could drive radial and vertical mixing, which may influence the
temperature structure of the disk, dust dynamics and, potentially, planet
formation.Comment: 16 pages, 5 figures. MNRAS Accepted. V2: cosmetic changes to bring in
line with MNRAS versio
The proper motion and energy distribution of the isolated neutron star RX J0720.4-3125
ESO 4m class telescope and VLT deep imaging of the isolated neutron star RX
J0720.4-3125 reveals a proper motion of mu = 97 +/-12 mas/yr and a blue U-B
color index. We show that a neutron star atmosphere model modified to account
for a limited amount of hydrogen on the star's surface can well represent both
the optical and X-ray data without invoking any additional components. The
large proper motion almost completely excludes the possibility that accretion
from interstellar medium is the powering mechanism of the X-ray emission. It
also implies that the proposed spin down is entirely due to magnetic dipole
losses. RX J0720.4-3125 is thus a very likely middle aged cooling neutron star.
Its overall properties are quite similar to some of the long period radio
pulsars recently discovered, giving further support to the idea that RX
J0720.4-3125 may be a pulsar whose narrow radio beam does not cross the Earth.Comment: Submitted to Astronomy and Astrophysics, 8 pages 5 figure
Three-Dimensional Hydrodynamic Simulations of Accretion Tori in Kerr Spacetimes
This paper presents results of three-dimensional simulations of global
hydrodynamic instabilities in black hole tori, extending earlier work by Hawley
to Kerr spacetimes. This study probes a three-dimensional parameter space of
torus angular momentum, torus size, and black hole angular momentum. We have
observed the growth of the Papaloizou-Pringle instability for a range of torus
configurations and the resultant formation of m=1 planets. We have also
observed the quenching of this instability in the presence of early accretion
flows; however, in one simulation both early accretion and planet formation
occurred. Though most of the conclusions reached in Hawley's earlier work on
Schwarzschild black holes carry over to Kerr spacetime, the presence of frame
dragging in the Kerr geometry adds an element of complexity to the simulations;
we have seen especially clear examples of this phenomenon in the accretion
flows that arise from retrograde tori.Comment: Accepted for publication in ApJ. 23 pages, 11 figures, animations
available at http://www.astro.virginia.edu/~jd5v/h91redux/h91redux.htm
Stability of general-relativistic accretion disks
Self-gravitating relativistic disks around black holes can form as transient
structures in a number of astrophysical scenarios such as binary neutron star
and black hole-neutron star coalescences, as well as the core-collapse of
massive stars. We explore the stability of such disks against runaway and
non-axisymmetric instabilities using three-dimensional hydrodynamics
simulations in full general relativity using the THOR code. We model the disk
matter using the ideal fluid approximation with a -law equation of
state with . We explore three disk models around non-rotating black
holes with disk-to-black hole mass ratios of 0.24, 0.17 and 0.11. Due to metric
blending in our initial data, all of our initial models contain an initial
axisymmetric perturbation which induces radial disk oscillations. Despite these
oscillations, our models do not develop the runaway instability during the
first several orbital periods. Instead, all of the models develop unstable
non-axisymmetric modes on a dynamical timescale. We observe two distinct types
of instabilities: the Papaloizou-Pringle and the so-called intermediate type
instabilities. The development of the non-axisymmetric mode with azimuthal
number m = 1 is accompanied by an outspiraling motion of the black hole, which
significantly amplifies the growth rate of the m = 1 mode in some cases.
Overall, our simulations show that the properties of the unstable
non-axisymmetric modes in our disk models are qualitatively similar to those in
Newtonian theory.Comment: 30 pages, 21 figure
Simulations of Magnetorotational Turbulence with a Higher-Order Godunov Scheme
(abridged) We apply a second-order Godunov code, Athena, to studies of the
magnetorotational instability using unstratified shearing box simulations with
a uniform net vertical field and a sinusoidally varying zero net vertical
field. The Athena results agree well with similar studies that used different
numerical algorithms. We conduct analyses to study the flow of energy from
differential rotation to turbulent fluctuations to thermalization. A study of
the temporal correlation between the time derivatives of volume-averaged energy
components shows that energy injected into turbulent fluctuations dissipates on
a timescale of , where is the orbital frequency of the
local domain. Magnetic dissipation dominates over kinetic dissipation, although
not by as great a factor as the ratio of magnetic to kinetic energy. We
Fourier-transform the magnetic and kinetic energy evolution equations and,
using the assumption that the time-averaged energies are constant, determine
the level of numerical dissipation as a function of length scale and
resolution. By modeling numerical dissipation as if it were physical in origin,
we characterize numerical resistivity and viscosity in terms of effective
Reynolds and Prandtl numbers. The resulting effective magnetic Prandtl number
is , independent of resolution or initial field geometry. MRI
simulations with effective Reynolds and Prandtl numbers determined by numerical
dissipation are not equivalent to those where these numbers are set by physical
resistivity and viscosity. These results serve, then, as a baseline for future
shearing box studies where dissipation is controlled by the inclusion of
explicit viscosity and resistivity.Comment: Accepted by ApJ; version 2 - minor changes following review; 59 pages
(preprint format), 19 figure
Quasars and the Big Blue Bump
We investigate the ultraviolet-to-optical spectral energy distributions
(SEDs) of 17 active galactic nuclei (AGNs) using quasi-simultaneous
spectrophotometry spanning 900-9000 Angstrom (rest frame). We employ data from
the Far Ultraviolet Spectroscopic Explorer (FUSE), the Hubble Space Telescope
(HST), and the 2.1-meter telescope at Kitt Peak National Observatory (KPNO).
Taking advantage of the short-wavelength coverage, we are able to study the
so-called "big blue bump," the region where the energy output peaks, in detail.
Most objects exhibit a spectral break around 1100 Angstrom. Although this
result is formally associated with large uncertainty for some objects, there is
strong evidence in the data that the far-ultraviolet spectral region is below
the extrapolation of the near-ultraviolet-optical slope, indicating a spectral
break around 1100 Angstrom. We compare the behavior of our sample to those of
non-LTE thin-disk models covering a range in black-hole mass, Eddington ratio,
disk inclination, and other parameters. The distribution of ultraviolet-optical
spectral indices redward of the break, and far-ultraviolet indices shortward of
the break, are in rough agreement with the models. However, we do not see a
correlation between the far-ultraviolet spectral index and the black hole mass,
as seen in some accretion disk models. We argue that the observed spectral
break is intrinsic to AGNs, although intrinsic reddening as well as
Comptonization can strongly affect the far-ultraviolet spectral index. We make
our data available online in digital format.Comment: 32 pages (10pt), 12 figures. Accepted for publication in Ap
On the stability of thick accretion disks around black holes
Discerning the likelihood of the so-called runaway instability of thick
accretion disks orbiting black holes is an important issue for most models of
cosmic gamma-ray bursts. To this aim we investigate this phenomenon by means of
time-dependent, hydrodynamical simulations of black hole plus torus systems in
general relativity. The evolution of the central black hole is assumed to be
that of a sequence of Kerr black holes of increasing mass and spin, whose
growth rate is controlled by the transfer of mass and angular momentum from the
material of the disk spiralling in through the event horizon of the black hole.
The self-gravity of the disk is neglected. We find that when the black hole
mass and spin are allowed to increase, constant angular momentum disks undergo
a runaway instability on a dynamical timescale (a few orbital periods).
However, our simulations show that a slight increase of the specific angular
momentum of the disk outwards has a dramatic stabilizing effect. Our results,
obtained in the framework of general relativity, are in broad agreement with
earlier studies based both on stationary models and on time-dependent
simulations with Newtonian and pseudo-Newtonian gravitational potentials.Comment: 12 pages, 3 figures, accepted for publication in ApJ
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