564 research outputs found
A Note on the Slim Accretion Disk Model
We show that when the gravitational force is correctly calculated in dealing
with the vertical hydrostatic equilibrium of black hole accretion disks, the
relationship that is valid for geometrically thin disks, i.e., constant, where is the sound speed, is the Keplerian
angular velocity, and is the half-thickness of the disk, does not hold for
slim disks. More importantly, by adopting the correct vertical gravitational
force in studies of thermal equilibrium solutions, we find that there exists a
maximally possible accretion rate for each radius in the outer region of
optically thick accretion flows, so that only the inner region of these flows
can possibly take the form of slim disks, and strong outflows from the outer
region are required to reduce the accretion rate in order for slim disks to be
realized.Comment: 14 pages, 5 figures, accepted by Ap
AQUATIC INVERTEBRATE COMMUNITY STRUCTURE IN THE NIOBRARA RIVER, AGATE FOSSIL BEDS NATIONAL MONUMENT, NEBRASKA, 1996-2009
Aquatic invertebrates were sampled annually in the Niobrara River, Nebraska, during the period 1996-2009 using Hester-Dendy multiplate samplers. Collections indicated the invertebrate community in the river has shifted from one dominated by Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa to one dominated by Chironomidae and Amphipoda. Generally, EPT richness and percentage abundance of EPT of the total community, as well as percentage abundance of Heptageniidae and Leptophlebiidae, has declined across the years. During that same period, percentage abundance of Amphipoda and Chironomidae, taxa evenness, Shannon\u27s Index, and the Hilsenhoff Biotic Index have increased. Stream discharge decreased significantly during the 15-year period when invertebrates were collected (Mann-Kendall trend test, P = 0.04). Water-surface elevation of the river measured with staff gages also gradually increased over time, although the trends were not significant (P \u3e/= 0.15). Although not analyzed statistically, water temperature (Degrees C), conductivity (uS/cm), and pH gradually increased over time while dissolved oxygen concentration (mg/liter) decreased over time. Although other factors cannot be ruled out, a proposed reason for the observed changes in invertebrate community structure may be related to changes in the physical habitat condition in the Niobrara River associated with encroachment of the nonnative and invasive emergent aquatic plant, yellow flag (Iris pseudacorus L.). This weed has expanded into the stream channel, where it blocks streamflow
Stability of accretion disk around rotating black holes: a pseudo-general-relativistic fluid dynamical study
We discuss the solution of accretion disk when the black hole is chosen to be
rotating. We study, how the fluid properties get affected for different
rotation parameters of the black hole. We know that no cosmic object is static
in Universe. Here the effect of the rotation of the black hole to the
space-time is considered following an earlier work of the author, where the
pseudo-Newtonian potential was prescribed for the Kerr geometry. We show that,
with the inclusion of rotation of the black hole, the valid disk parameter
region dramatically changes and disk becomes unstable. Also we discuss about
the possibility of shock in accretion disk around rotating black holes. When
the black hole is chosen to be rotating, the sonic locations of the accretion
disk get shifted or disappear, making the disk unstable. To bring it in the
stable situation, the angular momentum of the accreting matter has to be
reduced/enhanced (for co/counter-rotating disk) by means of some physical
process.Comment: 24 Latex pages including 7 figures; Accepted for publication in
Astrophysical Journa
Global Dynamics of Advection-Dominated Accretion Revisited
We numerically solve the set of dynamical equations describing
advection-dominated accretion flows (ADAF) around black holes, using a method
similar to that of Chakrabarti (1996a). We choose the sonic radius of the flow
and the integration constant in angular momentum equation as free
parameters, and integrate the equations from the sonic point inwards to see if
the solution can extend supersonically to the black hole horizon, and outwards
to see if and where an acceptable outer boundary of the flow can be found. We
recover the ADAF-thin disk solution constructed in Narayan, Kato, & Honma
(1997, NKH97), an representative paper of the previous works on global ADAF
solutions, although in that paper an apparently very different procedure was
adopted. We obtain a complete picture in the form of parameter space
which sums up the situation of ADAF solution at a glance. For comparison we
also present the distribution of global solutions for inviscid flows in the
space, which supports the view that there should be some similarities
between the dynamical behavior of ADAF and that of adiabatic flows, and that
there should be a continuous change from the properties of viscous flows to
those of inviscid ones.Comment: 24 pages with 15 figures, to appear in ApJ Vol. 52
Secular instability in quasi-viscous disc accretion
A first-order correction in the -viscosity parameter of Shakura and
Sunyaev has been introduced in the standard inviscid and thin accretion disc. A
linearised time-dependent perturbative study of the stationary solutions of
this "quasi-viscous" disc leads to the development of a secular instability on
large spatial scales. This qualitative feature is equally manifest for two
different types of perturbative treatment -- a standing wave on subsonic
scales, as well as a radially propagating wave. Stability of the flow is
restored when viscosity disappears.Comment: 15 pages, 2 figures, AASTeX. Added some new material and upgraded the
reference lis
Evolution of transonicity in an accretion disc
For inviscid, rotational accretion flows driven by a general pseudo-Newtonian
potential on to a Schwarzschild black hole, the only possible fixed points are
saddle points and centre-type points. For the specific choice of the Newtonian
potential, the flow has only two critical points, of which the outer one is a
saddle point while the inner one is a centre-type point. A restrictive upper
bound is imposed on the admissible range of values of the angular momentum of
sub-Keplerian flows through a saddle point. These flows are very unstable to
any deviation from a necessarily precise boundary condition. The difficulties
against the physical realisability of a solution passing through the saddle
point have been addressed through a temporal evolution of the flow, which gives
a non-perturbative mechanism for selecting a transonic solution passing through
the saddle point. An equation of motion for a real-time perturbation about the
stationary flows reveals a very close correspondence with the metric of an
acoustic black hole, which is also an indication of the primacy of
transonicity.Comment: 18 page
Line Emission from an Accretion Disk around a Black hole: Effects of Disk Structure
The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide
strong evidence for an accretion disk near a supermassive black hole as a
source of the line emission. These lines serve as powerful probes for examining
the structure of inner regions of accretion disks. Previous studies of line
emission have considered geometrically thin disks only, where the gas moves
along geodesics in the equatorial plane of a black hole. Here we extend this
work to consider effects on line profiles from finite disk thickness, radial
accretion flow and turbulence. We adopt the Novikov and Thorne (1973) solution,
and find that within this framework, turbulent broadening is the dominant new
effect. The most prominent change in the skewed, double-horned line profiles is
a substantial reduction in the maximum flux at both red and blue peaks. The
effect is most pronounced when the inclination angle is large, and when the
accretion rate is high. Thus, the effects discussed here may be important for
future detailed modeling of high quality observational data.Comment: 21 pages including 8 figures; LaTeX; ApJ format; accepted by ApJ;
short results of this paper appeared before as a conference proceedings
(astro-ph/9711214
Boson Stars as Gravitational Lenses
We discuss boson stars as possible gravitational lenses and study the lensing
effect by these objects made of scalar particles. The mass and the size of a
boson star may vary from an individual Newtonian object similar to the Sun to
the general relativistic size and mass of a galaxy close to its Schwarzschild
radius. We assume boson stars to be transparent which allows the light to pass
through them though the light is gravitationally deflected. We assume boson
stars of the mass to be on non-cosmological distance from
the observer. We discuss the lens equation for these stars as well as the
details of magnification. We find that there are typically three images of a
star but the deflection angles may vary from arcseconds to even degrees. There
is one tangential critical curve (Einstein ring) and one radial critical curve
for tangential and radial magnification, respectively. Moreover, the deflection
angles for the light passing in the gravitational field of boson stars can be
very large (even of the order of degrees) which reflects the fact they are very
strong relativistic objects. We also propose a suitable formula for the lens
equation for such large deflection angles, and with the reservation that large
deflection angle images are highly demagnified but in the area of the
tangential critical curve, their existence may help in observational detection
of suitable lenses possessing characteristic features of boson stars which
could also serve as a direct evidence for scalar fields in the universe.Comment: accepted by Astrophys. J., 31 pages, AASTeX, 6 figure
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