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., $c_s/\Omega_K H =$ constant, where $c_s$ is the sound speed, $\Omega_K$ is the Keplerian angular velocity, and $H$ 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 $R_s$ and the integration constant in angular momentum equation $j$ 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 $R_s-j$ 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 $R_s-j$ 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

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

Secular instability in quasi-viscous disc accretion

A first-order correction in the $\alpha$-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

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 $M = 10^{10}M_\odot$ 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