Simulation of Thick Accretion Disks with Standing Shocks by Smoothed Particle Hydrodynamics

We present results of numerical simulation of inviscid thick accretion disks and wind flows around black holes. We use Smoothed Particle Hydrodynamics (SPH) technique for this purpose. Formation of thick disks are found to be preceded by shock waves travelling away from the centrifugal barrier. For a large range of the parameter space, the travelling shock settles at a distance close to the location obtained by a one-and-a-half dimensional model of inviscid accretion disks. Occasionally, it is observed that accretion processes are aided by the formation of oblique shock waves, particularly in the initial transient phase. The post-shock region (where infall velocity suddenly becomes very small) resembles that of the usual model of thick accretion disk discussed in the literature, though they have considerable turbulence. The flow subsequently becomes supersonic before falling into the black hole. In a large number of cases which we simulate, we find the formation of strong winds which are hot and subsonic when originated from the disk surface very close to the black hole but become supersonic within a few tens of the Schwarzschild radius of the blackhole. In the case of accretion of high angular momentum flow, very little amount of matter is accreted directly onto the black hole. Most of the matter is, however, first squeezed to a small volume close to the black hole, and subsequently expands and is expelled as a strong wind. It is quite possible that this expulsion of matter and the formation of cosmic radio jets is aided by the shock heating in the inner parts of the accretion disks.Comment: LaTeX, 16 pages, Astrophysical Journal (in press

Simple absorbing layer conditions for shallow wave simulations with Smoothed Particle Hydrodynamics

We study and implement a simple method, based on the Perfectly Matched Layer approach, to treat non reflecting boundary conditions with the Smoothed Particles Hydrodynamics numerical algorithm. The method is based on the concept of physical damping operating on a fictitious layer added to the computational domain. The method works for both 1D and 2D cases, but here we illustrate it in the case of 1D and 2D time dependent shallow waves propagating in a finite domain

Numerical Simulations of Standing Shocks in Accretion Flows around Black Holes: A Comparative Study

We compare the results of numerical simulations of thin and quasi-spherical (thick) accretion flows with existing analytical solutions. We use a Lagrangian code based on the Smooth Particle Hydrodynamics (SPH) scheme and an Eulerian finite difference code based on the Total Variation Diminishing (TVD) scheme. In one-dimensional thin flows, the results of the simulations, with or without shocks, agree very well with each other and with analytical solutions. In two-dimensional thick flows, the general features, namely the locations and strengths of centrifugal and turbulent pressure supported shocks, centrifugal barriers, and the funnel walls which are expected from analytical models, agree very well, though the details vary. Generally speaking, the locations of the shocks may be better obtained by SPH since the angular momentum is strictly preserved in SPH, but the shocks themselves are better resolved by TVD. The agreement of these code test results with analytical solutions provides us with confidence to apply these codes to more complex problems which we will discuss elsewhere.Comment: ApJ accepted, 16 pages with 7 figures, using aaspp4.sty available upon request to [email protected] or from http://astro1.chungnam.ac.kr/Homepage/ryu/mypage.htm

Resonance Oscillation Of Radiative Shock Waves In Accretion Disks Around Compact Objects

We extend our previous numerical simulation of accretion disks with shock waves when cooling effects are also included. We consider bremsstrahlung and other power law processes: $\Lambda \propto T^{\alpha} \rho^2$ to mimic cooling in our simulation. We employ {\it Smoothed Particle Hydrodynamics} technique as in the past. We observe that for a given angular momentum of the flow, the shock wave undergoes a steady, radial oscillation with the period is roughly equal to the cooling time. Oscillations seem to take place when the disk and cooling parameters (i.e., accretion rate, cooling process) are such that the infall time from shock is of the same order as the post-shock cooling time. The amplitude of oscillation could be up to ten percent of the distance of the shock wave from the black hole when the black hole is accreting. When the accretion is impossible due to the centrifugal barrier, the amplitude variation could be much larger. Due to the oscillation, the energy output from the disk is also seen to vary quasi-periodically. We believe that these oscillations might be responsible for the quasi periodic oscillation (QPO) behaviors seen in several black hole candidates, in neutron star systems as well as dwarf novae outbursts such as SS Cygni and VW Hyi.Comment: 24 pages of text plus 6 figures (in tar-compressed form); Astrophysical journal (in press

On the Azimuthal Stability of Shock Waves around Black Holes

Analytical studies and numerical simulations of time dependent axially symmetric flows onto black holes have shown that it is possible to produce stationary shock waves with a stable position both for ideal inviscid and for moderately viscous accretion disks. We perform several two dimensional numerical simulations of accretion flows in the equatorial plane to study shock stability against non-axisymmetric azimuthal perturbations. We find a peculiar new result. A very small perturbation seems to produce an instability as it crosses the shock, but after some small oscillations, the shock wave suddenly transforms into an asymmetric closed pattern, and it stabilizes with a finite radial extent, despite the inflow and outflow boundary conditions are perfectly symmetric. The main characteristics of the final flow are: 1) The deformed shock rotates steadily without any damping. It is a permanent feature and the thermal energy content and the emitted energy vary periodically with time. 2) This behavior is also stable against further perturbations. 3) The average shock is still very strong and well defined, and its average radial distance is somewhat larger than that of the original axially symmetric circular shock. 4) Shocks obtained with larger angular momentum exhibit more frequencies and beating phenomena. 5) The oscillations occur in a wide range of parameters, so this new effect may have relevant observational consequences, like (quasi) periodic oscillations, for the accretion of matter onto black holes. Typical time scales for the periods are 0.01 and 1000 seconds for black holes with 10 and 1 million solar mass, respectively.Comment: 15 pages, 7 figures, accepted by the Astrophysical Journa

Zero-energy rotating accretion flows near a black hole

We characterize the nature of thin, axisymmetric, inviscid accretion flows of cold adiabatic gas with zero specific energy in the vicinity of a black hold by the specific angular momentum. Using two-dimensional hydrodynamic simulations in cylindrical geometry, we present various regimes in which the accretion flows behave distinctly differently. When the flow has a small angular momentum (lambda less than or similar to lambda(b)), most of the material is accreted into the black hold, forming a quasi-spherical flow or a simple disklike structure around it. When the flow has a large angular momentum (typically, larger than the marginally bound, value, lambda greater than or similar to(mb)), almost no accretion into the black hole occurs. Instead, the flow produces a stable shock with one or more vortices behind it and is deflected away at the shock as a conical, outgoing wind of higher entropy. If the flow has an angular momentum somewhat smaller than lambda(mb) (lambda less than or similar to lambda less than or similar to lambda(mb)), a fraction (typically 5%-10%) of the incoming material is accreted into the black hole, but the flow structure formed is similar to that for lambda greater than or similar to lambda(mb). Some of the deflected material is accreted back into the black hole while the rest is blown away as an outgoing wind. These two cases with lambda greater than or similar to lambda(u) correspond those studied in the previous works by Molteni, Lanzafame, & Chakrabarti, and Ryu et al. However, the flow with angular momentum close to the marginally stable value (lambda(ms)) is found to be unstable. More specifically, if lambda(b) less than or similar to lambda similar to lambda(ms) less than or similar to lambda(u), the flow displays a distinct periodicity in the sense that the inner part of the disk is built and destroyed regularly. The period is roughly equal to (4-6) x 10(3) R(g)/c, depending on the angular momentum of the flow. In this case, the internal energy of the flow around the black hold becomes maximum when the structure with the accretion shock and vortices is fully developed. But the mass accretion rate into the black hole reaches a maximum value when the structure collapses. Averaged over periods, more than half the incoming material is accreted into the black hole. We suggest the physical origin of these separate regimes from a global perspective. Then we discuss the possible relevance of the instability work to quasi-periodic oscillationsopen716

Políticas de Planeamiento en la Educación Superior: presupuesto, planes estratégicos y metas educativas globales

La perspectiva es comprender el desarrollo de las políticas de planeamiento en la Educación Superior desde el análisis de los presupuestos y los planes estratégicos en función de metas educativas globales que han sido consensuadas y aceptadas para constituir el norte macroinstitucional del sistema. En el desarrollo de los estudios sobre la Educación Superior, hay una preeminencia de indagaciones o ensayos referidos centralmente a la historicidad del campo, a la organización de datos económicos y financieros en tablas comparativas sin mucho contexto de sus respectivas aplicaciones, a la presentación de indicadores o metas sólo educativas o sólo institucionales. Esta constatación sobre los antecedentes se origina, por supuesto, en una apreciación crítica pero también tributaria de dichos aportes y desarrollos, porque consideramos necesario hacer un aporte que vincule las tres fuentes principales de datos concretos para lograr una propuesta integrada hacia el planeamiento prospectivo de la Educación Superior: los presupuestos (fuentes presupuestarias, fondos materiales, ejecuciones coyunturales), los planes estratégicos (los deseos y planes institucionales sobre la actividad inmediata y mediata de la organización) y las metas educativas globales (los acuerdos y consensos de organizaciones supraeducativas o interinstitucionales). La formulación investigativa también es precisa acerca del sentido del análisis y los posibles aportes: al final del proceso, el trabajo pondrá en circulación una reflexión compleja acerca del lugar de la UNLP en el sistema de Educación Superior argentino y la discusión por los modelos de desarrollo integrales de las políticas públicas en el contexto de las intencionalidades internacionalistas, globalizadoras y cooperativo-interinstitucionalistas.Instituto de Investigaciones en Educación Superio