1,268 research outputs found
Steady shocks around black holes produced by sub-keplerian flows with negative energy
We discuss a special case of formation of axisymmetric shocks in the
accretion flow of ideal gas onto a Schwarzschild black hole: when the total
energy of the flow is negative. The result of our analysis enlarges the
parameter space for which these steady shocks are exhibited in the accretion of
gas rotating around relativistic stellar objects. Since keplerian disks have
negative total energy, we guess that, in this energy range, the production of
the shock phenomenon might be easier than in the case of positive energy. So
our outcome reinforces the view that sub-keplerian flows of matter may
significantly affect the physics of the high energy radiation emission from
black hole candidates. We give a simple procedure to obtain analytically the
position of the shocks. The comparison of the analytical results with the data
of 1D and 2D axisymmetric numerical simulations confirms that the shocks form
and are stable.Comment: 5 pages, 5 figures, accepted by MNRAS on 10 November 200
Ab initio simulations of accretion disks instability
We show that accretion disks, both in the subcritical and supercritical
accretion rate regime, may exhibit significant amplitude luminosity
oscillations. The luminosity time behavior has been obtained by performing a
set of time-dependent 2D SPH simulations of accretion disks with different
values of alpha and accretion rate. In this study, to avoid any influence of
the initial disk configuration, we produced the disks injecting matter from an
outer edge far from the central object. The period of oscillations is 2 - 50 s
respectively for the two cases, and the variation amplitude of the disc
luminosity is 10^38 - 10^39 erg/s. An explanation of this luminosity behavior
is proposed in terms of limit cycle instability: the disk oscillates between a
radiation pressure dominated configuration (with a high luminosity value) and a
gas pressure dominated one (with a low luminosity value). The origin of this
instability is the difference between the heat produced by viscosity and the
energy emitted as radiation from the disk surface (the well-known thermal
instability mechanism). We support this hypothesis showing that the limit cycle
behavior produces a sequence of collapsing and refilling states of the
innermost disk region.Comment: 11 pages, 15 Postscript figures, uses natbib.sty, accepted for
publication in MNRA
Smoothed Particle Hydrodynamic Simulations of Viscous Accretion Discs Around Black Holes
Viscous Keplerian discs become sub-Keplerian close to a black hole since they
pass through sonic points before entering into it. We study the time evolution
of polytropic viscous accretion discs (both in one and two dimensional flows)
using Smoothed Particle Hydrodynamics. We discover that for a large region of
the parameter space, when the flow viscosity parameter is less than a critical
value, standing shock waves are formed. If the viscosity is very high then the
shock disappears. In the intermediate viscosity the disc oscillates very
significantly in viscous time-scale. Our simulations indicate that these
centrifugally supported high density region close to a black hole plays an
active role in the flow dynamics, and consequently, the radiation dynamics.Comment: MNRAS style 6 pages of output, macros included. MNRAS (submitted
QPOs from Radial and Vertical Oscillation of Shocks in Advective Accretion Flows
We present results of several numerical simulations of two dimensional
advective flows which include cooling processes. We show that the computed
light curve is similar to the state in GRS 1915+105. The power density
spectrum (PDS) also shows presence of QPOs near the break frequency.Comment: 4 pages, 2 figures To be published in the Proceedings of 10th Marcel
Grossman Meeting, Ed. R. Ruffini et al. (World Scientific: Singapore
Hydrodynamic Simulations of Oscillating Shock Waves in a Sub-Keplerian Accretion Flow Around Black Holes
We study the accretion processes on a black hole by numerical simulation. We
use a grid based finite difference code for this purpose. We scan the parameter
space spanned by the specific energy and the angular momentum and compare the
time-dependent solutions with those obtained from theoretical considerations.
We found several important results (a) The time dependent flow behaves close to
a constant height model flow in the pre-shock region and a flow with vertical
equilibrium in the post-shock region. (c) The infall time scale in the
post-shock region is several times higher than the free-fall time scale. (b)
There are two discontinuities in the flow, one being just outside of the inner
sonic point. Turbulence plays a major role in determining the locations of
these discontinuities. (d) The two discontinuities oscillate with two different
frequencies and behave as a coupled harmonic oscillator. A Fourier analysis of
the variation of the outer shock location indicates higher power at the lower
frequency and lower power at the higher frequency. The opposite is true when
the analysis of the inner shock is made. These behaviours will have
implications in the spectral and timing properties of black hole candidates.Comment: 19 pages, 13 figures, 1 Table MNRAS (In press
Steady state shocks in accretion disks around a Kerr black hole
Results of numerical simulations of shock solutions in a geometrical thin
accretion disk around a Kerr black hole (BH) are presented. Using the smoothed
particle hydrodynamics (SPH) technique, the influence of the central object is
included by means of an effective potential, We first present the theory of
standing shock formation in accretion disks around a Kerr black hole, and show
that the results of our numerical simulation agree very well with the
theoretical results. We find that the shocks in an inviscid flow are very
stable. We also remove the ambiguity prevalent regarding the location and
stability of shocks in adiabatic flows. Finally we sketch some of the
astrophysical consequences of our findings in relation to accretion disks in
Active Galactic Nuclei (AGN) and Quasars.Comment: 9 pages (figures included) self-extracting compressed PostScript, in
press (MNRAS
Backscattering Differential Ghost Imaging in Turbid Media
In this Letter we present experimental results concerning the retrieval of
images of absorbing objects immersed in turbid media via differential ghost
imaging (DGI) in a backscattering configuration. The method has been applied,
for the first time to our knowledge, to the imaging of small thin black objects
located at different depths inside a turbid solution of polystyrene nanospheres
and its performances assessed via comparison with standard imaging techniques.
A simple theoretical model capable of describing the basic optics of DGI in
turbid media is proposed.Comment: 5 pages, 6 figure
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