324 research outputs found
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
Shock oscillation model for quasi-periodic oscillations in stellar mass and supermassive black holes
We numerically examine centrifugally supported shock waves in 2D rotating accretion flows
around a stellar mass (10 M) and a supermassive (106 M) black holes over a wide range of
input accretion rates of 107 M\u2d9 /M\u2d9 E 10 124. The resultant 2D shocks are unstable with time
and the luminosities show quasi-periodic oscillations (QPOs) with modulations of a factor of
2\u20133 and with periods of a tenth of a second to several hours, depending on the black hole
masses. The shock oscillation model may explain the intermediate frequency QPOs with 1\u2013
10 Hz observed in the stellar mass black hole candidates and also suggest the existence of
QPOs with the period of hours in active galactic nuclei. When the accretion rate M\u2d9 is low, the
luminosity increases in proportion to the accretion rate. However, when M\u2d9 greatly exceeds
the Eddington critical rate M\u2d9 E, the luminosity is insensitive to the accretion rate and is kept
constantly around 3c3LE. On the other hand, the mass-outflow rate M\u2d9 loss increases in proportion
to M\u2d9 and it amounts to about a few per cent of the input mass-flow rat
BUILDING AN ENTANGLEMENT MEASURE ON PHYSICAL GROUND
We introduce on physical grounds a new measure of multipartite entanglement for pure states. The function we define is discriminant and monotone under LOCC; moreover, it can be expressed in terms of observables of the system.We introduce on physical grounds a new measure of multipartite entanglement for pure
states. The function we define is discriminant and monotone under LOCC; moreover, it
can be expressed in terms of observables of the system
3D SPH Simulations of Shocks in Accretion Flows around black holes
We present the simulation of 3D time dependent flow of rotating ideal gas
falling into a Schwarzschild black hole. It is shown that also in the 3D case
steady shocks are formed in a wide range of parameters (initial angular
momentum and thermal energy). We therefore highlight the stability of the
phenomenon of shock formation in sub keplerian flows onto black holes, and
reenforce the role of the shocks in the high luminosity emission from black
hole candidates. The simulations have been performed using a parallelized code
based on the Smoothed Particles Hydrodynamics method (SPH). We also discuss
some properties of the shock problem that allow its use as a quantitative test
of the accuracy of the used numerical method. This shows that the accuracy of
SPH is acceptable although not excellent.Comment: 9 pages, 22 figure
Radiative Shocks in Rotating Accretion Flows around Black Holes
It is well known that the rotating inviscid accretion flows with adequate
injection parameters around black holes could form shock waves close to the
black holes, after the flow passes through the outer sonic point and can be
virtually stopped by the centrifugal force.
We examine numerically such shock waves in 1D and 2D accretion flows, taking
account of cooling and heating of the gas and radiation transport.
The numerical results show that the shock location shifts outward compared
with that in the adiabatic solutions and that the more rarefied ambient density
leads to the more outward shock location.
In the 2D-flow, we find an intermediate frequency QPO behavior of the shock
location as is observed in the black hole candidate
GRS 1915+105.Comment: 11pages, 5 Postscript figures, to appear in PASJ, Vol.56, No.3, 200
SPH simulations of Shakura-Sunyaev instability at intermediate accretion rates
We show that a standard Shakura-Sunyaev accretion disc around a black hole
with an accretion rate lower than the critical Eddington limit does show the
instability in the radiation pressure dominated zone. We obtain this result
performing time-dependent simulations of accretion disks for a set of values of
the viscosity parameter and accretion rate. In particular we always find the
occurrence of the collapse of the disc: the instability develops always towards
a collapsed gas pressure dominated disc and not towards the expansion. This
result is valid for all initial configurations we tested. We find significant
convective heat flux that increases the instability development time, but is
not strong enough to inhibit the disc collapse. A physical explanation of the
lack of the expansion phase is proposed considering the role of the radial heat
advection. Our finding is relevant since it excludes the formation of the hot
comptonizing corona -often suggested to be present- around the central object
by the mechanism of the Shakura-Sunyaev instability. We also show that, in the
parameters range we simulated, accretion disks are crossed by significant
amplitude acoustic waves.Comment: 8 pages, 12 Postscript figures, uses natbib.sty, accepted for
publication in MNRA
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
ADIPONECTIN, LEPTIN, RESISTIN LEVELS IN CYSTIC FIBROSIS ADOLESCENTS
INTRODUCTION: Patients with Cystic Fibrosis, especially in adolescence, could develop endocrine and metabolic complications, related to nutritional state and chronic inflammation. They develop a progressive decrease in lean body mass correlated with the progression of lung disease. Adipose tissue is involved as well and adipocytokines are a possible link between malnutrition and long term complications. PATIENTS AND METHODS: In 24 Cystic Fibrosis adolescents we studied auxological, nutritional, glycometabolic, endocrine patterns, together with leptin, adiponectin and resistin levels. We selected patients not affected by diabetes, insulin resistance, malnutrition, acute inflammatory states so as to avoid possible influences on the adipocytokines. RESULTS: All patients presented with an adequate BMI centile, with no statistically significant difference versus controls. HOMA IR and HOMA B% are in the normal range but lower than in controls, expression of a lower insulin-resistance with a lower insulin secretion. Leptin is significantly higher than in controls and maintains a correlation with BMI and gender. Resistin levels are more elevated in Cystic Fibrosis than in controls, with a statistically significant direct correlation with CRP (C-reactive protein) and insulinemia T0\u2019 and T120\u2019.Adiponectin is significantly higher in Cystic Fibrosis, inversely correlated with CRP and insulinemia and directly with cholesterol and HDL-cholesterol. CONCLUSIONS:The detection of adipocytokines levels could improve the metabolic follow-up of these patients: the maintenance of an adequate leptin gender difference is useful in the follow-up of puberty in adolescence. Adiponectin could be a marker of insulin sensitivity and prevent protein catabolism and loss of lean body mass. Resistin levels may be used as a marker of insulin resistance and may indicate the severity of chronic inflammation. This is the first study available in literature about adiponectin levels in pediatric Cystic Fibrosis patients and about resistin in Cystic Fibrosis
A simple model of radiative emission in M87
We present a simple physical model of the central source emission in the M87
galaxy. It is well known that the observed X-ray luminosity from this galactic
nucleus is much lower than the predicted one, if a standard radiative
efficiency is assumed. Up to now the main model invoked to explain such a
luminosity is the ADAF (Advection-Dominated-Accretion-Flow) model. Our approach
supposes only a simple axis-symmetric adiabatic accretion with a low angular
momentum together with the bremsstrahlung emission process in the accreting
gas. With no other special hypothesis on the dynamics of the system, this model
agrees well enough with the luminosity value measured by Chandra.Comment: 11 pages, 6 figures, accepted for publication in The Astrophysical
Journa
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