0000000000246442
AUTHOR
V. Teresi
Black hole accretion discs and jets at super-Eddington luminosity
Super-Eddington accretion discs with 3 and 15 dot M_E around black holes with mass 10 M_sun are examined by two-dimensional radiation hydrodynamical calculations extending from the inner disc edge to 5*10^4 r_g and lasting up to \sim 10^6 r_g/c. The dominant radiation-pressure force in the inner region of the disc accelerates the gas vertically to the disc plane, and jets with 0.2 -- 0.4$c$ are formed along the rotational axis. In the case of the lower accretion rate, the initially anisotropic high-velocity jet expands outward and becomes gradually isotropic flow in the distant region. The mass-outflow rate from the outer boundary is as large as \sim 10^{19} -- 10^{23} g s^{-1}, but it is v…
A model of M87 nuclear emission without ADAF
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.
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.
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 …
QPOs expected in rotating accretion flows around a supermassive black hole
AbstractIt is well known that 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 numerically examine such shock waves in 2D accretion flows with 10−5 to 106 Eddington critical accretion rates around a supermassive black hole with 106M⊙. As the results, the luminosities show QPO phenomena with modulations of a factor 2–3 and with quasi-periods of a few to several hours.
Can gravity perturbations explain QPOs?
We show the results of some numerical simulations trying to reproduce the QPO behavior in black hole and neutron star sources. Our simulations are based on the idea that a nearly periodic luminosity oscillation can be obtained from a perturbation of the source gravitational field with a sinusoidal time behavior. We find that some specific features of the QPO phenomenon can be described by this simple model, but the required amplitude of the gravity perturbation is more than 1% of the unperturbed gravitational field. If the hypothesis is formed that such a perturbation is due to a density fluctuation (of the accretion disk or the source itself, in the case of the neutron star) going around t…
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…
Shock waves and QPOs in 2D rotating accretion flows around black holes
We examine numerically shock waves formed in 2D rotating accretion flows around a stellar‐mass and a supermassive black holes, while taking account of the cooling and heating of the gas and the radiation transport. As the results, we obtain general properties of the shock oscillations and the luminosity behaviors as QPOs independent of the black hole masses.
Radiative 2D Shocks, Super-Eddington Disks and Jets around Black Holes
It is well known that 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. Such shock waves in 2D accretion flows are examined by 2D radiation hydrodynamical calculations. We also examine super‐Eddington accretion disks with 15 ṀE around black holes, focusing on a small collimation degree of the jet and a large mass‐outflow rate observed in the X‐ray source SS 433.
Perspectives
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. …
Numerical Simulations of the Thermal Instability Collapse in Radiation Pressure Dominated Disks
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 α and accretion rate. 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 r…
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. …
Ab initiosimulations of accretion disc 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 o…
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 (10M_sun) and a supermassive (10^6M_sun) black holes over a wide range of input accretion rates of 10^7 >\dot M/\dot M_E>10^{-4}. The resultant 2D-shocks are unstable with time and the luminosities show quasi-periodic oscillations (QPOs) with modulations of a factor of 2-3 andwith periods of a tenth seconds to several hours, depending on the black hole masses. The shock oscillation model may explain the intermediate frequency QPOs with 1-10 Hz observed in the stellar-mass black hole candidates and also suggest the existence of QPOs with the period of hours in AGNs. When the accret…