Magnetized 1.5-dimensional advective accretion flows around black holes
Tushar Mondal, Banibrata Mukhopadhyay
备注：
6 pages including 1 figure; to appear in the Proceedings of the Fifteenth Marcel Grossmann Meeting (MG15) held at "La Sapienza" University in Rome, Italy, July 1-7, 2018; based on the talk given at the Parallel Session AC2 "MHD processes near compact objects"
We address the role of large scale magnetic stress in the angular momentum transport, as well as the formation of different kinds of magnetic barrier in geometrically thick, optically thin, vertically averaged 1.5-dimensional advective accretion flows around black holes. The externally generated magnetic fields are captured by the accretion process from the environment, say, companion stars or interstellar medium. This field becomes dynamically dominant near the event horizon of a black hole due to continuous advection of the magnetic flux. In such magnetically dominated accretion flows, the accreting matter either decelerates or faces magnetic barrier in vicinity of the black hole depending on the magnetic field geometry. We find that the accumulated strong poloidal fields along with certain toroidal field geometry help in the formation of magnetic barrier which may knock the matter to infinity. When matter is trying to go back to infinity after getting knocked out by the barrier, in some cases it is prevented being escaped due to cumulative action of strong magnetic tension and gravity, and hence another magnetic barrier. We suggest, this kind of flow may be responsible for the formation of episodic jets in which magnetic field can lock the matter in between these two barriers. We also find that for the toroidally dominated disc, the accreting matter rotates very fast and decelerates towards the central black hole.
Ultra-luminous X-ray sources (ULXs) have been puzzling us with a debate whether they consist of an intermediate mass black hole or super-Eddington accretion by a stellar mass black hole. Here we suggest that in the presence of large scale strong magnetic fields and non-negligible vertical motion, the luminosity of ULXs, particularly in their hard states, can be explained with sub-Eddington accretion by stellar mass black holes. In this framework of 2.5D magnetized advective accretion flows, magnetic tension plays the role of transporting matter (equivalent to viscous shear via turbulent viscos…
A 2.5-dimensional viscous, resistive, advective magnetized accretion-outflow coupling in black hole systems: A higher order polynomial approximation. I
We study the two-temperature magnetized advective accretion flow around the Kerr black holes. During accretion, ions are heated up due to viscous dissipation, and when Coulomb coupling becomes effective, they transfer a part of their energy to the electrons. On the contrary, electrons lose energy due to various radiative cooling processes, namely bremsstrahlung, synchrotron, and Comtonization processes, respectively. To account for the magnetic contribution inside the disc, we consider the toroidal magnetic fields which are assumed to be dominant over other components. Moreover, we adopt the r…
In order to explain unusually high luminosity and spectral nature of ultra-luminous X-ray sources (ULXs), some of the underlying black holes are argued to be of intermediate mass, between several tens to million solar masses. Indeed, there is a long standing question of missing mass of intermediate range of black holes. However, as some ULXs are argued to be neutron stars too, often their unusual high luminosity is argued by super-Eddington accretions. Nevertheless, all the models are based on non-magnetized or weakly magnetized accretion. There are, however, evidences that magnetic fields in …
The $Fermi$ blazar observations show a strong correlation between $γ$-ray luminosities and spectral indices. BL Lac objects are less luminous with harder spectra than flat-spectrum radio quasars (FSRQs). Interestingly FSRQs are evident to exhibit a Keplerian disc component along with a powerful jet. We compute the jet intrinsic luminosities by beaming corrections determined by different cooling mechanisms. Observed $γ$-ray luminosities and spectroscopic measurements of broad emission lines suggest a correlation of the accretion disc luminosity with jet intrinsic luminosity. Also, theoretical a…
We study the time evolution of sub-Keplerian transonic accretion flow onto a non-rotating black hole using a three-dimensional, inviscid hydrodynamics simulation code. Prior two-dimensional simulations show that centrifugal barrier in the accreting matter may temporarily halt the nearly free-falling matter and produce a stable, geometrically thick disk which may contain turbulent eddies. Our goal in this work is to investigate whether the disk develops any instability because of this turbulence when we dynamically activate all three dimensions. We find that the disk remains stable and axisymme…
Numerous studies on hydrodynamics of the Keplerian as well as the sub-Keplerian accretion disc around a compact object (e.g., white dwarf (WD), neutron star (NS), or a black hole (BH)) attempted to explain the observed UV, soft and hard X-ray spectra. Although, when the compact object (e.g., a WD or an NS) has a finite surface, its rapid rotation, the stellar magnetic field could cause deformation of the spherical symmetry. Earlier studies for Keplerian disc showed that a deviation from the spherical symmetry of the compact object could affect the observed light curve and spectra at high frequ…
Outward transport of angular momentum, as well as viscous and thermal stability, are the necessary criteria for the formation of accretion disc and to radiate steadily. Turbulent motions originating from magneto-rotational instability or hydrodynamic instability can do the required transport. We explore the effect of a large-scale magnetic field (LSMF) over the turbulent transport in an optically thin advective disc. In this work, turbulent transport is represented through the usual Shakura-Sunyaev $α$-viscosity. The evolution of the magnetic field and other variables is found by solving verti…
We examine the properties of the viscous dissipative accretion flow around rotating black holes in presence of mass loss. Considering thin disc approximation, we self-consistently calculate the inflow-outflow solutions and observe that the mass outflow rates decreases with the increase of viscosity parameter ($α$). Further, we carry out the model calculation of Quasi-periodic Oscillation frequency ($ν_{\rm QPO}$) that is frequently observed in black hole sources and observe that $ν^{\rm max}_{\rm QPO}$ increases with the increase of black hole spin ($a_k$). Then, we employ our model in order t…
Magnetic fields advected along with low angular momentum accretion flows predominantly become toroidal due to the strong azimuthal velocity close to a black hole. We study self-consistently the movements of these flux tubes inside an advective disc and how they dynamically influence the flow. We find that the centrifugal barrier slows down the radial motion of the flux tubes. In this case, the large magnetic flux tubes with a significant drag force escape along the vertical axis due to buoyancy. Magnetic pressure rises close to the black hole and together with the centrifugal force, it combats…
Tidal disruption events (TDEs) show a correlation between the UV to X-ray spectral index and the Eddington ratio, with non-thermal X-ray emission at the low Eddington ratio. We consider the corona surrounding the accretion disc as a non-thermal X-ray source. We construct a time-dependent and non-relativistic advective accretion disc-corona model for TDEs. The infalling debris is assumed to form a seed disc in time $t_c$, that evolves due to the mass gain from the infalling debris at the constant outer radius with a mass fallback rate $\dot{M}_{\rm fb}$ and the mass loss through accretion onto …
We discuss the importance of large scale strong magnetic field in the removal of angular momentum outward, as well as the possible origin of different kinds of magnetic barrier in advective, geometrically thick, sub-Keplerian accretion flows around black holes. The origin of this large scale strong magnetic field near the event horizon is due to the advection of the magnetic flux by the accreting gas from the environment, say, the interstellar medium or a companion star, because of flux freezing. In this simplest vertically averaged, 1.5-dimensional disc model, we choose the maximum upper limi…
The presence of a surface at the inner boundary, such as in a neutron star or a white dwarf, allows the existence of a standing shock in steady spherical accretion.The standing shock can become unstable in 2D or 3D; this is called the {\em standing accretion shock instability} (SASI).Two mechanisms -- advective-acoustic and purely acoustic -- have been proposed to explain SASI. Using axisymmetric hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations, we find that the advective-acoustic mechanism better matches the observed oscillation timescales in our simulations. The global shock oscil…
We studied the general advective accretion solutions around the Kerr black hole (BH) with investigating two types of inflow gases at the outer accretion boundary (AB). We classified these two types of gases as a \cm and a \hm inflow gas at the outer AB on the basis of their temperatures and solutions. We found that the \hm gas is more efficient for angular momentum transportation around the outer AB than the \cm gas. The \hm gas can give global multiple \cite[popular as shock solution][]{c89} or single sonic point solutions and the \cm can give smooth global solution \cite[popular as ADAF][]{n…
Advection is believed to be the dominant cooling mechanism in optically thin advection-dominated accretion flows (ADAF's). When outflow is considered, however, the first impression is that advection should be of opposite sign in the inflow and the outflow, due to the opposite direction of radial motion. Then how is the energy balance achieved simultaneously? We investigate the problem in this paper, analysing the profiles of different components of advection with self-similar solutions of ADAF's in spherical coordinates ($rθϕ$). We find that for $n < 3γ/2-1$, where $n$ is the density index in …
Outflows are common in many astrophysical systems. In the Two Component Advective Flow ({\fontfamily{qcr}\selectfont TCAF}) paradigm which is essentially a generalized Bondi flow including rotation, viscosity and cooling effects, the outflow is originated from the hot, puffed up, post-shock region at the inner edge of the accretion disk. We consider this region to be the base of the jet carrying away matter with high velocity. In this paper, we study the spectral properties of black holes using {\fontfamily{qcr}\selectfont TCAF} which includes also a jet ({\fontfamily{qcr}\selectfont JeTCAF}) …
We have studied shock in magnetized accretion flow/funnel flow in case of neutron star with bremsstrahlung cooling and cyclotron cooling. All accretion solutions terminate with a shock close to the neutron star surface, but at some region of the parameter space, it also harbours a second shock away from the star surface. We have found that cyclotron cooling is necessary for correct accretion solutions which match the surface boundary conditions.
The fundamental difference between accretion around black holes and neutron stars is the inner boundary condition, which affects the behavior of matter very close to the compact objects. This leads to formation of additional shocks and boundary layers for neutron stars. Previous studies on the formation of such boundary layers focused on Keplerian flows that reached the surface of the star, either directly or through the formation of a transition layer. However, behavior of sub-Keplerian matter near the surface of a neutron star has not been studied in detail. Here, we study the effect of visc…
We present the accretion of magnetized supersonic winds onto a rotating black hole in three dimensions. We select representative spin-wind orientations in order to illustrate its effects on the evolution and morphology of the shock cone. The most important finding in the magnetized case, unlike the purely hydrodynamical scenario, is the formation of rarified spots where the magnetic field pressure dominates over the gas pressure. In these rarified spots we find the formation of eddies within the shock cone.
Outflow Behavior from the Transonic Advective Disks: A Hydrodynamical Simulation Study
Aryabhatta Research Institute of Observational Sciences（阿亚巴塔观测科学研究所）；Department of Applied Physics, Mahatma Jyotiba Phule Rohilkhand University（马哈特马·乔蒂巴·普尔罗希勒坎德大学应用物理系）；Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences（波兰科学院尼古拉·哥白尼天文学中心）
Two temperature accretion around rotating black holes: Description of general advective flow paradigm in presence of various cooling processes to explain low to high luminous sources
We use global magnetohydrodynamic simulations to study the influence of net vertical magnetic fields on the structure of geometrically thin ($H/r \approx 0.05$) accretion disks in the Newtonian limit. We consider initial mid-plane gas to magnetic pressure ratios $β_0 = 1000,\, 300$ and $100$, spanning the transition between weakly and strongly magnetized accretion regimes. We find that magnetic pressure is important for the disks' vertical structure in all three cases, with accretion occurring at $z/R\approx 0.2$ in the two most strongly magnetized models. The disk midplane shows outflow rathe…
An extended magnetosphere of a strongly magnetized accreting white dwarf (known as a polar) prevents the formation of an accretion disk and the matter is channelled to the magnetic pole(s). A few such sources show quasi-periodic oscillations in their optical light curves. These high-frequency oscillations are thought to be generated from the post-shock accretion column. The kinetic energy of the accretion flow is finally emitted from this post-shock region and the involved radiation processes decide the state of the matter. Here we study the structure and the dynamical properties of such accre…
We explore the global structure of the accretion flow around a Schwarzschild black hole where the accretion disc is threaded by toroidal magnetic fields. The accretion flow is optically thin and advection dominated. Synchrotron radiation is considered to be the active cooling mechanism in the flow. With this, we obtain the global transonic accretion solutions and show that centrifugal barrier in the rotating magnetized accretion flow causes a discontinuous transition of the flow variables in the form of shock waves. The shock properties and the dynamics of the post-shock corona are affected by…
Two Component Advective Flow (TCAF) successfully explains the spectral and tem- poral properties of outbursting or persistent sources. Images of static TCAF with Compton cloud or CENtrifugal pressure supported Boundary Layer (CENBOL) due to gravitational bending of photons have been studied before. In this paper, we study time dependent images of advective flows around a Schwarzschild black hole which include cooling effects due to Comptonization of soft photons from a Keplerian disks well as the self-consistently produced jets and outflows. We show the overall image of the disk-jet system aft…
We solved analytically viscous two-dimensional (2D) fluid equations for accretion and outflows in spherical polar coordinates ($r, θ, ϕ$) and obtained explicitly flow variables in $r-$ and $θ-$directions around black holes (BHs). We investigated global transonic advection-dominated accretion flow (ADAF) solutions in $r-$direction on an equatorial plane with using Paczyński-Wiita potential. We used radial flow variables of ADAFs with symmetric conditions on the equatorial plane, as initial values for integration in $θ-$direction. In the study of 2D disk structure, we used two-azimuthal componen…
