We investigate the dynamical structure of advective accretion flow around stationary as well as rotating black holes. For a suitable choice of input parameters, such as, accretion rate ($\dot {\cal M}$) and angular momentum ($λ$), global accretion solution may include a shock wave. The post shock flow is located at few tens of Schwarzchild radius and it is generally very hot and dense. This successfully mimics the so called Compton cloud which is believed to be responsible for emitting hard radiations. Due to the radiative loss, a significant energy from the accreting matter is removed and the shock moves forward towards the black hole in order to maintain the pressure balance across it. We identify the effective area of the parameter space ($\dot {\cal M} - λ$) which allows accretion flows to have some energy dissipation at the shock $(Δ{\cal E})$. As the dissipation is increased, the parameter space is reduced and finally disappears when the dissipation is reached its critical value. The dissipation has a profound effect on the dynamics of post-shock flow. By moving forward, an unstable shock whose oscillation causes Quasi-Periodic Oscillations (QPOs) in the emitted radiation, will produce oscillations of high frequency. Such an evolution of QPOs has been observed in several black hole candidates during their outbursts.
We present the global structure of magnetized advective accretion flow around the rotating black holes in presence of dissipation. By considering accretion flow to be threaded by toroidal magnetic fields and by assuming synchrotron radiative mechanism to be the dominant cooling process, we obtain global transonic accretion solutions in terms of dissipation parameters, such as viscosity ($α_B$), accretion rate (${\dot m}$) and plasma-$β$, respectively. In the rotating magnetized accretion flow, centrifugal barrier is developed in the nearby region of the black hole that triggers the discontinuo…
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…
A core-collapse supernova is generated by the passage of a shockwave through the envelope of a massive star, where the shock wave is initially launched from the ``bounce'' of the neutron star formed during the collapse of the stellar core. Instead of successfully exploding the star, however, numerical investigations of core-collapse supernovae find that this shock tends to ``stall'' at small radii ($\lesssim$ 10 neutron star radii), with stellar material accreting onto the central object through the standing shock. Here, we present time-steady, adiabatic solutions for the density, pressure, an…
Using the relativistic MHD code MPI-AMRVAC and a radiative transfer code in post-processing, we explore the influence of the magnetic-field configuration and transverse stratification of an over-pressured jet on its morphology, on the moving shock dynamics, and on the emitted radio light curve. First, we investigate different large-scale magnetic fields with their effects on the standing shocks and on the stratified jet morphology. Secondly, we study the interaction of a moving shock wave with the standing shocks. We calculate the synthetic synchrotron maps and radio light curves and analyse t…
I present a scenario by which an accretion flow with alternating angular momentum sense on to a newly born neutron star in core collapse supernovae (CCSNe) efficiently amplifies magnetic fields and by that launches jets. The accretion flow of a collapsing core on to the newly born neutron star suffers the spiral standing accretion shock instability (SASI). This instability leads to a stochastically variable angular momentum of the accreted gas, that in turn forms an accretion flow with alternating directions of the angular momentum, hence alternating shear, at any given time. I study the shear…
We study two dimensional low angular momentum flow around the black hole using the resistive magnetohydrodynamic module of PLUTO code. Simulations have been performed for the flows with parameters of specific angular momentum, specific energy, and magnetic field which may be expected for the flow around Sgr A*. For flows with lower resistivity $η=10^{-6}$ and $0.01$, the luminosity and the shock location on the equator vary quasi-periodically. The power density spectra of luminosity variation show the peak frequencies which correspond to the periods of $5 \times 10^5$, $1.4 \times 10^5$, and $…
Accretion on length-scales of interstellar or even intergalactic is particularly relevant, in the context of spherical or quasi-spherical hot mode accretion, supposedly powering low excitation radio galaxies in the maintenance-mode feedback paradigm. In the present study, we aim to analyze such a spherical-type flow around the host active nucleus in the backdrop of a five-component galactic system (SMBH, stellar, dark matter, diffuse hot gas, $Λ$), with the principal intent to address the issue of {\it galactic potential induced shock} formation in the flow, that may contain (dissipative) isot…
We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We exami…
Through numerical simulations, it is predicted that the gravitational waves (GWs) reflect the characteristics of the core-collapse supernova (CCSN) explosion mechanism. There are multiple GW excitation processes that occur inside a star before its explosion, and it is suggested that the GWs originating from the CCSN have a mode for each excitation process in terms of time-frequency representation. Therefore, we propose an application of the Hilbert-Huang Transform (HHT), which is a high-resolution time-frequency analysis method, to analyze these GW modes for theoretically probing and increasin…
We investigate the global structure of the advection dominated accretion flow around a Schwarzschild black hole where the accretion disc is threaded by toroidal magnetic fields. We consider synchrotron radiative process as an effective cooling mechanism active in the flow. With this, we obtain the global transonic accretion solutions by exploring the variety of boundary conditions and dissipation parameters, namely accretion rate (${\dot m}$) and viscosity ($α_B$). The fact that depending on the initial parameters, steady state accretion flows can possess centrifugally supported shock waves. T…
We show that the standing accretion shock instability (SASI) that has been used to ease the shock revival in core collapse supernovae (CCSNe) neutrino-driven explosion models, might play a much more decisive role in supplying the stochastic angular momentum required to trigger an explosion with jittering jets. We find that if the kinetic energy associated with the transverse (non radial) motion of the SASI is larger than about ten percent of the energy associated with the energy of the accreted gas, then the stochastic angular momentum can reach about five percent of the Keplerian specific ang…
We present results from fully relativistic three-dimensional core-collapse supernova (CCSN) simulations of a non-rotating 15 Msun star using three different nuclear equations of state (EoSs). From our simulations covering up to ~350 ms after bounce, we show that the development of the standing accretion shock instability (SASI) differs significantly depending on the stiffness of nuclear EoS. Generally, the SASI activity occurs more vigorously in models with softer EoS. By evaluating the gravitational-wave (GW) emission, we find a new GW signature on top of the previously identified one, in whi…
Numerical hydrodynamic simulation of inviscid and viscous flows have shown that significant outflows could be produced from the CENtrifugal pressure supported BOundary Layer or CENBOL of an advective disk. However, this barrier is weakened in presence of viscosity, more so, if there are explicit energy dissipations at the boundary layer itself. We study effects of viscosity and energy dissipation theoretically on the outflow rate and show that as the viscosity or energy dissipation (or both) rises, the prospect of formation of outflows is greatly reduced, thereby verifying results obtained thr…
We study the properties of two-temperature accretion flow around a non-rotating black hole in presence of various dissipative processes where pseudo-Newtonian potential is adopted to mimic the effect of general relativity. The flow encounters energy loss by means of radiative processes acted on the electrons and at the same time, flow heats up as a consequence of viscous heating effective on ions. We assumed that the flow is exposed with the stochastic magnetic fields which leads to Synchrotron emission of electrons and these emissions are further strengthen by Compton scattering. We obtain th…
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…
This paper is a sequel to Takahashi et al. (2016), in which the authors investigated the influences of fluctuations in pre-shock accreting matter on the linear stability of the standing accretion shock in core-collapse supernovae (CCSNe). If one understands that this was concerning the effect of the outer boundary condition for the post-shock accretion flows, the present paper should be regarded as an investigation on the possible influences of the inner boundary conditions. More specifically, we impose a time-dependent, oscillating condition at the inner boundary, having in mind the injection…
Despite the sphericity of the collapsing stellar core, the birth conditions of neutron stars can be highly non spherical due to a hydrodynamical instability of the shocked accretion flow. Here we report the first laboratory experiment of a shallow water analogue, based on the physics of hydraulic jumps. Both the experiment and its shallow water modeling demonstrate a robust linear instability and nonlinear properties of symmetry breaking, in a system which is one million times smaller and about hundred times slower than its astrophysical analogue.
We investigate the dynamical behaviour of a magnetized, dissipative, accretion flow around a rapidly rotating black hole. We solve the magnetohydrodynamic equations and calculate the transonic accretion solutions which may contain discontinuous shock transitions. We investigate the effect of $ζ-$ parameter (parametrizing the radial variation of the toroidal magnetic flux advection rate) on the dynamical behaviour of shocks. For a rapidly rotating black hole and for fixed injection parameters at the outer edge, we show that stationary shocks are sustained in the global magnetized accretion solu…
Joint Detection and Characterization of the Standing Accretion Shock Instability for Core-Collapse Supernovae with cWB XP
Universidad de Guadalajara（瓜达拉哈拉大学）；Sun Yat-Sen University（中山大学）；University of Tennessee Knoxville（田纳西大学诺克斯维尔分校）；University of New Hampshire（新罕布什尔大学）；Embry-Riddle Aeronautical University（埃姆布里-里德航空大学）；Tecnológico de Monterrey（蒙特雷理工学院）；University of Warsaw（华沙大学）
Rotation of core-collapse supernovae (SNe) affects the neutrino emission characteristics. By comparing the neutrino properties of three three-dimensional SN simulations of a 15 M_sun progenitor (one non-rotating model and two models rotating at different velocities), we investigate how the neutrino emission varies with the flow dynamics in the SN core depending on the different degrees of rotation. The large-amplitude sinusoidal modulations due to the standing accretion-shock instability (SASI) are weaker in both the rotating models than in the non-rotating case. The SN progenitor rotation red…
We study the global accretion-ejection solutions around a rotating black hole considering three widely accepted pseudo-Kerr potentials that satisfactorily mimic the space-time geometry of rotating black holes. We find that all the pseudo potentials provide standing shock solutions for large range of flow parameters. We identify the effective region of the shock parameter space spanned by energy ($\mathcal{E}_{\text{in}}$) and angular momentum ($λ_{\text{in}}$) measured at the inner critical point ($x_{\text{in}}$) and find that the possibility of shock formation becomes feeble when the viscosi…
We examine radiative standing shocks in advective accretion flows around stellar-mass black holes by 2D radiation hydrodynamic simulations, focusing on the super-Eddington accreting flow. Under a set of input flow parameters responsible for the standing shock, the shock location on the equator decreases toward the event horizon with an increasing accretion rate. The optically thin and hot gas in the narrow funnel region along the rotational axis changes gradually into a dense and optically thick state with the increasingly dense gas transported from the base of the radiative shock near the equ…
Depending on the values of the energy and angular momentum per unit mass in the gas supplied at large radii, inviscid advection-dominated accretion flows can display velocity profiles with either preshock deceleration or preshock acceleration. Nakayama has shown that these two types of flow configurations are expected to have different stability properties. By employing the Chevalier \& Imamura linearization method and the Nakayama instability boundary conditions, we discover that there are regions of parameter space where disks/shocks with outflows can be stable or unstable. In regions of ins…
Aims. We wish to establish a firm link between jet simulations and analytical studies of magnetically-driven steady-state jets from Keplerian accretion disks. In particular, the latter have predicted the existence of recollimation shocks due to the dominant hoop-stress, so far never observed in platform simulations. Methods. We perform a set of axisymmetric MHD simulations of non-relativistic jets using the PLUTO code. The simulations are designed to reproduce the boundary conditions generally expected in analytical studies. We vary two parameters: the magnetic flux radial exponent $α$ and the…
Hydrodynamical instabilities, such as the standing accretion-shock instability (SASI), play an essential role in the dynamics of core-collapse supernovae, with observable imprints in the neutrino and gravitational wave signals. Yet, the impact of stellar rotation on the development of SASI is poorly explored. We investigate the conditions favoring the growth of SASI in the presence of rotation through a perturbative analysis. The properties of SASI are compared in two stationary configurations, cylindrical and spherical equatorial, which mainly differ by their advection timescales from the sho…
The explosion mechanism of core-collapse supernovae has not been fully understood yet but multi-dimensional fluid instabilities such as standing accretion shock instability (SASI) and convection are now believed to be crucial for shock revival. Another multi-dimensional effect that has been recently argued is the asymmetric structures in progenitors, which are induced by violent convections in silicon/oxygen layers that occur before the onset of collapse, as revealed by recent numerical simulations of the last stage of massive star evolutions. Furthermore, it has been also demonstrated numeric…
Low-angular-momentum accretion shocks can power weak-to-moderate X-ray flares from SgrA*
International Center for Theoretical Sciences, Tata Institute of Fundamental Research（塔塔基础研究所国际理论科学中心）；Center for Theoretical Physics, Polish Academy of Sciences（波兰科学院理论物理中心）；Department of Theoretical Physics and Astrophysics, Masaryk University（马萨里克大学理论与天体物理学系）；McGill University（麦吉尔大学）；Trottier Space Institute（特罗蒂尔太空研究所）
We study the relativistic, inviscid, advective accretion flow around the black holes and investigate a key feature of the accretion flow, namely the shock waves. We observe that the shock-induced accretion solutions are prevalent and such solutions are commonly obtained for a wide range of the flow parameters, such as energy (${\cal E}$) and angular momentum ($λ$), around the black holes of spin value $0\le a_{\rm k} < 1$. When the shock is dissipative in nature, a part of the accretion energy is released through the upper and lower surfaces of the disc at the location of the shock transition.…
Bondi accretion assumes that there is a sink of mass at the center -- which in case of a black hole (BH) corresponds to the advection of matter across the event horizon. Other stars, such as a neutron star (NS), have surfaces and hence the infalling matter has to slow down at the surface. We study the initial value problem in which the matter distribution is uniform and at rest at $t=0$. We consider different inner boundary conditions for BHs and NSs: outflow boundary condition (mimicking mass sink at the center) valid for BHs; and {\em reflective} and steady-shock (allowing gas to cross the i…
We investigate axisymmetric steady solutions of (magneto)hydrodynamics equations that describe approximately accretion flows through a standing shock wave and discuss the effects of rotation and magnetic field on the revival of the stalled shock wave in supernova explosions. We develop a new powerful numerical method to calculate the 2-dimensional (2D) steady accretion flows self-consistently. We first confirm the results of preceding papers that there is a critical luminosity of irradiating neutrinos, above which there exists no steady solution in spherical models. If a collapsing star has ro…
Hot, tenuous advection-dominated accretion flows around black holes are ideal sites for the Fermi acceleration of relativistic particles at standing shock waves in the accretion disc. Previous work has demonstrated that the shock-acceleration process can be efficient enough to power the observed, strong outflows in radio-loud active galaxies such as M87. However, the dynamical effect (back-reaction) on the flow, exerted by the pressure of the relativistic particles, has not been previously considered, and this effect can have a significant influence on the disc structure. We reexamine the prob…
