We show that the hard X-ray (HXR) emission observed from several galaxy clusters is naturally explained by a simple model, in which the nonthermal emission is produced by inverse Compton scattering of cosmic microwave background photons by electrons accelerated in cluster accretion shocks: The dependence of HXR surface brightness on cluster temperature is consistent with that predicted by the model, and the observed HXR luminosity is consistent with the fraction of shock thermal energy deposited in relativistic electrons being \lesssim 0.1. Alternative models, where the HXR emission is predicted to be correlated with the cluster thermal emission, are disfavored by the data. The implications of our predictions to future HXR observations (e.g. by NuStar, Simbol-X) and to (space/ground based) gamma-ray observations (e.g. by Fermi, HESS, MAGIC, VERITAS) are discussed.
In this Letter, we present a new self-consistent theory for the production of the relativistic outflows observed from radio-loud black hole candidates and active galaxies as a result of particle acceleration in hot, viscous accretion disks containing standing, centrifugally-supported isothermal shocks. This is the first work to obtain the structure of such disks for a relatively large value of the Shakura-Sunyaev viscosity parameter ($α=0.1$), and to consider the implications of the shock for the acceleration of relativistic particles in viscous disks. In our approach, the hydrodynamics and th…
Accretion of gas onto already virialized structures like galaxy clusters should give rise to accretion shocks which can potentially accelerate cosmic rays. Here, we use the radio emission detected from Coma cluster and models of evolution of cosmic accretion shocks, to constrain the possible contribution of unresolved galaxy clusters to the cosmic radio background. We assume that Coma is a typical galaxy cluster and that its entire radio emission is produced by cosmic rays accelerated in accretion shocks, making our prediction an upper limit. Our models predict that at lower frequencies accret…
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…
We present the global solutions of low angular momentum, inviscid, advective accretion flow around Kerr-Taub-NUT (KTN) black hole in presence and absence of shock waves. These solutions are obtained by solving the governing equations that describe the relativistic accretion flow in KTN spacetime which is characterized by the Kerr parameter ($a_{\rm k}$) and NUT parameter ($n$). During accretion, rotating flow experiences centrifugal barrier that eventually triggers the discontinuous shock transition provided the relativistic shock conditions are satisfied. In reality, the viability of shocked …
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 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…
Simulation based parameter space for shock in transonic, sub-Keplerian accretion flow onto non-rotating black holes
Indian Institute of Science Education and Research Kolkata（印度科学教育研究所加尔各答分校）；Center of Excellence in Space Sciences India, Indian Institute of Science Education and Research Kolkata（印度空间科学卓越中心，印度科学教育研究所加尔各答分校）
We study viscous accretion disc around black holes, and all possible accretion solutions, including shocked as well as shock free accretion branches. Shock driven bipolar outflows from a viscous accretion disc around a black hole has been investigated. One can identify two critical viscosity parameters $α_{cl}$ and $α_{cu}$, within which the stationary shocks may occur, for each set of boundary conditions. Adiabatic shock has been found for upto viscosity parameter $α=0.3$, while in presence of dissipation and massloss we have found stationary shock upto $α=0.15$. The mass outflow rate may inc…
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 perform 1D/2D/3D relativistic hydrodynamical simulations of accretion flows with low angular momentum, filling the gap between spherically symmetric Bondi accretion and disc-like accretion flows. Scenarios with different directional distributions of angular momentum of falling matter and varying values of key parameters such as spin of central black hole, energy and angular momentum of matter are considered. In some of the scenarios the shock front is formed. We identify ranges of parameters for which the shock after formation moves towards or outwards the central black hole or the long las…
We simulate shock-free and shocked viscous accretion flow onto a black hole in a two dimensional cylindrical geometry, where initial conditions were chosen from analytical solutions. The simulation code used the Lagrangian Total Variation Diminishing (LTVD) and remap routine, which enabled us to attain high accuracy in capturing shocks and to handle the angular momentum distribution correctly. Inviscid shock-free accretion disk solution produced a thick disk structure, while the viscous shock-free solution attained a Bondi-like structure, but in either case, no jet activity nor any QPO-like ac…
This paper is a sequel to our previous work for accretion onto a Schwarzschild black hole and the so-called standing accretion shock instability (SASI), in this paper we investigate non-axisymmetric perturbations for a Kerr black hole. The linear and non-linear phases for the shock evolution are analyzed in detail by both 2D general relativistic hydrodynamical simulations and linear analysis. Since the structure of steady axisymmetric accretion flows with a standing shock wave is very sensitive to the inner transonic flow, their properties such as Mach numbers, which are important for the stab…
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.…
Short-term variability and multiple velocity components in the powerful highly ionized wind of the archetypal UFO PG1211+143 are indicative of inner disc instabilities or short-lived accretion events. The recent detection of a high velocity inflow offered the first direct observational support for the latter scenario, where matter approaching at a high inclination to the black hole spin plane may result in warping and tearing of the inner accretion disc, with subsequent inter-ring collisions producing shocks, loss of rotational support and rapid mass infall. Here we identify a variable continu…
Simulations of hot, pressure supported, tilted black hole accretion flows, in which the angular momentum of the flow is misaligned with the black hole spin axis, can exhibit two non-axisymmetric shock structures in the inner regions of the flow. We analyze the strength and significance of these shock structures in simulations with tilt angles of 10 and 15 degrees. By integrating fluid trajectories in the simulations through the shocks, and tracking the variations of fluid quantities along these trajectories, we show that these shocks are strong, with substantial compression ratios, in contrast…
Cosmology models predict that external accretion shocks form in the outer region of galaxy clusters due to supersonic gas infall from filaments and voids in the cosmic web. They are characterized by high sonic and Alfvénic Mach numbers, $M_s\sim10-10^2$ and $M_A\sim10^2-10^3$, and propagate into weakly magnetized plasmas of $β\equiv P_g/P_B\gtrsim10^2$. Although strong accretion shocks are expected to be efficient accelerators of cosmic rays (CRs), nonthermal signatures of shock-accelerated CRs around clusters have not been confirmed, and detailed acceleration physics at such shocks has yet to…
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…
We show for the first time the direct time variable radio images in the context of shocked accretion flows around a black hole under the general relativistic treatment of both hydrodynamics and radiation transfer. Time variability around a black hole can be induced by the non-axisymmetric standing accretion shock instability (namely Black Hole SASI). Since the spiral arm shock waves generate the density and temperature waves at the post shock region, they cause time variability in the black hole vicinity. Based on our dynamical simulations, we discuss a possibility of detection for the time va…
We examine the properties of spiral shocks from a steady, adiabatic, non-axisymmetric accretion disk around a compact star in binary. We first time incorporate all the possible influences from binary through adopting the Roche potential and Coriolis forces in the basic conservation equations. In this paper, we assume the spiral shocks to be point-wise self-similar, and the flow is in vertical hydrostatic equilibrium to simplify the study. We also investigate the mass outflow due to the shock compression and apply it to the accreting white dwarf in binary. We find that our model will be benefic…
We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion - ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von - Zeipel surfaces and projected the jet equations of motion on to the streamline and solved them simultaneously with the accretion disc equations of motion. We found that steady shock cannot exist {for $α\gsim0.06$} in the general relativistic prescription, but is lo…
In this study, we conduct three-dimensional hydrodynamic simulations systematically to investigate the flow patterns behind the accretion shock waves that are commonly formed in the post-bounce phase of core-collapse supernovae. Adding small perturbations to spherically symmetric, steady, shocked accretion flows, we compute the subsequent evolutions to find what flow pattern emerges as a consequence of hydrodynamical instabilities such as convection and standing accretion shock instability (SASI) for different neutrino luminosities and mass accretion rates. Depending on these two controlling p…
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.
We analyse flows around a rotating black hole and obtain self-consistent accretion-ejection solutions in full general relativistic prescription. Entire energy-angular momentum parameter space is investigated in the advective regime to obtain shocked and shock-free accretion solutions. Jet equations of motion are solved along the von-Zeipel surfaces computed from the post-shock disc, simultaneously with the equations of accretion disc along the equatorial plane. For a given spin parameter, the mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, max…
AIMS. We investigate the stability and dynamics of accretion shocks in CTTSs, considering the case of beta >= 1 in the post-shock region. In these cases the 1D approximation is not valid and a multi-dimensional MHD approach is necessary. METHODS. We model an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere, by performing 2D axisymmetric MHD simulations. The model takes into account the stellar magnetic field, the gravity, the radiative cooling, and the thermal conduction (including the effects of heat flux saturation). RESULTS. The dynamics and …
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 study numerically the axisymmetric relativistic Bondi-Hoyle accretion of a supersonic ideal gas onto a fixed Schwarzschild background space-time described with horizon penetrating coordinates. We verify that a nearly stationary shock cone forms and that the properties of the shock cone are consistent with previous results in Newtonian gravity and former relativistic studies. The fact that the evolution of the gas is tracked on a spatial domain that contains a portion of the inner part of the black hole avoids the need to impose boundary conditions on a time-like boundary as done in the past…
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…
Low-frequency quasi-periodic oscillations (LFQPOs) have been routinely observed in black hole X-ray binaries (BHXRBs). These LFQPOs can be explained by axisymmetric shock oscillation in accretion flow around a rotating black hole. We address the physical origin of Type-C LFQPOs in BHXRBs observed by the Rossi X-ray Timing Explorer satellite considering a minimum number of free parameters, namely, specific energy and specific angular momentum of the infalling matter for a given set of BH mass and spin parameter. We apply the solution for a large number of BH candidates to further strengthen the…
Outflow from unmagnetized shocked radiative transonic accretion disk around a black hole
Indian Institute of Science Education and Research Kolkata（印度科学教育研究所加尔各答分校）；Center of Excellence in Space Sciences India, Indian Institute of Science Education and Research Kolkata（印度太空科学卓越中心，印度科学教育研究所加尔各答分校）
We study numerically the relativistic Bondi-Hoyle accretion of an ideal gas onto a Kerr fixed background space-time on the equatorial plane with s-lab symmetry. We use both Kerr-Schild (KS) and Boyer-Lindquist (BL) coordinates. We particularly focus on the study of the flip-flop motion of the shock cone formed when the gas is injected at supersonic speed. The development of the flip-flop instability of the shock cone in the relativistic regime was reported recently for the first time. We reproduce the flip-flop behaviour found in the past when BL coordinates are used, and perform similar numer…
We present here the solutions of magnetized accretion flows on to a compact object with hard surface such as neutron stars. The magnetic field of the central star is assumed dipolar and the magnetic axis is assumed to be aligned with the rotation axis of the star. We have used an equation of state for the accreting fluid in which the adiabatic index is dependent on temperature and composition of the flow. We have also included cooling processes like bremsstrahlung and cyclotron processes in the accretion flow. We found all possible accretion solutions. All accretion solutions terminate with a …
A set of strongly magnetized accreting white dwarfs (polars) shows quasi-periodic oscillations (QPOs) with frequency about a Hz in their optical luminosity. These Hz-frequency QPOs are thought to be generated by intensity variations of the emitted radiation originating at the post-shock accretion column. Thermal instability in the post-shock region, triggered by efficient cooling process at the base, is believed to be the primary reason behind the temporal variability. Here we study the structure and the dynamical properties of the post-shock accretion column including the effects of bremsstra…
We study the spatial and temporal behavior of fluid in fully three-dimensional, general relativistic, magnetohydrodynamical simulations of both tilted and untilted black hole accretion flows. We uncover characteristically greater variability in tilted simulations at frequencies similar to those predicted by the formalism of trapped modes, but ultimately conclude that its spatial structure is inconsistent with a modal interpretation. We find instead that previously identified, transient, over-dense clumps orbiting on roughly Keplerian trajectories appear generically in our global simulations, i…
The secular evolution of the purely general relativistic low angular momentum accretion flow around a spinning black hole is shown to exhibit hysteresis effects. This confirms that a stationary shock is an integral part of such an accretion disc in the Kerr metric. The equations describing the space gradient of the dynamical flow velocity of the accreting matter have been shown to be equivalent to a first order autonomous dynamical systems. Fixed point analysis ensures that such flow must be multi-transonic for certain astrophysically relevant initial boundary conditions. Contrary to the exist…
Context. High resolution X-ray observations of classical T Tauri stars (CTTSs) show a soft X-ray excess due to high density plasma (n_e=10^11-10^13 cm^-3). This emission has been attributed to shock-heated accreting material impacting onto the stellar surface. Aims. We investigate the observability of the shock-heated accreting material in the X-ray band as a function of the accretion stream properties (velocity, density, and metal abundance) in the case of plasma-beta<<1 in the post-shock zone. Methods. We use a 1-D hydrodynamic model describing the impact of an accretion stream onto the chro…
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…
In our study of the timing properties of few Galactic black hole candidates evolutions of the low and intermediate frequency quasi-periodic oscillations (LIFQPOs) are observed. In 2005, for explaining evolution of QPO frequency during rising phase of 2005 GRO J1655-40 outburst, Chakrabarti and his students introduced a new model, namely propagating oscillatory shock (POS) model. Here we present the results obtained from the same POS model fitted QPO evolutions during both the rising and declining phases of the outbursts of 2005 GRO J165540, 2010-11 GX 339-4, and 2010 & 2011 H 1743-322.
Two Mpc-size contact discontinuities have recently been identified in the XMM-Newton and Suzaku X-ray observations of the outskirts of the Perseus cluster (Walker et al. 2020). These structures have been tentatively interpreted as "sloshing cold fronts", which are customarily associated with differential motions of the cluster gas, perturbed by a merger. In this study we consider an alternative scenario, namely, that the most prominent discontinuity near the cluster virial radius is the result of the collision between the accretion shock and the "runaway" merger shock. We also discuss the poss…
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…
Core-collapse supernovae span a wide range of energies, from much less than to much greater than the binding energy of the progenitor star. As a result, the shock wave generated from a supernova explosion can have a wide range of Mach numbers. In this paper, we investigate the propagation of shocks with arbitrary initial strengths in polytropic stellar envelopes using a suite of spherically symmetric hydrodynamic simulations. We interpret these results using the three known self-similar solutions for this problem: the Sedov-Taylor blastwave describes an infinitely strong shock and the self-sim…
We study the relativistic viscous accretion flows around the Kerr black holes. We present the governing equations that describe the steady state flow motion in full general relativity and solve them in 1.5D to obtain the complete set of global transonic solutions in terms of the flow parameters, namely specific energy (${\cal E}$), specific angular momentum (${\cal L}$) and viscosity ($α$). We obtain a new type of accretion solution which was not reported earlier. Further, we show for the first time to the best of our knowledge that viscous accretion solutions may contain shock waves particula…
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…
Models and observations indicate that the impact of matter accreting onto the surface of young stars produces regions at the base of accretion columns, in which optically thin and thick plasma components coexist. Thus an accurate description of these impacts requires to account for the effects of absorption and emission of radiation. We study the effects of radiation emerging from shock-heated plasma in impact regions on the structure of the pre-shock downfalling material. We investigate if a significant absorption of radiation occurs and if it leads to a pre-shock heating of the accreting gas…
Accretion of gas during the large scale structure formation has been thought to give rise to shocks that can accelerate cosmic rays. This process then results in an isotropic extragalactic gamma-ray emission contributing to the extragalactic gamma-ray background observed by the Fermi-LAT. Unfortunately this emission has been difficult to constrain and thus presents an uncertain foreground to any attempts to extract potential dark matter signal. Recently, IceCube has detected high-energy isotropic neutrino flux which could be of an extragalactic origin. In general, neutrinos can be linked to ga…
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-fa…
We develop and discuss global accretion solutions for viscous ADAF disks containing centrifugally supported isothermal shock waves. The fact that such shocks can exist at all in ADAF disks is a new result. Interestingly, we find that isothermal shocks can form even when the level of viscous dissipation is relatively high. In order to better understand this phenomenon, we explore all possible combinations of the fundamental flow parameters, such as specific energy, specific angular momentum, and viscosity, to obtain the complete family of global solutions. This procedure allows us to identify t…
A new target design is presented to model high-energy radiative accretion shocks in polars. In this paper, we present the experimental results obtained on the GEKKO XII laser facility for the POLAR project. The experimental results are compared with 2D FCI2 simulations to characterize the dynamics and the structure of plasma flow before and after the collision. The good agreement between simulations and experimental data confirm the formation of a reverse shock where cooling losses start modifying the post-shock region. With the multi-material structure of the target, a hydrodynamic collimatio…
We investigated accretion on to black holes in presence of viscosity and cooling, by employing an equation of state with variable adiabatic index and multi-species fluid. We obtained the expression of generalized Bernoulli parameter which is a constant of motion for an accretion flow in presence of viscosity and cooling. We obtained all possible transonic solutions for a variety of boundary conditions, viscosity parameters and accretion rates. We identified the solutions with their positions in the parameter space of generalized Bernoulli parameter and the angular momentum on the horizon. We s…
We present solutions for the self-similar propagation of a shock wave in a hydrostatic, adiabatic medium with a point mass gravitational field. In contrast to the well-known, Sedov-Taylor blastwave, these solutions apply to the case when the shock Mach number is of order a few, and the energy of the shocked fluid is not conserved but self-consistently modified by the binding energy of the ambient medium that is swept up by the passage of the shock. Furthermore, we show that there is one solution (for a given ambient density profile) that smoothly passes through a sonic point in the post-shock …
