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Solutions of black hole accretion flows with axisymmetric shocks are obtained self-consistently when the dissipation at the post-shock flow is taken into account. The Rankine-Hugoniot relationships had to be modified suitably to incorporate the energy loss as well as possible matter loss due to outflows in the post-shock region. The outflow rate from the post-shock region is also computed self-consistently. This was done by considering the quantities in the subsonic post-shock flow as the initial condition for the conical outflow. We have several major results: we find the analytical expression of the ratio of the outflow rate and the inflow rate Rṁ. We find that Rṁ strongly depends on the model assumptions which govern the flow geometry. It appears that, (a) the outflow rate is at most a few percent of the inflow rate, (b) the outflow is absent when the shock is relatively weak, (c) the outflow rate decreases with the increase in the energy loss at the post-shock region. These conclusions are very important as they have direct bearings on the observational effects. Since spectrally soft states are generally believed to be caused by the dominance of the soft photons and almost total loss of thermal energy of the Compton cloud by inverse Comptonization, a spectrally softer state should have less outflows. The opposite is generally true: A spectrally harder state will have a stronger outflow, but the result depends on the compression ratio and the adopted model. The other major result is that the model independence of the transonic properties of the flow does not hold in presence of the loss of the energy (radiation) and mass (outflow). [ABSTRACT FROM AUTHOR]

The Galactic microquasar GRS 1915 + 105 exhibits various types of light curves. There is, however, no understanding of when a certain type of light curve will be exhibited and only in a handful of cases, the transitions from one type to another have actually been observed. We study the detailed spectral properties in these cases to show that different classes have different ratio of the power-law photon and the blackbody photon. Since the power-law photons are from the Compton cloud, and the intensity of the power-law photon component depends on the degree of interception of the soft photons by the Compton cloud, we conclude that not only the accretion rate, but the accretion flow geometry must also change during a class transition. [ABSTRACT FROM AUTHOR]

Exactly three decades ago, it was realized that an accretion flow onto a black hole should be transonic. Since then, the subject has matured considerably and several new and well established concepts and methodologies have replaced earlier ways of studying accretion and winds. Not surprisingly, with the advent of the faster computers as well as better space-based telescopes, the results of numerical simulations and the observations have also improved along with the theory. Today, it is more than satisfying that the results of theory and numerical simulations, even in the context of nonmagnetic flows, agree in details of the observations exceedingly well. I present here several new concepts and intricacies which one has to get familiar with when one talks about the behavior of the transonic flows, either in accretion or in the outflows. [ABSTRACT FROM AUTHOR]

We study the properties of a steady, multi-species, low angular momentum accretion flow around a Schwarzschild black hole. Each species is described by a relativistic equation of state. We find that the transonic properties depend strongly on the composition of the flow. We find that an electron-positron pair plasma is the least relativistic one. This flow produces only one sonic point very close to the event horizon and does not show multiple critical points for any angular momentum or energy. When the baryons are present, the number of critical points depend on the specific energy content. Since the number of critical points decide whether the flow will have nonlinearities or shock waves, our results imply that whether standing shocks will form or not depends on the flow composition. Thus, for instance, a pure electron-positron pair plasma will never undergo a shock transition, while mixing it with some baryons (common in outflows and jets, for example) as in a completely ionized gas, will have shocks. We study in detail how the baryon loading affects the shock properties and discuss the implications in astrophysical observations. [ABSTRACT FROM AUTHOR]

Accretion flow on a horizon is supersonic, no matter what the flow angular momentum or the spin of the black hole is. This means that a black hole accretion can always be viewed as a flow in a flat space-time through one or more convergent-divergent ducts. In this paper, we study how the area of cross-sections must vary in order that the flow has the same properties in both systems. We show that the accretion flow experiencing a shock is equivalent to having two ducts connected back-to-back, both with a neck where the flow becomes supersonic. We study the pressure and Mach number variations for corotating, contrarotating flows and flows around a black hole with evolving spin. [ABSTRACT FROM AUTHOR]

A black hole accretion may have both the Keplerian and the sub-Keplerian component. In the so-called Chakrabarti–Titarchuk scenario, the Keplerian component supplies low-energy (soft) photons while the sub-Keplerian component supplies hot electrons which exchange their energy with the soft photons through Comptonization or inverse Comptonization processes. In the sub-Keplerian component, a shock is generally produced due to the centrifugal force. The postshock region is known as the CENtrifugal pressure–supported BOundary Layer (CENBOL). In this paper, we compute the effects of the thermal and the bulk motion Comptonization on the soft photons emitted from a Keplerian disk by the CENBOL, the preshock sub-Keplerian disk and the outflowing jet. We study the emerging spectrum when the converging inflow and the diverging outflow (generated from the CENBOL) are simultaneously present. From the strength of the shock, we calculate the percentage of matter being carried away by the outflow and determine how the emerging spectrum depends on the outflow rate. The preshock sub-Keplerian flow is also found to Comptonize the soft photons significantly. The interplay between the up-scattering and down-scattering effects determines the effective shape of the emerging spectrum. By simulating several cases with various inflow parameters, we conclude that whether the preshock flow, or the postshock CENBOL or the emerging jet is dominant in shaping the emerging spectrum depends strongly on the geometry of the flow and the strength of the shock in the sub-Keplerian flow. [ABSTRACT FROM AUTHOR]

We compute the effects of thermal Comptonization of soft photons emitted from a Keplerian disk around a black hole by the postshock region of a sub-Keplerian flow, known as the CENtrifugal-pressure-dominated BOundary Layer (CENBOL). We show that the spectral state transitions of black hole candidates could be explained either by varying the outer boundary of the CENBOL, which also happens to be the inner edge of the Keplerian disk, or by changing the central density of the CENBOL, which is governed by the rate of the sub-Keplerian flow. We confirm the conclusions of the previous theoretical studies that the interplay between the intensity of the soft photons emitted by the Keplerian flow, the optical depth and electron temperature of the Comptonizing cloud is responsible for the state transitions in a black hole. [ABSTRACT FROM AUTHOR]

Black hole accretion is necessarily transonic and the number of physical sonic points depends on the angular momentum of the flow. We study the properties of such a flow by recasting this idea into an engineering problem in which a flow has a subsonic to supersonic transition when it passes through a de Laval nozzle, i.e. a converging and diverging duct in a flat geometry in the presence of sufficient end pressure difference. Particularly interesting is the case of the centrifugal pressure supported standing shock formation inside an accretion flow, because the flow passes through at least two saddle type sonic points, one before and one after the shock. In this case, the duct itself has two minima and a maximum. We study the properties of such a duct as a function of the inflow parameters and classify all possible types of the flow through this composite nozzle. [ABSTRACT FROM AUTHOR]

Non-linearities such as shock waves are common in accretion flows around compact objects. Exact quantification of these non-linearities will help testing time-dependent numerical codes. In this paper, we study the detailed properties of these non-linear waves in a steady accretion or wind flows around a rotating black hole. We use a pseudo-Kerr geometry for this purpose. In the context of energy preserving standing shocks, we find that there are two shock locations for a given pair of conserved flow parameters, such as specific energy and angular momentum. We also show that as the Kerr parameter is increased, the shock location moves closer to the black hole. We discuss the astrophysical implications of such solutions. [ABSTRACT FROM AUTHOR]

We compute the effects of the centrifugal pressure supported shock waves on the emitted spectrum from an accretion disk primarily consisting of low angular momentum matter. Electrons are very efficiently accelerated by the accretion shock and acquire power-law distribution. The accelerated particles in turn emit synchrotron radiation in the presence of a stochastic magnetic field in equipartition with the gas. Efficient cooling of the electrons by these soft photons reduces its temperature in comparison to the protons. We explore the nature of the broadband spectra by using Comptonization, bremsstrahlung and synchrotron emission. We then show that there could be two crossing points in a broadband spectrum, one near ~10 keV and the other ~300–400 keV. [ABSTRACT FROM AUTHOR]

Low angular momentum accretion flows can have standing and oscillating shock waves. We study the region of the parameter space in which multiple sonic points occur in viscous flows in presence of various cooling effects such as bremsstrahlung and Comptonization. We also quantify the parameter space in which shocks are steady or oscillating. We find that cooling induces effects opposite to heating by viscosity even in modifying the topology of the solutions, though one can never be exactly balanced by the other due to their dissimilar dependence on dynamic and thermodynamic parameters. We show that beyond a critical value of cooling, the flow ceases to contain a shock wave. [ABSTRACT FROM AUTHOR]

Astrophysical jets are observed to come out with relativistic speed and are extremely collimated. Here we study the effect of momentum deposition by hot photons from the torus like, puffed up inner region of the accretion disc on radial outflows. We calculate the radiative momentum deposition force on the outflows and find that there is a focusing effect. The size of the radiating torus completely determines the location where the radiative force is maximum. We solve the steady state equations of motion of the radial outflow, in Schwarzschild geometry to show that the acceleration achieved is appreciable, when it is compared with Bondi-type outflow. We also show that, the photon momentum deposition force results in bringing the sonic point closer to the black hole thereby resulting in a more energetic outflows as compared to purely thermally driven outflow. Particularly interesting is that in many cases, where bound matters are freed and driven off as outflows after sufficient momentum is deposited. [ABSTRACT FROM AUTHOR]

Winds and outflows form in active galaxies and in binary systems which are known to harbour compact objects such as black holes. Matter starting subsonically from a disc must be accelerated very close to the black hole in order to reach a velocity comparable to the velocity of light, which is actually observed. In the absence of magnetic fields, winds forming in inner regions of accretion discs could primarily be accelerated by radiations emitted from this region where centrifugal force is important. We study critical point behaviour of outflows in presence of this radiative acceleration. We show that the momentum deposition term changes the character of the solution drastically depending on the magnitude and the location of the deposition. We discuss the implications of these solutions in detail. Particularly important is the fact that matter were found to be pushed to infinity, even when they were originally bound energetically. We perform numerical simulations by smoothed particle hydrodynamics (SPH), and show that these new solutions are stable. [ABSTRACT FROM AUTHOR]