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Two-component advective flow (TCAF) successfully explains the spectral and temporal properties of outbursting or persistent sources. Images of static TCAF with Compton cloud or centrifugal pressure supported boundary layer 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 Keplerian discs as well as the self-consistently produced jets and outflows. We show the overall image of the disc-jet system after convolving with a typical beamwidth. A long exposure image with time-dependent system need not to show the black hole horizon conspicuously, unless one is looking at a soft state with no jet or the system along the jet axis. Assuming these disc-jet configurations are relevant to radio emitting systems also, our results would be useful to look for event horizons in high accretion rate supermassive black holes in Seyfert galaxies, RL Quasars. [ABSTRACT FROM AUTHOR]

We carry out a time-dependent numerical simulation where both the hydrodynamics and the radiative transfer are coupled together. We consider a two-component accretion flow in which the Keplerian disc is immersed inside an accreting low angular momentum flow (halo) around a black hole. The injected soft photons from the Keplerian disc are reprocessed by the electrons in the halo. We show that in presence of an axisymmetric soft-photon source the spherically symmetric Bondi flow loses its symmetry and becomes axisymmetric. The low angular momentum flow was observed to slow down close to the axis and formed a centrifugal barrier which added new features into the spectrum. Using the Monte Carlo method, we generated the radiated spectra as functions of the accretion rates. We find that the transitions from a hard state to a soft state is determined by the mass accretion rates of the disc and the halo. We separate out the signature of the bulk motion Comptonization and discuss its significance. We study how the net spectrum is contributed by photons suffering different number of scatterings and spending different amounts of time inside the Compton cloud. We study the directional dependence of the emitted spectrum as well. [ABSTRACT FROM AUTHOR]

We study the spectral properties of the accretion disc in the Galactic black hole candidate XTE J 564 during the 1998–1999 outburst when the source exhibited double-peaked eruptions. This outburst lasted for 250 d and the 2.5–25.0 keV spectral state varied smoothly from one to another several times. We show that the spectral features of the 1998–1999 outburst could be clearly understood by a two-component (Keplerian and sub-Keplerian) advective flow (TCAF). We concentrate on the spectral data from a Proportional Counter Array instrument on the RXTE satellite for the black hole XTE J 564 and fit them quite satisfactorily using TCAF model. From the spectral fit we calculate the disc parameters, such as the Keplerian rate, the sub-Keplerian rate (halo rate), the shock location and the inner edge of the Keplerian disc. This observation points to the presence of two independent components in the accretion flow and that the accretion rate at all radii need not be constant in an evolving disc. [ABSTRACT FROM AUTHOR]

We study the accretion processes on a black hole by a 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-fall time-scale. (b) There are two discontinuities in the flow, one being just outside of the inner sonic point. Turbulence plays a major role in determining the locations of these discontinuities. (d) The two discontinuities oscillate with two different frequencies and behave as a coupled harmonic oscillator. A Fourier analysis of the variation of the outer shock location indicates higher power at the lower frequency and lower power at the higher frequency. The opposite is true when the analysis of the inner shock is made. These behaviours will have implications in the spectral and timing properties of black hole candidates. [ABSTRACT FROM AUTHOR]

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 and angular momentum (λ), a global accretion solution may include a shock wave. The post-shock flow is located at a few tens times the Schwarzchild radius and is generally very hot and dense. This successfully mimics the so-called Compton cloud, which is believed to be responsible for emitting hard radiation. Owing to the radiative loss, significant energy is removed from the accreting matter and the shock moves forward towards the black hole in order to maintain the pressure balance across it. We identify the effective area of parameter space that allows accretion flows to have some energy dissipation at the shock . As the dissipation is increased, the parameter space is reduced and finally disappears when the dissipation reaches a critical value. The dissipation has a profound effect on the dynamics of post-shock flow. By moving forward, an unstable shock, the oscillation of which 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. [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]

Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot regions embedded in Keplerian discs. Observational evidence of evolutions of QPOs would therefore be very important as they throw lights on the dynamics of the hotter region. Our aim is to find systems in which there is a well-defined correlation among the frequencies of the QPOs over a range of time so as to understand the physical picture. In this paper, we concentrate on the archival data of XTE J1550−564 obtained during 1998 outburst, and study the systematic drifts during the rising phase from the 1998 September 7 to the 1998 September 19, when the QPO frequency increased monotonically from 81 mHz to 13.1 Hz. Immediately after that, QPO frequency started to decrease and on the 1998 September 26, the QPO frequency became 2.62 Hz. After that, its value remained almost constant. This frequency drift can be modelled satisfactorily with a propagatory oscillating shock solution where the post-shock region behaves as the Comptonized region. Comparing with the nature of a more recent 2005 outburst of another black hole candidate GRO 1655−40, where QPOs disappeared at the end of the rising phase, we conjecture that this so-called ‘outburst’ may not be a full-fledged outburst. [ABSTRACT FROM AUTHOR]

In a recent outburst which lasted for 260 days, the black hole candidate GRO J1655-40 exhibited a behaviour similar to its last outburst observed almost eight years ago. We analyze a total of 150 observational spells in 122 days of data spreaded over the entire outburst phase of Feb. 2005 to Oct. 2005. From our study, a comprehensive understanding of the detailed behaviour of this black hole candidate has emerged. Based on the degree of importance of the black body and the power-law components we divide the entire episode in four spectral states, namely, hard, soft, very soft and intermediate. Quasi-Periodic oscillations (QPOs) were found in two out of these four states, namely, in the hard and the intermediate states. In the hard state, at the rising phase of the outburst, QPO frequency ranged from 0.034 - 17.78Hz and the spectra was fitted by a disk black body, power-law and iron emission line at 6.2 - 6.5 keV. In the intermediate state, QPOs vary from 13.17Hz to 19.04Hz and the QPO frequency modulation in this state was not significant. The spectra in this state are well fitted by the disk black body and the power-law components. In the hard state of the declining phase of the outburst, we found QPOs of decreasing frequency from 13.14 Hz to 0.034 Hz. The spectra of this state were fitted by a disk black body and power-law components, but in the initial few days a cooler Comptonized component was required for a better fit. In the soft/very soft states, the spectral states are mostly dominated by the strong disk black body component. [ABSTRACT FROM AUTHOR]

We study the dynamical structure of a cooling dominated rotating accretion flow around a spinning black hole. We show that non-linear phenomena such as shock waves can be studied in terms of only three flow parameters, namely the specific energy , the specific angular momentum (λ) and the accretion rate of the flow. We present all possible accretion solutions. We find that a significant region of the parameter space in the plane allows global accretion shock solutions. The effective area of the parameter space for which the Rankine–Hugoniot shocks are possible is maximum when the flow is dissipation-free. It decreases with the increase of cooling effects and finally disappears when the cooling is high enough. We show that shock forms further away when the black hole is rotating compared to the solution around a Schwarzschild black hole with identical flow parameters at a large distance. However, in a normalized sense, the flow parameters for which the shocks form around the rotating black holes are produced shocks closer to the black hole. The location of the shock is also dictated by the cooling efficiency in that higher the accretion rate , the closer is the shock location. We believe that some of the high-frequency quasi-periodic oscillations may be due to the flows with higher accretion rate around the rotating black holes. [ABSTRACT FROM AUTHOR]

Gravitational wave signal characteristics from a binary black hole system in which the companion moves through the accretion disc of the primary are studied. We chose the primary to be a super-massive Kerr black hole and the companion to be a massive black hole to clearly demonstrate the effects. We show that the drag exerted on the companion by the disc is sufficient to reduce the coalescence time of the binary. The drag is primarily due to the fact that the accretion disc on a black hole deviates from a Keplerian disc and becomes sub-Keplerian due to inner boundary condition on the black hole horizon. We consider two types of accretion rates on to the companion. The companion is deeply immersed inside the disc and it can accrete at the Bondi rate which depends on the instantaneous density of the disc. However, an accretion disc can also form around the smaller black hole and it can accrete at its Eddington rate. Thus, this case is also studied and the results are compared. We find that the effect of the disc will be significant in reducing the coalescence time and one needs to incorporate this while interpreting gravitational wave signals emitted from such a binary system. [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]

TAUVEX brings us a unique opportunity to explore the temporal variability of UV emitting objects in the sky. One of the questions that we intend to resolve with TAUVEX is whether the 'variabilities' detected in active galaxies and quasars and in radiations around massive black holes in general are just random variations of the intensities or these are intrinsic to the disk system, and possibly due to the quasi-periodic oscillations (QPOs) which are well known to be observed in smaller black holes (nano-quasars). In this article, we present a physical mechanism for the QPOs and show that this is a generic mechanism which should be manifested in all types of active compact objects, ranging from quasars to nano-quasars. We propose some tests by which we may be able to tell if these are QPOs, even without waiting for a large number of cycles to test the periodicity. We present a few examples to impress that perhaps we have already seen QPOs in some objects. Multi-wavelength observation capabilities in TAUVEX may be used to pinpoint the nature of the variable sources more accurately. [ABSTRACT FROM AUTHOR]

Magnetized accretion flows around black holes which include standing or oscillating shock waves can produce very realistic spectrum till a few MeV. These shocks accelerate hot electrons which produce power-law spectrum. The post-shock region intercepts soft-photons from an external source, namely, a Keplerian disk and also from distributed sources such as the synchrotron photons emitted from thermal and non-thermal electrons originated in the pre-shock and post-shock flow. These photons are inverse Comptonized by the thermal and the non-thermal electrons present in the CENBOL region. Computations show that the emitted radiation is extended till a few MeV. We include the bulk motion Comptonization as well and discuss its importance vis-a-vis the power-law spectrum produced by non-thermal electrons. [ABSTRACT FROM AUTHOR]

We solve the magnetohydrodynamic (MHD) equations governing axisymmetric flows around compact objects and found all possible classes of solutions for non-relativistic adiabatic accretion flows. We divide the parameter space in terms of these classes. We study the possibility of the formation of the MHD shock waves and show how the strength of the shocks depends on the flow parameters. We also show regions of the parameter space where the shock conditions are not satisfied and therefore the shocks may oscillate. These solutions are astrophysically interesting as they could give rise to quasi-periodic oscillations seen in hard X-rays. [ABSTRACT FROM AUTHOR]

Standing, propagating or oscillating shock waves are common in accretion and winds around compact objects. We study the topology of all possible solutions using the pseudo-Kerr geometry. We present the parameter space spanned by the specific energy and angular momentum and compare it with that obtained from the full general relativity to show that the potential can work satisfactorily in fluid dynamics also, provided the polytropic index is suitably modified. We then divide the parameter space depending on the nature of the solution topology. We specifically study the nature of the standing Rankine–Hugoniot shocks. We also show that as the Kerr parameter is increased, the shock location generally moves closer to the black hole. In future, these solutions can be used as guidelines to test numerical simulations around compact objects. [ABSTRACT FROM AUTHOR]

In this series of papers, we shall present a simplistic approach to the study of particle dynamics, fluid dynamics and numerical simulations of accretion flows and outflows around rotating black holes. We show that with a suitably modified effective potential of the central gravitating rotating object, one can carry out these studies very accurately. In this approach, one need not use the full general relativistic equations to obtain the salient features of the general relativistic flows provided the Kerr parameter remains within . We present the equatorial and the non-equatorial particle trajectories from our potential and compare salient properties in Kerr and in pseudo-Kerr geometries. Our potential naturally produces accurate results for motions around the Schwarzschild geometry when the black hole angular momentum is set to zero. [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]

Accretion flows having low angular momentum and low viscosity can have standing shock waves. These shocks arise because of the presence of multiple sonic points in the flow. We study the region of the parameter space in which multiple sonic points occur in viscous flows in the absence of cooling. We also separate the parameter space into regions allowing steady shocks and oscillating shocks. We quantify the nature of two critical viscosities which separate the flow topologies. The post-shock region being hotter, it emits harder X-rays and oscillating shocks cause oscillating X-ray intensities giving rise to quasi-periodic oscillations. We show that with the increase in viscosity parameter, the shock always moves closer to the black hole. This implies an enhancement of the quasi-periodic oscillation frequency as viscosity is increased. [ABSTRACT FROM AUTHOR]

Matter accreting on to black holes has long been known to have standing or oscillating shock waves. The post-shock matter puffs up in the form of a torus, which intercepts soft photons from the outer Keplerian disc and inverse Comptonizes to produce hard photons. The post-shock region also produces jets. We study the interaction of both hard photons and soft photons, with on-axis electron–positron jets. We show that the radiation from post-shock torus accelerates the flow to relativistic velocities, while that from the Keplerian disc has marginal effect. We also show that the velocity at infinity or the terminal velocity ϑ depends on the shock location in the disc. [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]