Narrow Results

Regular black holes are crucially important as approaches to solving the singularity problem, and in this paper, the accretion disk of Bardeen and Hayward models has been studied. For this purpose, we calculated the physical properties of black holes, including radiant energy, luminosity derivative, temperature, and conversion efficiency of accretion mass into radiation. The obtained results show that the nonzero-free parameters of regular black holes cause the radius of the innermost stable circular orbit of the disk to shift to smaller values. As a result of this displacement, we saw an increase in the profiles of radiant energy, luminosity derivative, and temperature. We also find that Bardeen and Hayward’s black holes are more efficient in converting mass to radiation than Schwarzschild. Finally, we compared the free parameter of these two black holes with the spin of the rotating black hole and found that the Bardeen and Hayward black holes can mimic the slowly rotating Kerr black hole.

The shocked wave created on the accretion disk after different physical phenomena (accretion flows with pressure gradients, star-disk interaction etc.) may be responsible observed Quasi Periodic Oscillations (QPOs) in X-ray binaries. We present the set of characteristics frequencies associated with accretion disk around the rotating and non-rotating black holes for one particle case. These persistent frequencies are results of the rotating pattern in an accretion disk. We compare the frequency's from two different numerical results for fluid flow around the non-rotating black hole with one particle case. The numerical results are taken from Refs. 1 and 2 using fully general relativistic hydrodynamical code with non-selfgravitating disk. While the first numerical result has a relativistic tori around the black hole, the second one includes one-armed spiral shock wave produced from star-disk interaction. Some physical modes presented in the QPOs can be excited in numerical simulation of relativistic tori and spiral waves on the accretion disk. The results of these different dynamical structures on the accretion disk responsible for QPOs are discussed in detail.

We review the development of the equations of gravitoelectromagnetism and summarize how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near the accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism to produce collimated jets, it is a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

We consider that electromagnetic pulses produced in the jets of this innermost part of the accretion disk accelerate charged particles (protons, ions, electrons) to very high energies via wakefield acceleration, including energies above 10${}^{20}$ eV for the case of protons and nucleus and 10${}^{12-15}$ eV for electrons by electromagnetic wave-particle interaction. Thereby, the wakefield acceleration mechanism supplements the pervasive Fermi’s stochastic acceleration mechanism (and overcomes its difficulties in the highest energy cosmic ray generation). The episodic eruptive accretion in the disk by the magneto-rotational instability gives rise to the strong Alfvenic pulses, which acts as the driver of the collective accelerating pondermotive force. This pondermotive force drives the wakes. The accelerated hadrons (protons and nuclei) are released to the intergalactic space to be ultra-high energy cosmic rays. The high-energy electrons, on the other hand, emit photons to produce various non-thermal emissions (radio, IR, visible, UV, and gamma-rays) of active galactic nuclei in an episodic manner, giving observational telltale signatures.

In this paper, we have investigated the motion of photons as well as the frequency shift of photons emitted by geodesic particles influenced by the central black hole (BH) described by Bardeen solution surrounded by perfect fluid dark matter (PFDM). It has been shown that in general there are two stable photon spheres formed due to the gravitational attraction of photons by the central gravitating compact object. It has been obtained that with the increase of the space–time parameters these photon spheres come close to each other until they merge forming unique photon sphere around central BH. Investigation of the frequency shift of photons radiated by the particles geodesically moving in the equatorial plane has shown that the former becomes stronger for the bigger values of the intensity of PFDM and smaller values of the magnetic charge of the Bardeen solution.

The 2D time-dependent solution of the thin and stable accretion disk with two-armed spiral shock waves in a closed binary system have been presented on the equatorial plane around the Schwarzschild black hole in Donmez (2004).² The subject of this paper is to study the influence of two different boundary conditions, far away from a black hole called the outer boundary, on an accretion disk around the black hole during the time evolution. We have started with a stable accretion disk after the point where two-armed spiral shock waves were created (Donmez, 2004).² The initial data which is also called the freezing boundary is used as a first boundary condition. As a second one, we use the outflow boundary condition. In both cases, the accretion disk is created and gases on the disk made closed trajectories. As a stable tori close to the black hole is created by using the first boundary, freezing condition, which has a ~10M radius where M is the mass of black hole, and the other boundary, outflow, creates stable two-armed spiral shock waves. The last stable circular orbit around the Schwarzschild black hole for this type of accretion disk is located around 11M in the case of the freezing boundary condition. The results of these simulations show that the tori and spiral shock waves are created in each case using freezing and the outflow boundary, respectively, and it also suggests that spiral waves are a robust feature of accretion disks in binary systems, and that these spiral shocks can indeed transfer the gravitational energy to the radiation energy observed by different X-ray satellites.

Recently, in a series of papers, Mukhopadhyay and his collaborators have argued for possible pure hydrodynamic turbulence in a Keplerian accretion disk. This is essentially important to solving the puzzle of the transport mechanism in cold accretion disk systems where the temperature could be lower than 5000 K, where magnetorotational instability seems not to be working to trigger turbulence. Here we quantify the corresponding instability and turbulence in terms of turbulent viscosity and obtain the famous Shakura–Shunyaev viscosity parameter, α. It is exciting that the range of α obtained from our analysis is 0.1 ≳ α ≳ 0.0001 for a realistic parameter region. This range also suggests that once the hydrodynamic instability described by Mukhopadhyay and his collaborators leads to turbulence — an effect which should exist in systems independent of being hot or cold — the effect may compete with the magnetohydrodynamic effect even in hot accretion disks and thus may be effective in transporting matter in hot gas systems as well.

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.

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.

We discuss the possibility of thermal flares in centers of AGNs and microquasars. We present preliminary results of an ongoing study trying to assess the feasibility of a hypothesis suggesting that certain flares observed in these sources originate in the very centers of the systems and not in the relativistic jets. Using a simple toy model we reproduce optical flares with light curves very similar to those observed in the sources. The model suits especially well those cases where only the latter peak of a double-peaked optical flare has a radio counterpart.

Microquasar outbursts are characterized by spectral state transitions. The transitions between states characterized by a hard spectrum and those characterized by a soft spectrum are of particular interest. Besides drastic spectral and timing changes, these transitions often show discrete ejections detectable in the radio domain. The mechanisms giving birth to the ejections, the links with accretion and the exact nature of the ejected material are still largely unknown. We present systematic X–ray spectral analysis prior to the ejection in several microquasars, and show that, in each case, the properties of the corona drastically evolve, while that of the disc remain roughly constant. We intepret this behavior as possibly due to an ejection of the corona at the spectral transition.

In the present study, we perform the numerical simulation of a relativistic thin accretion disk around the nonrotating and rapidly rotating black holes using the general relativistic hydrodynamic code with Kerr in Kerr–Schild coordinate that describes the central rotating black hole. Since the high energy X-rays are produced close to the event horizon resulting the black hole–disk interaction, this interaction should be modeled in the relativistic region.

We have set up two different initial conditions depending on the values of thermodynamical variables around the black hole. In the first setup, the computational domain is filled with constant parameters without injecting gas from the outer boundary. In the second, the computational domain is filled with the matter which is then injected from the outer boundary. The matter is assumed to be at rest far from the black hole. Both cases are modeled over a wide range of initial parameters such as the black hole angular momentum, adiabatic index, Mach number and asymptotic velocity of the fluid. It has been found that initial values and setups play an important role in determining the types of the shock cone and in designating the events on the accretion disk. The continuing injection from the outer boundary presents a tail shock to the steady state accretion disk. The opening angle of shock cone grows as long as the rotation parameter becomes larger. A more compressible fluid (bigger adiabatic index) also presents a bigger opening angle, a spherical shock around the rotating black hole, and less accumulated gas in the computational domain. While results from [J. A. Font, J. M. A. Ibanez and P. Papadopoulos, Mon. Not. R. Astron. Soc.305 (1999) 920] indicate that the tail shock is warped around for the rotating hole, our study shows that it is the case not only for the warped tail shock but also for the spherical and elliptical shocks around the rotating black hole. The warping around the rotating black hole in our case is much smaller than the one by [J. A. Font, J. M. A. Ibanez and P. Papadopoulos, Mon. Not. R. Astron. Soc.305 (1999) 920], due to the representation of results at the different coordinates. Contrary to the nonrotating black hole, the tail shock is slightly warped around the rotating black hole. The filled computational domain without any injection leads to an unstable accretion disk. However much of it reaches a steady state for a short period of time and presents quasi-periodic oscillation (QPO). Furthermore, the disk tends to loose mass during the whole dynamical evolution. The time-variability of these types of accretion flowing close to the black hole may clarify the light curves in Sgr A*.

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.

In this paper, we study spin effects in the neutrino gravitational scattering by a supermassive black hole with a magnetized accretion disk having a finite thickness. We exactly describe the propagation of ultrarelativistic neutrinos on null geodesics and solve the spin precession equation along each neutrino trajectory. The interaction of neutrinos with the magnetic field is due to the nonzero diagonal magnetic moment. Additionally, neutrinos interact with plasma of the accretion disk electroweakly within the Fermi approximation. These interactions are obtained to change the polarization of incoming neutrinos, which are left particles. The fluxes of scattered neutrinos, proportional to the survival probability of spin oscillations, are derived for various parameters of the system. In particular, we are focused on the matter influence on the outgoing neutrinos flux. The possibility to observe the predicted effects for astrophysical neutrinos is briefly discussed.

The effects of a gravitomagnetic charge on the thermal radiation properties of a thin accretion disk surrounding a non-rotating black hole are studied. The studied system consists of a non-rotating black hole with a non-zero gravitomagnetic charge and a Novikov–Thorne disk that is thin and optically thick. It is found that the gravitomagnetic charge enhances the gravitational field of the central black hole, resulting in an increase in the event horizon and innermost stable circular orbit (ISCO) radii. However, the maximum flux of radiant energy from the accretion disk is reduced and shifted outward from the central object due to the effect of the gravitomagnetic charge. The thermal profile of the accretion disk also exhibits a similar dependence on the radial coordinate and the gravitomagnetic charge of the black hole. The radiative efficiency of the accretion disk decreases from around 6% to approximately 2% with an increase in the value of the gravitomagnetic charge by $l\simeq 3M$. The thermal spectra of the accretion disk are also shifted towards lower frequencies, corresponding to the gravitational redshift of electromagnetic radiation coming from the disk, with an increase in the value of the gravitomagnetic charge. One may conclude that the effect of the gravitomagnetic charge is opposite to the effect of black hole spin.

Massive gravity offers an interesting modification of general relativity by considering a nonzero mass for the graviton. We present a de Rham, Gabadadze and Tolley (dRGT) massive gravity model in the presence of higher order curvature gravity. We obtain a spherical solution for the field equations in this theory. The spherical solution possesses an electric charge and a cosmological constant, and it reduces to the Schwarzschild solution in the limit of a negligible graviton mass and the absence of the higher order term. We study the thermodynamics of this black hole and find that the mass of the graviton and the higher order term of gravity have a significant effect on the thermodynamic properties of the black hole. We also show the entropy of a black hole is independent of the mass of the graviton. Using the observational constraints on the coefficient of the higher order term, we determine an upper limit for the graviton mass. We compute the other thermodynamic quantities, such as heat capacity and Helmholtz free energy. Assuming that the spherical solutions are the modified Schwarzschild, we consider the relativistic thin accretion disk and study the effects of higher order term on thermal properties of the disk at the infrared limit in an asymptotic safety scenario.

The optical behaviour of the Be star in the high mass X-ray transient A0535+26/ HDE245770 shows that at the periastron typically there is an enhancement in the luminosity of order 0.05 to few tenths mag, and the X- ray outburst happens about 8 days after the periastron. We construct a quantitative model of this event, basing on a nonstationary accretion disk behavior, connected with a high ellipticity of the orbital motion. We explain the observed time delay between the peaks of the optical and X-ray outbursts in this system by the time of radial motion of a matter in the accretion disk, after increase of the mass flux in the vicinity of a periastral point in the binary. This time is determined by the turbulent viscosity, with the parameter α = 0.1 – 0.3, estimated from the comparison of the model with observational data.

We consider that electromagnetic pulses produced in the jets of this innermost part of the accretion disk accelerate charged particles (protons, ions, electrons) to very high energies via wakefield acceleration, including energies above 10²⁰ eV for the case of protons and nucleus and 10¹²⁻¹⁵ eV for electrons by electromagnetic wave-particle interaction. Thereby, the wakefield acceleration mechanism supplements the pervasive Fermi’s stochastic acceleration mechanism (and overcomes its difficulties in the highest energy cosmic ray generation). The episodic eruptive accretion in the disk by the magneto-rotational instability gives rise to the strong Alfvenic pulses, which acts as the driver of the collective accelerating pondermotive force. This pondermotive force drives the wakes. The accelerated hadrons (protons and nuclei) are released to the intergalactic space to be ultra-high energy cosmic rays. The high-energy electrons, on the other hand, emit photons to produce various non-thermal emissions (radio, IR, visible, UV, and gamma-rays) of active galactic nuclei in an episodic manner, giving observational telltale signatures.

An accretion flow around a black hole has a saddle type sonic point just outside the event horizon to guarantee that the flow enters the black hole supersonically. This feature exclusively present in the strong gravity limit makes its marks in every observation of black hole candidates. Another physical sonic point is present (as in a Bondi flow) even in weak gravity. Every aspect of spectral or temporal properties of every black hole can be understood using this transonic or advective flow having more than one saddle type points. This most well known and generalized solution with viscosity and radiative transfer has been verified by numerical simulations also. Spectra, computed for various combinations of the standard Keplerian, and advective sub-Keplerian components match accurately with those from satellite observations. Standing, oscillating and propagatory oscillating shocks are produced due to centrifugal barrier of the advective component. The post-shock region acts as the Compton cloud producing the power-law spectra. Jets and outflows are also produced from this post-shock region, commonly known as the CENtrifugal barrier supported BOundary Layer or CENBOL. In soft states, the CENBOL is cooled down by soft photons from the Keplerian disk, and thus the outflow is absent. Type-C and Type-B QPOs are generated respectively due to strong and weak resonance oscillations of the CENBOL. Away from resonance, oscillation may be triggered when Rankine-Hugoniot conditions are not satisfied and Type-A QPOs could be seen.

We study time variability properties of black hole transients densely monitored by the RXTE instruments. We systematically study the time/phase lag at QPO frequency. We find hard lag, intregated over Quasi Periodic Oscillation (QPO) frequency for the low inclination source (such as GX 339-4). The hard lag monotonically decrease and become negative (i.e., soft lag happens) close to 3.0 Hz for the high inclination sources (e.g., XTE J1550-564). Thus we find two different behaviours for the high inclination and the low inclination systems. We also find that the evolution properties of low-frequency quasi-periodic oscillations (QPOs) do not depend on the orbital inclination though the amplitude of low-frequency quasi-periodic oscillations (QPOs) depends on the orbital inclination. We conclude these evolutions could be due to the systematic movement of the Comptonizing region itself confirming the propagatory shock oscillation model.