Rotational Energy Of Black Hole Research Articles

NEUTRINO-COOLED ACCRETION MODEL WITH MAGNETIC COUPLING FOR X-RAY FLARES IN GAMMA-RAY BURSTS

The neutrino-cooled accretion disk, which was proposed to work as the central engine of gamma-ray bursts, encounters difficulty in interpreting the X-ray flares after the prompt gamma-ray emission. In this paper, the magnetic coupling (MC) between the inner disk and the central black hole (BH) is taken into consideration. For mass accretion rates around 0.001 ∼ 0.1 M☉ s−1, our results show that the luminosity of neutrino annihilation can be significantly enhanced due to the coupling effects. As a consequence, after the gamma-ray emission, a remnant disk with mass Mdisk ≲ 0.5 M☉ may power most of the observed X-ray flares with the rest frame duration less than 100 s. In addition, a comparison between the MC process and the Blandford–Znajek mechanism is shown on the extraction of BH rotational energy.

Floating and Sinking: The Imprint of Massive Scalars around Rotating Black Holes

We study the coupling of massive scalar fields to matter in orbit around rotating black holes. It is generally expected that orbiting bodies will lose energy in gravitational waves, slowly inspiraling into the black hole. Instead, we show that the coupling of the field to matter leads to a surprising effect: because of superradiance, matter can hover into "floating orbits" for which the net gravitational energy loss at infinity is entirely provided by the black hole's rotational energy. Orbiting bodies remain floating until they extract sufficient angular momentum from the black hole, or until perturbations or nonlinear effects disrupt the orbit. For slowly rotating and nonrotating black holes floating orbits are unlikely to exist, but resonances at orbital frequencies corresponding to quasibound states of the scalar field can speed up the inspiral, so that the orbiting body sinks. These effects could be a smoking gun of deviations from general relativity.

Hard X-Ray bursts in collapse of supermassive stars

The very first stars in the Universe can be very massive, up to $10^6M_\odot$. They would leave behind massive black holes that could act as seeds for growing super massive black holes of active galactic nuclei. Given the anticipated fast rotation such stars would end their live as super massive collapsars and drive powerful magnetically-dominated jets. In this paper we investigate the possibility of observing the bursts of high-energy emission similar to the Long Gamma Ray Bursts associated with normal collapsars. We show that during the collapse of supercollapsars, the Blandford-Znajek mechanism can produce jets as powerful as few$\times10^{51}$erg/s and release up to $10^{56}$erg of the black hole rotational energy. Due to the higher intrinsic time scale and higher redshift the initial bright phase of the burst can last for about $10^5$ seconds whereas the central engine would remain active for about 10 days. Due to the high redshift the burst spectrum is expected to be soft, with the spectral energy distribution peaking at around 60keV. The peak total flux density is relatively low, few$\times 10^{-7}erg\, cm^{-2} s^{-1}$, but not prohibitive. The such events should be rear 0.03 year$^{-1}$, the observations needs long term program and could be done in future.

Supercollapsars and their X-ray bursts

Abstract The very first stars in the Universe can be very massive, up to 103 M⊙. If born in large numbers, such massive stars can have a strong impact on the subsequent star formation, producing strong ionizing radiation and contaminating the primordial gas with heavy elements. They would leave behind massive black holes that could act as seeds for growing supermassive black holes of active galactic nuclei. Given the anticipated fast rotation, such stars would end their life as supermassive collapsars and drive powerful magnetically dominated jets. In this Letter, we investigate the possibility of observing the bursts of high-energy emission similar to the long gamma-ray bursts associated with normal collapsars. We show that during the collapse of supercollapsars, the Blandford–Znajek mechanism can produce jets as powerful as few ×1052 erg s−1 and release up to 1054 erg of the black hole rotational energy. Due to the higher intrinsic time-scale and higher redshift, the initial bright phase of the burst can last for about 104 s, whereas the central engine would remain active for about 1 d. Due to the high redshift the burst spectrum is expected to be soft, with the spectral energy distribution peaking at around 20–30 keV. The peak total flux density is relatively low, 10−7 erg cm−2 s−1, but not prohibitive. If one supercollapsar is produced per every minihalo of dark matter arising from the 3σ cosmological fluctuations, then the whole sky frequency of such bursts could reach several tens per year.

Blandford-Znajek Mechanism versus Penrose Process

During the three decades since its theoretical discovery the Blandford-Znajek process of extracting the rotational energy of black holes has become one of the foundation stones in the building of modern relativistic astrophysics. However, it is also true that for a long time its physics was not well understood, as evidenced by the controversy that surrounded it since 1990s. Thanks to the efforts of many theorists during the last decade the state of affairs is gradually improving. In this lecture I attempt to explain the key ingredients of this process in more or less systematic, rigorous, and at the same time relatively simple fashion. A particular attention is paid to the similarities and differences between the Blandford-Znajek and Penrose processes. To this purpose I formulate the notion of energy counter flow. The concept of horizon membrane is replaced with the concept of vacuum as an electromagnetically active medium. The effect of negative phase velocity of electromagnetic waves in the black hole ergosphere is also discussed.

Gamma-Ray Bursts and Hypernova Explosions of Some Galactic Sources

Knowing the Kerr parameters, we can make quantitative calculations of the rotational energy of black holes. We show that Nova Sco, Il Lupi, XTE J1550-564, and GS 2023+338 are relics of gamma-ray bursts (GRBs) and hypernova explosions. They had more than enough rotational energy to power themselves. In fact, they had so much energy that they would have disrupted the accretion disk of the black hole that powered them by the communicated rotational energy, so that the energy delivery would have been self-limiting. The most important feature in producing high rotational energy in the binary is low donor (secondary star) mass. We suggest that V4641 Sgr and GRS 1915+105 underwent less energetic explosions, because of their large donor masses. Cyg X-1 had an even less energetic explosion, because of an even larger donor mass. We find that in the evolution of the soft X-ray transient sources the donor is tidally locked with the helium star, which evolved from the giant, as the hydrogen envelope is stripped off in common-envelope evolution. Depending on the mass of the donor, the black hole can be spun up to the angular momentum necessary to power the GRB and hypernova explosion. The donor decouples, acting as a passive witness to the explosion which, for the given angular momentum, then proceeds as in the Woosley collapsar model. High-mass donors which tend to follow from low metallicity give long GRBs because their lower energy can be accepted by the central engine.

Simulations of jets driven by black hole rotation.

The origin of jets emitted from black holes is not well understood; however, there are two possible energy sources: the accretion disk or the rotating black hole. Magnetohydrodynamic simulations show a well-defined jet that extracts energy from a black hole. If plasma near the black hole is threaded by large-scale magnetic flux, it will rotate with respect to asymptotic infinity, creating large magnetic stresses. These stresses are released as a relativistic jet at the expense of black hole rotational energy. The physics of the jet initiation in the simulations is described by the theory of black hole gravitohydromagnetics.

Magnetic extraction of black hole rotational energy: Method and results of general relativistic magnetohydrodynamic simulations in Kerr space-time

We present the complete numerical method of general relativistic magnetohydrodynamic simulations in Kerr space-time, and then apply those techniques to the basic astrophysical problem of activity near a Kerr black hole immersed in a plasma with a large-scale magnetic field. Our numerical results show that a torsional Alfven wave is generated in the ergosphere of the Kerr black hole. This wave propagates outward along the magnetic field lines, extracting rotational energy from the plasma in the ergosphere. If the magnetic field is strong enough, the plasma energy in the ergosphere rapidly decreases and eventually becomes negative. When this negative energy plasma is swallowed by the black hole, the total energy of the black hole decreases, spinning it down. This energy extraction mechanism is similar to the ``Penrose process,'' in which the negative energy also plays an important role. The difference between the two is the force that causes the redistribution of the angular momentum (which is necessary to produce the negative energy). In the Penrose process, elementary particle interactions cause the redistribution, while in the present case it is performed by magnetic tension.

Discovery of Helical Magnetic Fields in AGN, and CME and Ejection of Large-Scale Magnetic Fields from AGN

Magnetic Fields are the crucial and most important ingredient involved in the processes of various violent activities in Active Galactic Nuclei and other celestial bodies. The generally accepted 2-sided symmetric-jets model of active galactic nuclei (AGN) does not include the magnetic fields. We present here the first direct detection of helical magnetic field in AGN, and the first direct detection of ejection of large scale magnetic fields from AGN. (CME) 2. The annular (helical) magnetic field is responsible for the collimation of the jet (through Pinch Effect) and may be of crucial importance for extraction of black hole rotational energy. 3. The CMEs are responsible for sporadic ejection of jet components in AGN, while the general accepted 2-sided jets pertain to the quiescent Jets. 4. Observations show that the CME with Asymmetric Jets can explain many phenomena and much of the morphological diversities in AGN.

Extraction of Black Hole Rotational Energy by a Magnetic Field

We have developed a numerical method for general relativistic magnetohydrodynamic simulations in Kerr space-time. The method is applied to the basic astrophysical problem of the Kerr black hole activity in the large-scale strong magnetic field. The numerical result shows that the magnetic field extracts the rotational energy of the black hole with negative energy-at-infinity and the torsional Alfven wave is induced from the ergosphere.

Accretion Disk Torqued by a Black Hole

If a Kerr black hole is connected to a disk rotating around it by a magnetic field, the rotational energy of the Kerr black hole provides an energy source for the radiation of the disk in addition to disk accretion. The black hole exerts a torque on the disk, which transfers energy and angular momentum between the black hole and the disk. If the black hole rotates faster than the disk, energy and angular momentum are extracted from the black hole and transferred to the disk. The energy deposited into the disk is eventually radiated away by the disk, which will increase the efficiency of the disk. If the black hole rotates slower than the disk, energy and angular momentum are transferred from the disk to the black hole, which will lower the efficiency of the disk. With suitable boundary conditions, quasi-steady state solutions are obtained for a thin Keplerian disk magnetically coupled to a Kerr black hole. By "quasi-steady state" we mean that any macroscopic quantity at a given radius in the disk slowly changes with time: the integrated change within one rotation period of the disk is much smaller than the quantity itself. We find that the torque produced by magnetic coupling propagates outward only in the disk, and the total radiation flux of the disk is a superposition of the radiation flux produced by magnetic coupling and that produced by accretion. Interestingly, a disk magnetically coupled to a rapidly rotating black hole can radiate without accretion: the total power of the disk comes from the rotational energy of the black hole. With a simple example that the magnetic field touches the disk at a single radius, we show that the radial radiation profile produced by magnetic coupling can be very different from that of a standard accretion disk: the emissivity index is significantly larger, and most radiation can come from a region that is closer to the center of the disk. While the shape of the radiation flux curve sensitively depends on the extension of the magnetic field in the disk, the spectral signature of magnetic coupling can be robust. The limitations of our model are briefly discussed, which include the assumption of a weak magnetic field, the ignorance of the instabilities of the disk and the magnetic field, and the ignorance of the radiation captured by the black hole and the radiation returning to the disk.

Extraction of black hole rotational energy by a magnetic field and the formation of relativistic jets.

Using numerical simulations, we modeled the general relativistic magnetohydrodynamic behavior of a plasma flowing into a rapidly rotating black hole in a large-scale magnetic field. The results show that a torsional Alfvén wave is generated by the rotational dragging of space near the black hole. The wave transports energy along the magnetic field lines outward, causing the total energy of the plasma near the hole to decrease to negative values. When this negative energy plasma enters the horizon, the rotational energy of the black hole decreases. Through this process, the energy of the spinning black hole is extracted magnetically

Direct numerical simulations of the Blandford-Znajek effect

The time-dependent general relativistic equations of degenerate electrodynamics are solved numerically in order to study the mechanism of the electromagnetic extraction of the rotational energy of black holes. We performed a series of 2D runs for black holes with specific angular momentum, a, from 0.1 to 0.9 and for a monopole magnetic field assuming axisymmetry. In the inner region of the wind, the solution quickly settles to a steady state with an outgoing Poynting flux. In all cases the angular velocity of the magnetic field lines is almost half the angular velocity of the black hole. Thus, at least for the configuration considered, the Blandford–Znajek mechanism operates near its maximum power output.

Self-force on a scalar charge in the spacetime of a stationary, axisymmetric black hole

We study the self force acting on a particle endowed with scalar charge, which is held static (with respect to an undragged, static observer at infinity) outside a stationary, axially-symmetric black hole. We find that the acceleration due to the self force is in the same direction as the black hole's spin, and diverges when the particle approaches the outer boundary of the black hole's ergosphere. This acceleration diverges more rapidly approaching the ergosphere's boundary than the particle's acceleration in the absence of the self force. At the leading order this self force is a (post)$^2$-Newtonian effect. For scalar charges with high charge-to-mass ratio, the acceleration due to the self force starts dominating over the regular acceleration already far from the black hole. The self force is proportional to the rate at which the black hole's rotational energy is dissipated. This self force is local (i.e., only the Abraham-Lorentz-Dirac force and the local coupling to Ricci curvature contribute to it). The non-local, tail part of the self force is zero.

Transient Optical Emission from a Nonstationary Accretion Disk: A Model for Optical Afterglow of Gamma Ray Bursts

We consider the time-dependent behavior of nonstationary accretion disk which is formed by the capture and disruption of a main-sequence star around a massive black hole. If an instability is developed in the inner edge of the nonstationary disk and produces a strong magnetic field, a large amount of the black hole rotational energy can be extracted via the Blandford-Znajek process, which results in a Gamma-Ray Burst (GRB) with a typical duration ∼ 10–102 s. Other parameters related to a typical GRB including burst energy, rise time of the burst, number of sub-burst etc are also estimated. The vertical structure and thermal stability properties of the disk are investigated in detail. In the early stage of forming transient accretion disk, the material supplied by the tidally disrupted star is clumped in the relatively cool disk. This eventually leads to a thermal instability, which results in an increase of the viscosity of the disk. As the instability propagates across the disk, the stability of a time-dependent disk and variability of the mass-deposition rate provide a possible explanation for transient optical emission. We speculate that this transient optical emission from a nonstationary accretion disk may relate to the behavior of GRB optical afterglow, and the transient optical afterglow can be reproduced if one takes into account the effect of an evolving viscous parameter, α.

Screw Instability and the Blandford-Znajek Mechanism.

When magnetic field lines thread a rotating black hole's horizon and connect with remote astrophysical loads, the rotational energy of the black hole can be extracted through the Blandford-Znajek mechanism. Due to the rotation of the black hole, the magnetic field lines are twisted and toroidal components are generated. Thus, poloidal electric currents are induced and the black hole's rotational energy is transported to the astrophysical loads through Poynting flux. The Blandford-Znajek mechanism has been considered to be a possible process for powering extragalactic jets. In this Letter, we show that because of the screw instability of the magnetic field, the toroidal components of the magnetic field, and thus the poloidal currents, cannot exceed the limits given by the Kruskal-Shafranov criterion. This significantly lowers the power of the Blandford-Znajek mechanism when the loads are far from the black hole. So the Blandford-Znajek mechanism can only work efficiently within the neighborhood of the black hole. The implications of the results for the scenario of extragalactic jets powered by the Blandford-Znajek mechanism are discussed.

The evolution of black hole mass and angular momentum

We show that neither accretion nor angular momentum extraction are likely to lead to significant changes in the mass M_1 or angular momentum parameter a_* of a black hole in a binary system with realistic parameters. Current values of M_1 and a_* therefore probably reflect those at formation. We show further that sufficiently energetic jet ejection powered by the black hole's rotational energy can stabilize mass transfer in systems with large adverse mass ratios, and even reduce the mass transfer rate to the point where the binary becomes transient.