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Register a new userRelativistic Jets from Black Holes: A Unified Picture
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C Sivaram Christ Junior College
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The current understanding of the formation of powerful bi-directional jets in systems such as radio galaxies and quasars is that the process involves a supermassive black hole that is being fed with magnetized gas through an orbiting accretion disc. In this paper we discuss the dynamics of the jet powered by rotating black holes, in the presence of a magnetic field, including the scaling of the jet length and their typical time scales. We consider a unified picture covering all phenomena involving jets and rotating black holes ranging from gamma ray bursts to extragalactic jets and discuss the relevant scaling laws. We have also discussed the acceleration of the particles in jets and consequent synchrotron and inverse Compton radiations. Accelerated protons from jets as possible sources of high energy cosmic rays are also discussed.
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It has for long been an article of faith among astrophysicists that black hole spin energy is responsible for powering the relativistic jets seen in accreting black holes. Two recent advances have strengthened the case. First, numerical general relativistic magnetohydrodynamic simulations of accreting spinning black holes show that relativistic jets form spontaneously. In at least some cases, there is unambiguous evidence that much of the jet energy comes from the black hole, not the disk. Second, spin parameters of a number of accreting stellar-mass black holes have been measured. For ballistic jets from these systems, it is found that the radio luminosity of the jet correlates with the spin of the black hole. This suggests a causal relationship between black hole spin and jet power, presumably due to a generalized Penrose process.
In 2009, Banados, Silk and West (BSW) pointed out the possibility of having an unbounded limit of centre-of-mass collision energy for test particles in the field of an extremal Kerr black hole, if one of them has fine-tuned parameters and the collision point is approaching the horizon. The possibility of this BSW effect attracted much attention: it was generalised to arbitrary (dirty) rotating black holes and an analogy was found for collisions of charged particles in the field of non-rotating charged black holes. Our work considers the unification of these two mechanisms, which have so far been studied only separately. Exploring the enlarged parameter space, we find kinematic restrictions that may prevent the fine-tuned particles from reaching the limiting collision point. These restrictions are first presented in a general form, which can be used with an arbitrary black-hole model, and then visualised for the Kerr-Newman solution by plotting the admissible region in the parameter space of critical particles, reproducing some known results and obtaining a number of new ones. For example, we find that (marginally) bounded critical particles with enormous values of angular momentum can, curiously enough, approach the degenerate horizon, if the charge of the black hole is very small. Such mega-BSW behaviour is excluded in the case of a vacuum black hole, or a black hole with large charge. It may be interesting in connection with the small Wald charge induced on rotating black holes in external magnetic fields.
High-resolution very long baseline interferometry (VLBI) radio observations have resolved the detailed emission structures of active galactic nucleus jets. General relativistic magnetohydrodynamic (GRMHD) simulations have improved the understanding of jet production physics, although theoretical studies still have difficulties in constraining the origin and distribution of jetted matter. We construct a new steady, axisymmetric GRMHD jet model to obtain approximate solutions of black hole (BH) magnetospheres, and examine the matter density distribution of jets. By assuming fixed poloidal magnetic field shapes that mimic force-free analytic solutions and GRMHD simulation results and assuming constant poloidal velocity at the separation surface, which divides the inflow and outflow, we numerically solve the force-balance between the field lines at the separation surface and analytically solve the distributions of matter velocity and density along the field lines. We find that the densities at the separation surface in our parabolic field models roughly follow $propto r_{ss}^{-2}$ in the far zone from the BH, where $r_{ss}$ is the radius of the separation surface. When the BH spin is larger or the velocity at the separation surface is smaller, the density at the separation surface becomes concentrated more near the jet edge. Our semi-analytic model, combined with radiative transfer calculations, may help interpret the high-resolution VLBI observations and understand the origin of jetted matter.
We present a model for launching relativistic jets in active galactic nuclei (AGN) from an accreting Kerr black hole (BH) as an effect of the rotation of the space-time, where the gravitational energy of the accretion disc inside the ergosphere, which can be increased by the BH rotational energy transferred to the disc via closed magnetic field lines that connect the BH to the disc (BH-disc magnetic connection), is converted into jet energy. The main role of the BH-disc magnetic connection is to provide the source of energy for the jets when the mass accretion rate is very low. We assume that the jets are launched from the disc inside the BH ergosphere, where the rotational effects of the space-time become much stronger, being further accelerated by magnetic processes. The rotation of the space-time channels a fraction of the disc energy (i.e., the gravitational energy of the disc plus the rotational energy of the BH which is deposited into the disc by magnetic connection) into a population of particles that escape from the disc surfaces, carrying away mass, energy and angular momentum in the form of jets, allowing the remaining disc gas to accrete. In the limit of the spin-down power regime, the model proposed here can be regarded as a variant of the Blandford-Znajek mechanism, where the BH rotational energy is transferred to the disc inside the ergosphere and then used to drive the jets. We use general-relativistic conservation laws to calculate the mass flow rate into the jets, the launching power of the jets and the angular momentum transported by the jets for BHs with a spin parameter $a_* geqslant 0.95$. We found that a stationary state of the BH ($a_* = $ const) can be reached if the mass accretion rate is larger than $dot{m} sim 0.001$. In addition, the maximum AGN lifetime can be much longer than $sim 10^{7}$ yr when using the BH spin-down power.
A common consequence of accretion onto black holes is the formation of powerful, relativistic jets that escape the system. In the case of supermassive black holes at the centres of galaxies this has been known for decades, but for stellar-mass black holes residing within galaxies like our own, it has taken recent advances to arrive at this conclusion. Here, a review is given of the evidence that supports the existence of jets from accreting stellar-mass black holes, from observations made at optical and infrared wavelengths. In particular it is found that on occasion, jets can dominate the emission of these systems at these wavelengths. In addition, the interactions between the jets and the surrounding matter produce optical and infrared emission on large scales via thermal and non-thermal processes. The evidence, implications and applications in the context of jet physics are discussed. It is shown that many properties of the jets can be constrained from these studies, including the total kinetic power they contain. The main conclusion is that like the supermassive black holes, the jet kinetic power of accreting stellar-mass black holes is sometimes comparable to their bolometric radiative luminosity. Future studies can test ubiquities in jet properties between objects, and attempt to unify the properties of jets from all observable accreting black holes, i.e. of all masses.
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