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Register a new userStriped Blandford/Znajek jets from advection of small scale magnetic field
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Added by
Jens Florian Mahlmann
Publication date
2020
fields
Physics
and research's language is
English
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Black hole - accretion disc systems are the central engines of relativistic jets from stellar to galactic scales. We numerically quantify the unsteady outgoing Poynting flux through the horizon of a rapidly spinning black hole endowed with a rotating accretion disc. The disc supports small-scale, concentric, flux tubes with zero net magnetic flux. Our General Relativistic force-free electrodynamics simulations follow the accretion onto the black hole over several hundred dynamical timescales in 3D. For the case of counter-rotating accretion discs, the average process efficiency reaches up to $leftlangleepsilonrightrangleapprox 0.43$, compared to a stationary energy extraction by the Blandford/Znajek process. The process efficiency depends on the cross-sectional area of the loops, i.e. on the product $ltimes h$, where $l$ is the radial loop thickness and $h$ its vertical scale height. We identify a strong correlation between efficient electromagnetic energy extraction and the quasi-stationary setting of ideal conditions for the operation of the Blandford/Znajek process (e.g. optimal field line angular velocity and fulfillment of the so-called Znajek condition). Remarkably, the energy extraction operates intermittently (alternating episodes of high and low efficiency) without imposing any large-scale magnetic field embedding the central object. Scaling our results to supermassive black holes, we estimate that the typical variability timescale of the system is of the order of days to months. Such timescales may account for the longest variability scales of TeV emission observed, e.g. in M87.
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We consider the conditions under which a rotating magnetic object can produce a magnetically powered outflow in an initially unmagnetized medium stratified under gravity. 3D MHD simulations are presented in which the footpoints of localized, arcade-shaped magnetic fields are put into rotation. It is shown how the effectiveness in producing a collimated magnetically powered outflow depends on the rotation rate, the strength and the geometry of the field. The flows produced by uniformly rotating, non-axisymmetric fields are found to consist mainly of buoyant plumes heated by dissipation of rotational energy. Collimated magnetically powered flows are formed if the field and the rotating surface are arranged such that a toroidal magnetic field is produced. This requires a differential rotation of the arcades footpoints. Such jets are well-collimated; we follow their propagation through the stratified atmosphere over 100 times the source size. The magnetic field is tightly wound and its propagation is dominated by the development of non-axisymmetric instabilities. We observe a Poynting flux conversion efficiency of over 75% in the longest simulations. Applications to the collapsar model and protostellar jets are discussed.
Recent Very Long Baseline Interferometry observations of the relativistic jet in the M87 radio galaxy at 43 GHz show gradual relativistic acceleration of the plasma and suggest a linear dependence of Lorentz factor on jet radius at scales up to 8 marcsec (0.65 pc) from the core (2.5 marcsec in projection). General analysis of integrals of motion being unaltered along the jet and reflecting fundamental conservation laws shows that the above dependence implies a polytropic equation of state with index 4/3. The inferred value of the polytropic index appears independent of the exact nature of forces sustaining the transverse balance of the jet and indicates exact conservation of the longitudinal electric current and hence the existence of a stable internal electromagnetic structure at the scales under consideration. At this index the flow is hot and corresponds to relativistic thermodynamic motion of particles. Considerable weakening of the acceleration efficiency after 8 marcsec with the jet form being unchanged can be related to the plasma cooling, when the polytropic index becomes 5/3. Such a sharp change in the index without intermediate delay at 1.44 during cooling favours the existence of an electron-positron plasma and requires at least partial participation of the Blandford-Znajek mechanism in the launching of the M87 jet.
Jitter radiation is produced by relativistic electrons moving in turbulent small-scale magnetic fields such as those produced by streaming Weibel-type instabilities at collisionless shocks in weakly magnetized media. Here we present a comprehensive study of the dependence of the jitter radiation spectra on the properties of, in general, anisotropic magnetic turbulence. We have obtained that the radiation spectra do reflect, to some extent, properties of the magnetic field spatial distribution, yet the radiation field is anisotropic and sensitive to the viewing direction with respect to the field anisotropy direction. We explore the parameter space of the magnetic field distribution and its effect on the radiation spectrum. Some important results include: the presence of the harder-than-synchrotron segment below the peak frequency at some viewing angles, the presence of the high-frequency power-law tail even for a monoenergetic distribution of electrons, the dependence of the peak frequency on the field correlation length rather than the field strength, the strong correlation of the spectral parameters with the viewing angle. In general, we have found that even relatively minor changes in the magnetic field properties can produce very significant effects upon the jitter radiation spectra. We consider these results to be important for accurate interpretation of prompt gamma-ray burst spectra and possibly other sources.
Various astrophysical studies have motivated the investigation of the transport of high energy particles in magnetic turbulence, either in the source or en route to the observation sites. For strong turbulence and large rigidity, the pitch-angle scattering rate is governed by a simple law involving a mean free path that increases proportionally to the square of the particle energy. In this paper, we show that perpendicular diffusion deviates from this behavior in the presence of a mean field. We propose an exact theoretical derivation of the diffusion coefficients and show that a mean field significantly changes the transverse diffusion even in the presence of a stronger turbulent field. In particular, the transverse diffusion coefficient is shown to reach a finite value at large rigidity instead of increasing proportionally to the square of the particle energy. Our theoretical derivation is corroborated by a dedicated Monte Carlo simulation. We briefly discuss several possible applications in astrophysics.
The energetics of the long duration GRB phenomenum is compared with the BZ mechanism. A rough estimate of the energy extracted from a rotating Black Hole with the Blandford-Znajek mechanism is evaluated with a very simple assumption: an inelastic collision between the rotating BH and an accreting torus. The GRB energetics requires an high magnetic field that breaks down the vacuum around the BH and gives origin to a e$^pm$ fireball.
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