Striped Blandford/Znajek jets from advection of small-scale magnetic field

Abstract

<jats:title>ABSTRACT</jats:title> <jats:p>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 on to the black hole over several hundred dynamical time-scales in 3D. For the case of counter-rotating accretion discs, the average process efficiency reaches up to 〈ϵ〉 ≈ 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 l × 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 time-scale of the system is of the order of days to months. Such time-scales may account for the longest variability scales of TeV emission observed, e.g. in M87.</jats:p>

Computational general relativistic force-free electrodynamics

“…In the following sections, we address the similarities and differences between numerical and physical resistivity in FFE. The aim of this discussion is to provide support for the interpretation of the numerical dissipation found in astrophysical applications of our code (Mahlmann et al 2019(Mahlmann et al , 2020) as a consistent model of physical dissipation. Along the way, we may also contribute to asses whether suitably extended FFE models (including physical dissipation, albeit not at the astrophysically expected levels) may be used to study a number of dissipative processes in relativistic astrophysics, for example, resistive solutions for pulsar magnetospheres (Li et al 2012), the dissipation induced by the kink instability in relativistic jets (Bromberg et al 2019), or the conversion between fast and Alfvén modes (Li et al 2019).…”

“…We note that for resolutions of fewer than eight points per wavelength, the values of the numerical resistivity are not too different. This justifies our choice of a fourth-order accurate discretization of j || (Mahlmann et al 2019(Mahlmann et al , 2020 in combination with MP7. The resolution employed in these global 3D models is smaller than p ∼ 8 zones per wavelength.…”

“…Abandoning part of the current ( j ) and only enforcing D • B = 0 algebraically leads to an inconsistency between the charge ρ and the electric field D. As we evolve the continuity equation of the charge density (and do not reconstruct ρ = divD in every timestep, as most other FFE codes do) the algebraic reset of the electric fields to enforce D • B = 0 without a consistent modeling of the respective Ohm's law in the source terms is no longer adequate. The possible mismatches between charge densities and electric fields become obvious in this test and justifies the additional evolution equation we include into our scheme to ensure the long-term validity of charge-densities and electromagnetic fields in global astrophysical simulations (as we found especially useful in Mahlmann et al 2019Mahlmann et al , 2020.…”

“…In the context of instabilities in magnetar magnetospheres, Parfrey et al (2013), Carrasco et al (2019), and Mahlmann et al (2019) have encountered current sheets at the stellar surface, and sensitivity to a (conservative) transport of charges throughout the domain. The longest variability timescales in the observed TeV emission, for example, in M87, have recently been linked to recurring periods of efficient Blandford/Znajek (Blandford & Znajek 1977) type outflows induced by the accretion of magnetic flux tubes (Parfrey et al 2015;Mahlmann et al 2020). Reconnection and plasmoid formation in BH accretion processes are likely to act on much shorter timescales (studied in the ideal limit by, e.g., Nathanail et al 2020) and involve relevant physical nonideal electric fields (analyzed in the resistive limit by, e.g., Ripperda et al 2020).…”

Polarimetric Properties of Event Horizon Telescope Targets from ALMA

“…Asada et al 2002;Gómez et al 2008;Gabuzda et al 2014). Systematic changes in the signs of these gradients, leading to RM sign reversals in unresolved measurements, can be explained with a number of models, including the magnetic "tower" model (Lynden-Bell 1996; Contopoulos & Kazanas 1998;Lico et al 2017), or the "striped" jet model (Parfrey et al 2015;Mahlmann et al 2020;Nathanail et al 2020). Nevertheless, it remains difficult to explain the rapid fluctuations observed in 2017 April with these models.…”