Magnetic Reconnection and Hot Spot Formation in Black Hole Accretion Disks

Abstract

Hot spots, or plasmoids, forming due to magnetic reconnection in current sheets, are conjectured to power frequent X-ray and near-infrared flares from Sgr A * , the black hole in the center of our Galaxy. It is unclear how, where, and when current sheets form in black-hole accretion disks. We perform axisymmetric general-relativistic resistive magnetohydrodynamics simulations to model reconnection and plasmoid formation in a range of accretion flows. Current sheets and plasmoids are ubiquitous features which form regardless of the initial magnetic field in the disk, the magnetization in the quasisteady-state phase of accretion, and the spin of the black hole. Within 10 Schwarzschild radii from the event horizon, we observe plasmoids forming, after which they can merge, grow to macroscopic scales of the order of a few Schwarzschild radii, and are ultimately advected along the jet's sheath or into the disk. Large plasmoids are energized to relativistic temperatures via reconnection and contribute to the jet's limb-brightening. We find that only hot spots forming in magnetically arrested disks can potentially explain the energetics of Sgr A * flares. The flare period is determined by the reconnection rate, which we find to be between 0.01c and 0.03c in all cases, consistent with studies of reconnection in isolated Harris-type current sheets. We quantify magnetic dissipation and non-ideal electric fields which can efficiently inject non-thermal particles. We show that explicit resistivity allows for converged numerical solutions, such that the electromagnetic energy evolution and dissipation become independent of the grid scale for the extreme resolutions considered here.

Reconnection-driven Particle Acceleration in Relativistic Shear Flows

“…The highest-energy particles resulting from reconnection are further energized by shear-driven acceleration, i.e., reconnection can mediate particle injection into shear acceleration. Our work lends support to spinesheath models of jet emission and can explain the origin of radioemitting electrons at the boundaries of relativistic jets (see Ripperda et al 2020 for an alternative explanation).…”

Visibility of black hole shadows in low-luminosity AGN

“…An important assumption made in our model is that we include only thermal electron populations. For sources such as Sgr A*, emission from electrons with a non-thermal energy-distribution function is thought to be important to explain the observed time variability and the high frequency emission (Ball et al 2016;Mao et al 2017;Gravity Collaboration et al 2018;Davelaar et al 2018;Ripperda et al 2020;Dexter et al 2020;Porth et al 2020;Petersen and Gammie 2020). However, at 230 GHz, time-averaged images of this model will look similar to the thermal case when adding nonthermal electrons (Davelaar et al 2018), although it is shown in that work that high-spin models of M87 do become more optically thin because of this addition.…”

“…In SANE simulations, on the other hand, no magnetic 'pile-up' occurs, causing a radical change in the observed source morphology. Whether an accretion flow is MAD or SANE depends on the initial conditions of the magnetic field and the size of the accretion disc (see, e.g., Ripperda et al (2020). The radiative models considered in this work consist of a single-electron-temperature, disc-dominated model (Mościbrodzka et al 2009), and a two-electron-temperature, jet-dominated model (Mościbrodzka et al 2014;Davelaar et al 2018).…”

Computational general relativistic force-free electrodynamics

“…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). A large array of work makes use of numerical laboratories set up in (GR)FFE in order to simulate the most extreme environments of the universe while constantly breaking their own limits.…”