Flares in the Galactic Centre – I. Orbiting flux tubes in magnetically arrested black hole accretion discs

Abstract

Recent observations of Sgr A* by the GRAVITY instrument have astrometrically tracked infrared (IR) flares at distances of ∼10 gravitational radii (rg). In this paper, we study a model for the flares based on 3D general relativistic magnetohydrodynamic (GRMHD) simulations of magnetically arrested accretion discs (MADs) that exhibit violent episodes of flux escape from the black hole magnetosphere. These events are attractive for flare modelling for several reasons: (i) the magnetically dominant regions can resist being disrupted via magnetorotational turbulence and shear; (ii) the orientation of the magnetic field is predominantly vertical as suggested by the GRAVITY data; and (iii) the magnetic reconnection associated with the flux eruptions could yield a self-consistent means of particle heating/acceleration during the flare events. In this analysis, we track erupted flux bundles and provide distributions of sizes, energies, and plasma parameter. In our simulations, the orbits tend to circularize at a range of radii from ${\sim} 5\hbox{ to }40\, r_{\rm g}$. The magnetic energy contained within the flux bundles ranges up to ${\sim} 10^{40}\,\rm erg$, enough to power IR and X-ray flares. We find that the motion within the magnetically supported flow is substantially sub-Keplerian, in tension with the inferred period–radius relation of the three GRAVITY flares.

Black hole flares: ejection of accreted magnetic flux through 3D plasmoid-mediated reconnection

“…The universal reconnection rate directly sets the magnetic flux decay rate at the horizon. Other studies have related flux decay at the horizon with flares (Ball et al 2018;Dexter et al 2020;Chashkina et al 2021;Scepi et al 2021) or observed orbiting flux tubes in retrograde disks (those rotating in the opposite sense to their black hole; Porth et al 2021). However, due to limited numerical resolution they did do not capture plasmoidmediated reconnection as the power source and did not identify a direct link between the magnetic flux decay at the event horizon and its origin in reconnection in the equatorial magnetospheric current.…”

“…General-relativistic magnetohydrodynamics (GRMHD) simulations show that a large amount of poloidal (pointing in the R-and z-directions) magnetic flux (proportional to the square root of the mass accretion rate) is forced into the black hole by the accreting gas, until the flux becomes dynamically important and strong enough to push the accreting gas away (Igumenshchev et al 2003;Igumenshchev 2008;Tchekhovskoy et al 2011). The MAD state is accompanied by large-amplitude fluctuations, caused by quasi-periodic accumulation and escape of the magnetic flux bundles in the vicinity of the black hole (Igumenshchev 2008;Tchekhovskoy et al 2011;Dexter et al 2020;Porth et al 2021).…”

The Jet–disk Boundary Layer in Black Hole Accretion

“…In MAD models, the inhomogeneous flow magnifies the effects of magnetic torques, since some equator-crossing field lines are lightly loaded (in contrast to SANE models, in which the equator-crossing field lines pass through a dense disk). Moreover, the more concentrated magnetic flux tubes in the MAD models can result in stronger torques (see Porth et al 2021): when matter in the accretion stream with u f > 0 interacts with a flux tube with u f < 0, the plasma is rapidly braked and its angular momentum is reversed. Figure 5 shows an example of this interaction as counterrotating field lines collide with the corotating field lines near the horizon.…”

GRMHD Simulations and Modeling for Jet Formation and Acceleration Region in AGNs

“…GRMHD simulations in the MAD regime exhibit violent episodes of flux escape from the black hole magnetosphere (e.g., [116,117]). These magnetic flux eruptions could explain the flare events observed in Sgr A* because they are associated with magnetic reconnection, which provides particle heating and acceleration during the flare events and contains enough energy to power flares.…”