Flares in the Galactic center I: orbiting flux tubes in Magnetically Arrested Black Hole Accretion Disks

Abstract

Recent observations of SgrA* by the GRAVITY instrument have astrometrically tracked infrared flares (IR) at distances of $\sim 10$ gravitational radii ($r_g$). In this paper, we study a model for the flares based on 3D general relativistic magnetohydrodynamic (GRMHD) simulations of magnetically arrested accretion disks (MADs) which exhibit violent episodes of flux escape from the black hole magnetosphere. These events are attractive for flare modeling for several reasons: i) the magnetically dominant regions can resist being disrupted via magneto-rotational turbulence and shear, ii) the orientation of the magnetic field is predominantly vertical as suggested by the GRAVITY data, iii) 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 $\sim 5-40r_g$. The magnetic energy contained within the flux bundles ranges up to $\sim 10^{40}$ 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.

accepted for publication by MNRAS, 18-Jan-2021

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