skip to main content


Title: The surprisingly small impact of magnetic fields on the inner accretion flow of Sagittarius A* fueled by stellar winds
ABSTRACT We study the flow structure in 3D magnetohydrodynamic (MHD) simulations of accretion on to Sagittarius A* via the magnetized winds of the orbiting Wolf–Rayet stars. These simulations cover over 3 orders of magnitude in radius to reach ≈300 gravitational radii, with only one poorly constrained parameter (the magnetic field in the stellar winds). Even for winds with relatively weak magnetic fields (e.g. plasma β ∼ 106), flux freezing/compression in the inflowing gas amplifies the field to β ∼ few well before it reaches the event horizon. Overall, the dynamics, accretion rate, and spherically averaged flow profiles (e.g. density, velocity) in our MHD simulations are remarkably similar to analogous hydrodynamic simulations. We attribute this to the broad distribution of angular momentum provided by the stellar winds, which sources accretion even absent much angular momentum transport. We find that the magneto-rotational instability is not important because of (i) strong magnetic fields that are amplified by flux freezing/compression, and (ii) the rapid inflow/outflow times of the gas and inefficient radiative cooling preclude circularization. The primary effect of magnetic fields is that they drive a polar outflow that is absent in hydrodynamics. The dynamical state of the accretion flow found in our simulations is unlike the rotationally supported tori used as initial conditions in horizon scale simulations, which could have implications for models being used to interpret Event Horizon Telescope and GRAVITY observations of Sgr A*.  more » « less
Award ID(s):
1715054
NSF-PAR ID:
10179838
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Volume:
492
Issue:
3
ISSN:
0035-8711
Page Range / eLocation ID:
3272 to 3293
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ABSTRACT

    Wind-fed models offer a unique way to form predictive models of the accretion flow surrounding Sagittarius A*. We present 3D wind-fed magnetohydrodynamic (MHD) and general relativistic magnetohydrodynamic (GRMHD) simulations spanning the entire dynamic range of accretion from parsec scales to the event horizon. We expand on previous work by including non-zero black hole spin and dynamically evolved electron thermodynamics. Initial conditions for these simulations are generated from simulations of the observed Wolf–Rayet stellar winds in the Galactic Centre. The resulting flow tends to be highly magnetized (β ≈ 2) with an ∼r−1 density profile independent of the strength of magnetic fields in the winds. Our simulations reach the magnetically arrested disc (MAD) state for some, but not all cases. In tilted flows, standard and normal evolution (SANE) jets tend to align with the angular momentum of the gas at large scales, even if that direction is perpendicular to the black hole spin axis. Conversely, MAD jets tend to align with the black hole spin axis. The gas angular momentum shows similar behaviour: SANE flows tend to only partially align while MAD flows tend to fully align. With a limited number of dynamical free parameters, our models can produce accretion rates, 230 GHz flux, and unresolved linear polarization fractions roughly consistent with observations for several choices of electron heating fraction. Absent another source of large-scale magnetic field, winds with a higher degree of magnetization (e.g. where the magnetic pressure is 1/100 of the ram pressure in the winds) may be required to get a sufficiently large rotation measure with consistent sign.

     
    more » « less
  2. ABSTRACT

    We interpret the 1.3 mm VLBI observations made by the Event Horizon Telescope of the black hole in M87. It is proposed that, instead of being a torus of accreting gas, the observed annular ring is a rotating, magnetically dominated ergomagnetosphere that can transmit electromagnetic angular momentum and energy outward to the disc through a combination of large scale magnetic torque and small scale instabilities. It is further proposed that energy may be extracted by magnetic flux threading the ergosphere through the efficient emission of long wavelength electromagnetic disturbances on to negative energy orbits, after the invariant B2 − E2 approaches zero on small scales. In this way, the spinning black hole and its ergosphere not only power the jets but also the ejection disc so as to drive away most of the gas supplied near the Bondi radius. This outflow takes the form of an MHD wind, extending over many decades of radius, with a unidirectional magnetic field, that is collimated by the infalling gas across a magnetopause. This wind, in turn, collimates the relativistic jets and the emission observed from the jet sheath may be associated with a return current. A model for the global flow of mass, angular momentum, energy, and current, on scales from the horizon to the Bondi radius, is outlined and discussed.

     
    more » « less
  3. null (Ed.)
    ABSTRACT We present a systematic shearing-box investigation of magnetorotational instability (MRI)-driven turbulence in a weakly collisional plasma by including the effects of an anisotropic pressure stress, i.e. anisotropic (Braginskii) viscosity. We constrain the pressure anisotropy (Δp) to lie within the stability bounds that would be otherwise imposed by kinetic microinstabilities. We explore a broad region of parameter space by considering different Reynolds numbers and magnetic-field configurations, including net vertical flux, net toroidal-vertical flux, and zero net flux. Remarkably, we find that the level of turbulence and angular-momentum transport are not greatly affected by large anisotropic viscosities: the Maxwell and Reynolds stresses do not differ much from the MHD result. Angular-momentum transport in Braginskii MHD still depends strongly on isotropic dissipation, e.g. the isotropic magnetic Prandtl number, even when the anisotropic viscosity is orders of magnitude larger than the isotropic diffusivities. Braginskii viscosity nevertheless changes the flow structure, rearranging the turbulence to largely counter the parallel rate of strain from the background shear. We also show that the volume-averaged pressure anisotropy and anisotropic viscous transport decrease with increasing isotropic Reynolds number (Re); e.g. in simulations with net vertical field, the ratio of anisotropic to Maxwell stress (αA/αM) decreases from ∼0.5 to ∼0.1 as we move from Re ∼ 103 to Re ∼ 104, while 〈4$\pi$Δp/B2〉 → 0. Anisotropic transport may thus become negligible at high Re. Anisotropic viscosity nevertheless becomes the dominant source of heating at large Re, accounting for ${\gtrsim } 50 {{\ \rm per\ cent}}$ of the plasma heating. We conclude by briefly discussing the implications of our results for radiatively inefficient accretion flows on to black holes. 
    more » « less
  4. ABSTRACT We present 3D general relativistic magnetohydrodynamic simulations of zero angular momentum accretion around a rapidly rotating black hole, modified by the presence of initially uniform magnetic fields. We consider several angles between the magnetic field direction and the black hole spin. In the resulting flows, the mid-plane dynamics are governed by magnetic reconnection-driven turbulence in a magnetically arrested (or a nearly arrested) state. Electromagnetic jets with outflow efficiencies ∼10–200 per cent occupy the polar regions, reaching several hundred gravitational radii before they dissipate due to the kink instability. The jet directions fluctuate in time and can be tilted by as much as ∼30○ with respect to black hole spin, but this tilt does not depend strongly on the tilt of the initial magnetic field. A jet forms even when there is no initial net vertical magnetic flux since turbulent, horizon-scale fluctuations can generate a net vertical field locally. Peak jet power is obtained for an initial magnetic field tilted by 40○–80○ with respect to the black hole spin because this maximizes the amount of magnetic flux that can reach the black hole. These simulations may be a reasonable model for low luminosity black hole accretion flows such as Sgr A* or M87. 
    more » « less
  5. Abstract

    A black hole (BH) traveling through a uniform, gaseous medium is described by Bondi–Hoyle–Lyttleton (BHL) accretion. If the medium is magnetized, then the black hole can produce relativistic outflows. We performed the first 3D, general-relativistic magnetohydrodynamic simulations of BHL accretion onto rapidly rotating black holes using theH-AMRcode, where we mainly varied the strength of a background magnetic field that threads the medium. We found that the ensuing accretion continuously drags the magnetic flux to the BH, which accumulates near the event horizon until it becomes dynamically important. Depending on the strength of the background magnetic field, the BHs can sometimes launch relativistic jets with high enough power to drill out of the inner accretion flow, become bent by the headwind, and escape to large distances. For stronger background magnetic fields, the jets are continuously powered, while at weaker field strengths they are intermittent, turning on and off depending on the fluctuating gas and magnetic flux distributions near the event horizon. We find that our jets reach extremely high efficiencies of ∼100%–300%, even in the absence of an accretion disk. We also calculated the drag forces exerted by the gas onto to the BH and found that the presence of magnetic fields causes the drag forces to be much less efficient than in unmagnetized BHL accretion. They can even sometimes become negative, accelerating the BH rather than slowing it down. Our results extend classical BHL accretion to rotating BHs moving through magnetized media, and demonstrate that accretion and drag are significantly altered in this environment.

     
    more » « less