7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Outflow from black hole accretion flows

Feng Yuan1
1
Shanghai Astronomical Observatory, Chinese Academy of Sciencese-mail (speaker): fyuan@shao.ac.cn
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Black hole accretion is accompanied by wind and jet. Inaddition, episodic ejection is also observed. In this talk, I
will review the current understanding of the wind fromhot accretion flows and episodic ejection.
The study of wind can be traced back to the first global
numerical simulation of black hole accretion where themass inflow rate is found to decrease with decreasing
radius (Fig. 1). This is surprising because people havebeen assuming that the inflow rate should be a constantof radius. Two models have been proposed to explain
this result, with one being convection another being
outflow (or wind). To study which model is correct, wehave performed numerical simulations of black holeaccretion. Using simulation data, we have analyzed theconvective stability and found that the MHD accretion
flow is convectively stable. We have also compared thevelocity, angular momentum, and Bernoulli parameter ofinflow and outflow and found that they aresystematically different. This indicates that inflow andoutflow are not due to convective turbulence. These
results suggest that systematic outflow must exist and is
the reason for the inward decrease of accretion flow.
The properties of wind such as their mass flux andvelocity are crucial for the study of AGN feedback. The
values of these two quantities are not easy to obtain since
they are contaminated by MRI turbulence. In theliterature many works do time-average to the simulation
data to filter the turbulence. But this will also filter out
wind since wind is instantaneous. Instead, we have tried
to obtain the trajectories of virtual test particles in theaccretion flow to judge whether the flow is wind orturbulence. In this way, we can loyally trace the windand obtain their properties. Using this approach, we havesuccessfully obtained the wind properties launched by
hot accretion flow of different modes, i.e., MAD andSANE, around black holes with various spin values. TheFig. 1. The mass fluxes of inflow, outflow,wind, and jet as a function of radius. Taken
from Yuan et al. (2015).

mass flux of wind as a function of radius is shown in Fig.
1. We have also compared the fluxes of energy andmomentum of wind and jet. The observational evidencesof wind and the comparison between theory andobservations will also be briefly discussed.
The second topic we will discuss is episodic ejection
from accretion flows. Such kind of ejection has been
observed in more and more AGNs, such as Sgr A*, M81,and M87, and in some cases the velocity of the ejected
blobs has been measured. By analogy with the coronalmass ejection in the Sun, we have proposed an MHD
model for episodic ejection from accretion flows. In this
scenario, flux ropes are formed in the coronal region of
the accretion flow due to the magnetic reconnection
driven by the turbulent and differential motion of theaccretion flow where magnetic field lines are rooted. The
flux rope will be further ejected out due to the magnetic
pressure force. We have performed 3D GRMHDsimulation and confirmed this scenario (Cemeljic et al.
2022). The trajectories of ejected flux rope are shown inFig. 2. During the reconnection, electrons will beaccelerated and they will emit strong radiation. This
explains why ejection is associated with flares, asobserved by GRAVITY in Sgr A* (Lin, Li & Yuan
2023).
References[1] Yuan, F., Bu, D. F., & Wu, M. C. 2012, ApJ, 761, 130[2] Yuan, F., Gan, Z., Narayan, R., et al. 2015, ApJ, 804,
101[3] Yuan, F., Lin, J., Wu, K., & Ho, L. C. 2009, MNRAS,395, 2183
[4] Cemeljic M., Yang H., Yuan F., Shang H., 2022, ApJ ,933, 55[5] Lin, X., Li, Y. & Yuan, F. 2023, MNRAS, 520, 1271

Fig. 2. The trajectories of ejected flux ropes.
Taken from Cemeljic et al. (2022).