The role of resistivity in hot accretion flows with anisotropic pressure: Comparing magnetic field models

Abstract

Abstract In hot accretion flows, such as the accretion flow in the Galactic center (Sgr A*) and in M 87, the collisional mean free path of the charged particles is significantly larger than the typical length-scale of the accretion flows. Under these conditions, the pressure perpendicular to the magnetic field and that parallel to the magnetic field are not the same; therefore, the pressure is anisotropic to magnetic field lines. On the other hand, the resistivity as a dissipative mechanism plays a key role in the structure and the heating of hot accretion flows. In the present paper, we study the dynamics of resistive hot accretion flows with anisotropic pressure when the magnetic fields have even z-symmetry about the midplane. By presenting a set of self-similar solutions, we find that if the magnetic fields have even z-symmetry or the viscosity form depends on the strength of magnetic field, the disc properties can be entirely different. In the presence of symmetric fields, the velocity components and the disc temperature increase considerably. Also, we show that the increase in infall velocity and temperature due to the anisotropic pressure can be more significant if the resistivity is taken into account. Our results indicate that the resistivity can be an effective mechanism for the heating of hot accretion flows in the high-limit of the magnetic diffusivity parameter. Moreover, the heating due to the anisotropic pressure is comparable to the resistive heating, only when the strength of anisotropic pressure is about unity. The increase of disc temperature can lead to the acceleration of the electrons in such flows. This helps us to explain the origin of phenomena such as the flares in Sgr A*. Our results predict that the presence of resistivity makes it easier for outflows to launch from hot accretion flows.