Accretion Structures around Kerr Black Holes in a Swirling Background

Abstract

We investigate thick accretion structures around Kerr black holes in a swirling background. This stationary and axisymmetric spacetime is composed of a rotating black hole, which is immersed in a rotating background. The swirling background is characterized by an odd $\mathcal{Z}_2$ symmetry, where the northern and southern hemispheres are rotating in opposite directions. The presence of the Kerr rotation leads to the emergence of complex spin-spin interactions with the background rotation, which heavily influence the spacetime properties. In order to study this influence, we analyze circular orbits and geometrically thick disks for different spacetime solutions, that are classified by their Kerr parameter $a$ and the swirling parameter $j$. We identify stabilizing effects on prograde circular orbits and destabilizing effects on retrograde circular orbits, which originate from the spin-spin interaction and depend mainly on the Kerr rotation. Furthermore, we discovered the emergence of static orbits, which appear due to the background rotation. The symmetry breaking with regard to the equatorial plane causes a concave (convex) distribution of prograde (retrograde) circular orbits and accordingly, bowl-like deformations of the accretion disk structures. The parameter space for disk solutions gets heavily downsized by the appearance of an outer marginally stable orbit. Due to the possibility of an outer and inner disk cusp, different types of disk solutions are possible. We classify the different types of disk solutions, which differ from each other by the properties of their cusps. Four different scenarios can be identified in which different accretion dynamics could arise.