https://doi.org/10.1140/epjc/s10052-025-14306-7
Regular Article - Theoretical Physics
Accretion structures around Kerr black holes in a swirling background
1
Department of Physics, Carl von Ossietzky University of Oldenburg, 26111, Oldenburg, Germany
2
Instituto de Física de São Carlos, Universidade de São Paulo, 13560-970, São Carlos, São Paulo, Brazil
Received:
13
March
2025
Accepted:
11
May
2025
Published online:
30
May
2025
In this paper, we investigate thick accretion structures around a Kerr black hole in a swirling background, that is, a rotating black hole immersed in a rotating background. This is a novel solution characterized by the black hole mass in addition to two distinct rotational parameters, the Kerr parameter a, which identifies the rotation of the black hole, and the swirling parameter j, which describes the background rotation. The swirling background is characterized by an odd 
symmetry, where the northern and southern hemispheres rotate in opposite directions. The rotation of the black hole embedded into this swirling background leads to non-trivial spin–spin interactions with the background rotation. The spacetime properties in the vicinity of the black hole are significantly influenced by this spin–spin interaction. In order to study the influence on the basic properties of this spacetime, we analyze circular orbits and geometrically thick disks for different spacetime solutions, which are classified by the black hole and swirling spins. 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 discover the emergence of static orbits, which appears due to the background rotation. The symmetry breaking of the spacetime rotation with regard to the equatorial plane highly influences the spatial distribution of circular orbits. This asymmetry causes a concave (convex) distribution of the prograde (retrograde) circular orbits and accordingly, bowl-like deformations of the accretion disk solutions. Moreover, due to the destabilizing effect of the swirling rotation, an outer marginally stable orbit appears, which heavily limits the range of the parameter space in which disk solutions can exist. 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.
© The Author(s) 2025
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