Dragged surfaces. On the accretion tori in the ergoregion

TL;DR

This paper investigates the existence, stability, and observational signatures of extended matter configurations called dragged tori orbiting near the ergoregion of Kerr black holes, including effects of magnetic fields and frame-dragging-induced instabilities.

Contribution

It introduces conditions for the formation and stability of magnetized and non-magnetized dragged tori in the ergoregion of Kerr black holes, highlighting potential observational features and instabilities.

Findings

Dragged tori can exist around highly spinning black holes with a > 0.9897M.

Frame-dragging can induce characteristic instabilities leading to torus destruction.

Magnetic fields influence the stability and observational signatures of the tori.

Abstract

We discuss the conditions for the existence of extended matter configurations orbiting in the ergoregion or close to the outer ergosurface of the Kerr black hole ("dragged" configurations). The corotating tori under consideration are perfect fluid configurations with barotropic equation of state, orbiting on the equatorial plane of the central Kerr black hole. The possibility of magnetized tori with a toroidal magnetic field is also discussed. Indications on the attractors where dragged tori can be observed are provided with the analysis of the fluid characteristics and geometrical features, relevant for the tori stability and their observations. QPOs emissions from the inner edges of the dragged tori are also discussed. We argue that the smaller dragged tori could be subjected to a characteristic instability, effect of the frame-dragging. This possibility is thoroughly explored. This can finally lead to the destruction of the torus (disk exfoliation) which can combine with accretion and processes present in the regions very close to the black hole horizon. Tori are characterized according to the central attractor dimensionless spin. These structures can be observed orbiting black holes with dimensionless spin a > 0.9897M.