Accretion process onto super-spinning objects

TL;DR

This study investigates how accretion onto super-spinning objects (with Kerr parameter |a| > M) differs from standard black holes, revealing suppressed accretion, cloud formation, and potential observational signatures of such exotic objects.

Contribution

It provides the first detailed numerical analysis of accretion onto super-spinning Kerr objects, identifying a critical spin parameter and associated accretion regimes.

Findings

Accretion is suppressed for |a| just above M, leading to matter accumulation around the object.

A quasi-steady accretion state forms for |a|/M ≥ 1.4, with a thin disk at small radii.

High-spin objects produce harder, more intense radiation, potentially observable as gamma-ray bursts.

Abstract

The accretion process onto spinning objects in Kerr spacetimes is studied with numerical simulations. Our results show that accretion onto compact objects with Kerr parameter (characterizing the spin) $|a| < M$ and $|a| > M$ is very different. In the super-spinning case, for $|a|$ moderately larger than $M$, the accretion onto the central object is extremely suppressed due to a repulsive force at short distance. The accreting matter cannot reach the central object, but instead is accumulated around it, forming a high density cloud that continues to grow. The radiation emitted in the accretion process will be harder and more intense than the one coming from standard black holes; e.g. $\gamma$-rays could be produced as seen in some observations. Gravitational collapse of this cloud might even give rise to violent bursts. As $|a|$ increases, a larger amount of accreting matter reaches the central object and the growth of the cloud becomes less efficient. Our simulations find that a quasi-steady state of the accretion process exists for $|a|/M \gtrsim 1.4$, independently of the mass accretion rate at large radii. For such high values of the Kerr parameter, the accreting matter forms a thin disk at very small radii. We provide some analytical arguments to strengthen the numerical results; in particular, we estimate the radius where the gravitational force changes from attractive to repulsive and the critical value $|a|/M \approx 1.4$ separating the two qualitatively different regimes of accretion. We briefly discuss the observational signatures which could be used to look for such exotic objects in the Galaxy and/or in the Universe.