The final stages of accretion onto non-Kerr compact objects

TL;DR

This paper explores the final accretion stages onto various compact objects, revealing that non-Kerr geometries can trap gas between the ISCO and the object, unlike the standard Kerr black hole scenario.

Contribution

It introduces new scenarios for accretion onto non-Kerr compact objects, highlighting differences in gas behavior near the ISCO based on spacetime geometry.

Findings

Gas can be trapped between ISCO and the compact object for prolate geometries.

Accretion onto non-Kerr objects may require additional angular momentum loss.

Different accretion behaviors depend on the object's spacetime shape.

Abstract

The $5 - 20 M_\odot$ dark objects in X-ray binary systems and the $10^5 - 10^9 M_\odot$ dark objects in galactic nuclei are currently thought to be the Kerr black holes predicted by General Relativity. However, direct observational evidence for this identification is still elusive, and the only viable approach to confirm the Kerr black hole hypothesis is to explore and rule out any other possibility. Here we investigate the final stages of the accretion process onto generic compact objects. While for Kerr black holes and for more oblate bodies the accreting gas reaches the innermost stable circular orbit (ISCO) and plunges into the compact object, we find that for more prolate bodies several scenarios are possible, depending on the spacetime geometry. In particular, we find examples in which the gas reaches the ISCO, but then gets trapped between the ISCO and the compact object. In this situation, accretion onto the compact object is possible only if the gas loses additional angular momentum, forming torus-like structures inside the ISCO.