Charge separation instability in an unmagnetized disk plasma around a Kerr black hole

TL;DR

This paper discovers a charge separation instability in unmagnetized plasma around a Kerr black hole's ISCO, which can lead to black hole charging under certain low-density conditions, challenging the usual charge neutrality assumption.

Contribution

It introduces a new instability mechanism causing charge separation in rotating plasma near a black hole, especially relevant at low plasma densities inside the ISCO.

Findings

Charge separation instability exists inside the ISCO.

Growth rate is smaller than disk instability but becomes comparable at low densities.

Potential for black hole charging due to plasma instability.

Abstract

In almost all of plasma theories for astrophysical objects, we have assumed the charge quasi-neutrality of unmagnetized plasmas in global scales. This assumption has been justified because if there is a charged plasma, it induces electric field which attracts the opposite charge, and this opposite charge reduces the charge separation. Here, we report a newly discovered instability which causes a charge separation in a rotating plasma inside of an innermost stable circular orbit (ISCO) around a black hole. The growth rate of the instability is smaller than that of the disk instability even in the unstable disk region and is forbidden in the stable disk region outside of the ISCO. However, this growth rate becomes comparable to that of the disk instability when the plasma density is much lower than a critical density inside of the ISCO. In such case, the charge separation instability would become apparent and cause the charged accretion into the black hole, thus charge the hole up.