Black holes shielded by magnetic fields

TL;DR

This paper explores how magnetic fields around black holes can prevent accretion disk formation and make the black holes undetectable, providing a theoretical explanation for the weak magnetic fields observed in some supermassive black holes.

Contribution

It introduces a theoretical framework showing that magnetic fields of certain strengths can shield black holes, affecting their detectability and the formation of accretion disks.

Findings

Magnetic fields of order B ~ R_g^{-1} can prevent regular particle motion around black holes.

Such magnetic shielding could render black holes undetectable by traditional observational methods.

The paper constrains magnetic field strengths for black holes of different masses to remain undetectable.

Abstract

Black holes (BHs) formed by collapsing and/or merging of magnetized progenitors, have magnetic fields penetrating the event horizon, and there are several possible scenarios. Thus, the no-hair theorem that assumes the outside medium is a vacuum, is not applicable in this case. Bearing this in mind and considering a Schwarzschild BH of mass $M$ immersed in a uniform magnetic field $B$, we show that all three frequencies related to the equatorial circular orbit of a test particle become imaginary for the orbits of radii $r_B > 2B^{-1}$. It signifies that if a BH is surrounded by a magnetic field of order $B \sim R_g^{-1}$ (where $R_g$ is the gravitational radius of the BH), a test particle could unable to continue its regular geodesic motion from/at $r > r_B$, hence the accretion disk could not be formed, and the motion of other stellar objects around the BH could be absent. As the BHs are generally detected by watching for their effects on nearby stars and gas, a magnetic field of order $B \sim R_g^{-1}$ could be able to shield a BH in such a way that it could remain undetectable. Motivated with this theoretical investigation and considering the sphere (of radius $r_f$) of magnetic influence around an astrophysical BH, we constrain $B$, above which a magnetized BH could remain undetectable. For example, $M=10^9M_{\odot}$ BH surrounded by $B > 10^6$ G and $M=10M_{\odot}$ BH surrounded by $B > 10^{14}$ G could remain undetectable for $r_f \sim 10^5R_g$. In other words, our result also explains why a detected SMBH has surprisingly weak magnetic field.