Black holes shielded by magnetic fields by Chandrachur Chakraborty. on Tuesday 22 November

Black holes (BHs) formed by collapsing of magnetized progenitors, have

magnetic fields penetrating the event horizon, and there are several possible

scenarios. 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 $r_B > 2/B$. It signifies that if a BH is surrounded by

a magnetic field of order $B \sim M^{-1}$, 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 $B \sim M^{-1}$

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 $r_f \sim 10^5M$.

In other words, our result also explains why a detected SMBH has surprisingly

weak magnetic field.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.11356v1