Black holes shielded by magnetic fields

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, $M=10^9{M}_{\odot }$ BH surrounded by $B>10^6$ G and $M=10M_{\odot }$ BH surrounded by $B>10^{14}$ G could remain undetectable for $r_f\sim 10^5{R}_g$. In other words, our result also explains why a detected SMBH has surprisingly weak magnetic field.

arXiv comment(6+3) pages including (3+1) figures, matches published version, dedicated to my mother Shree Snigdha Chakraborty

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Cited by in corpus (5)

• Magnetic Penrose process in the magnetized Kerr spacetime
• Hamiltonian Formulation of Relativistic Magnetohydrodynamic Accretion on a General Spherically Symmetric and Static Black Hole: Quantum effects on Shock States
• Classification of the trajectories of uncharged particles in the Schwarzschild-Melvin metric
• Spin Precession in magnetized Kerr spacetime
• Orbital Motion in Spacetimes Influenced by the Presence of Scalar and Electromagnetic Fields