Magnetic Penrose process in the magnetized Kerr spacetime

Abstract

Magnetic Penrose process (MPP) could be highly efficient (efficiency can even exceed $100\%$) for extracting the energy from a Kerr black hole, if it is immersed in a mG order magnetic field. Considering the exact solution of the magnetized Kerr spacetime, here we derive the exact expression of efficiency (${\eta }_{\mathrm{MPP}}$) for MPP, which is valid for both the Kerr black hole (BH) as well as Kerr superspinar (SS), and also from the weak magnetic field to an ultra-strong magnetic field $(B)$ which can even distort the original Kerr geometry. We show that although the value of ${\eta }_{\mathrm{MPP}}$ increases upto a certain value of ultra-strong magnetic field ($B_p$), it decreases to zero for $B>B_p$, in case of the Kerr BHs. On the other hand, ${\eta }_{\mathrm{MPP}}$ shows the opposite behavior in case of the Kerr SSs. One intriguing feature that emerges is, ${\eta }_{\mathrm{MPP}}$ acquires the maximum value for the Kerr parameter $a_{\ast }\approx 0.786$ (unlike $a_{\ast }=1$ for the ordinary PP), decreases for $0.786<a_{\ast }\le 1$. This indicates that the BH starts to expel the effect of magnetic field for $a_{\ast }>0.786$, and is fully expelled from the extremal Kerr BH due to the gravitational Meissner effect. As a special case of MPP, we also study the ordinary Penrose process (PP) for magnetized Kerr spacetime. We show that MPP for Kerr BHs, Kerr SSs and ordinary PP for Kerr SSs can be superefficient for the astrophysical applications to powering engines in the high-energy sources like active galactic nuclei and quasars, in the weak magnetic fields. Our strong magnetic field result of MPP could be important to the primordial BHs in the early Universe immersed in the primordial magnetic fields, and to the transmuted BHs which are formed by collapsing and/or by merging of the magnetized progenitors. It is almost impossible to extract the energy from a BH (SS) through MPP (PP) in the ultra-strong magnetic fields.

18 pages, 11 figures, revised version, accepted for publication in PRD, CC dedicates this paper to his mother Shree Snigdha Chakraborty

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