Abstract
In this work we have presented a special class of Kerr–Newman-NUT black hole, having its horizon located precisely at r=2M, for Q^{2}=l^{2}-a^{2}, where M, l, a and Q are respectively mass, NUT, rotation and electric charge parameters of the black hole. Clearly this choice radically alters the causal structure as there exists no Cauchy horizon indicating spacelike nature of the singularity when it exists. On the other hand, there is no curvature singularity for l^2 > a^2, however it may have conical singularities. Furthermore there is no upper bound on specific rotation parameter a / M, which could exceed unity without risking destruction of the horizon. To bring out various discerning features of this special member of the Kerr–Newman-NUT family, we study timelike and null geodesics in the equatorial as well as off the equatorial plane, energy extraction through super-radiance and Penrose process, thermodynamical properties and also the quasi-periodic oscillations. It turns out that the black hole under study radiates less energy through the super-radiant modes and Penrose process than the other black holes in this family.
Highlights
On the other hand, even though there is no observational evidence whatsoever for the existence of gravitomagnetic mass [12], investigation of the geodesics in Kerr– Newman-NUT spacetime has significance from both theoretical as well as conceptual points of view
1 Note that the results presented in [14,15] are based on the erroneous assumption that circular geodesics lie on the equatorial plane, see e.g., [16]
For a black hole given with specific NUT charge and rotation parameter, we can numerically solve the above equations and locate the circular orbits confined in a particular plane
Summary
Even though there is no observational evidence whatsoever for the existence of gravitomagnetic mass [12], investigation of the geodesics in Kerr– Newman-NUT spacetime has significance from both theoretical as well as conceptual points of view. For that we have studied quasi periodic oscillations for the black hole in question where fundamental frequency of oscillations depends upon it This could be one of the possible observational tests to unveil the existence of NUT parameter. The choice of Q2 + a2 = l2 indicates that repulsive effect due to charge and rotation is fully balanced by attractive effect due to the NUT parameter This is why the causal structure of spacetime has been radically altered [31]. With the choice l > a, there occurs no ring singularity as reflected in the fact that r 2 + (l + a cos θ )2 = 0 for any choices of r and θ It is a very interesting special case of the Kerr–NewmanNUT family of spacetimes, whose structure we wish to understand in this paper for studying its various interesting properties. All the Greek indices run over four dimensional spacetime coordinates, while the roman indices run over spatial three dimensional coordinates
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