https://doi.org/10.1140/epjc/s10052-021-09890-3
Regular Article - Theoretical Physics
Slowly rotating black holes in the novel Einstein–Maxwell-scalar theory
1
National Astronomical Observatories, Chinese Academy of Sciences, 100101, Beijing, China
2
School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, No. 19A, Yuquan Road, 100049, Beijing, China
Received:
12
October
2021
Accepted:
28
November
2021
Published online:
12
December
2021
We investigate a slowly rotating black hole solution in a novel Einstein–Maxwell-scalar theory, which is prompted by the classification of general Einstein–Maxwell-scalar theory. The gyromagnetic ratio of this black hole is calculated, and it increases as the second free parameter 
increases, but decreases with the increasing parameter 
. In the Einstein–Maxwell-dilaton (EMD) theory, the parameter vanishes but the free parameter 
governing the strength of the coupling between the dilaton and the Maxwell field remains. The gyromagnetic ratio is always less than 2, the well-known value for a Kerr–Newman (KN) black hole as well as for a Dirac electron. Scalar hairs reduce the magnetic dipole moment in dilaton theory, resulting in a drop in the gyromagnetic ratio. However, we find that the gyromagnetic ratio of two can be realized in this Einstein–Maxwell-scalar theory by increasing and the charge-to-mass ratio Q/M simultaneously (recall that the gyromagnetic ratio of KN black holes is independent of Q/M). The same situation also applies to the angular velocity of a locally non-rotating observer. Moreover, we analyze the period correction for circular orbits in terms of charge-to-mass ratio, as well as the correction of the radius of the innermost stable circular orbits. It is found the correction increases with but decreases with Q/M. Finally, the total radiative efficiency is investigated, and it can vanish once the effect of rotation is considered.
© The Author(s) 2021
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