Distinguishability of magnetic massive black holes from environmental mimics with inspiral gravitational waves

Abstract

In this work, we investigate the ppE waveform imprints induced by the external magnetic fields of Bertotti-Robinson and Bonnor-Melvin black holes, with the aim of distinguishing such magnetic effects from environmental influences. We first compute the ppE frequency-domain waveform for a small black hole inspiraling into a massive KBR black hole, which corresponds to a Kerr black hole embedded in an external magnetic field. We find that the leading-order correction arising from the magnetic field is at the $-2$ PN order relative to the quadrupole term, while the next-leading-order correction is at $-1.5$ PN, originating from the spin of the black hole. We further examine the effects of a spinning KBM black hole, whose leading-order magnetic correction is at $-3$ PN, whereas its spin-induced correction is also at $-1.5$ PN. The leading-order ppE corrections for both KBR and KBM black holes do not appear degenerate with any modified theory of gravity effects; nonetheless, we demonstrate that they resemble the gravitational pull contributions from additional matter with power-law distributions of index $\gamma=1$ and 0, respectively. To break the degeneracy with a single event, we adopt the statistic F in former research to discriminate between these two classes of beyond-vacuum GR effects using multiple gravitational wave events. We show even with multiple event statistic, it is not always efficient to distinguish real magnetic field effect from corresponding gravitational pull effect, especially for Bertotti-Robinson magnetic effect. For Bonnor-Melvin black hole, there is a transition value of $\rho_0$ estimated around $10^{-4}\text{kg}/\text{m}^3$ and corresponding $B\sim 10^{4}\text{T}$ above which real magnetic effect can be efficiently distinguished from gravitational pull and below the transition value it cannot.