Gravitational waves from highly charged binary black holes in Einstein-Maxwell-dilaton theory

Abstract

We perform fully nonlinear numerical-relativity simulations of electrically charged binary black hole mergers in Einstein-Maxwell-dilaton (EMD) theory, varying the magnitude and relative sign of the input charges and the dilaton coupling $a$. In the weak-charge sequence, the dominant gravitational waveforms remain close to the neutral vacuum baseline of general relativity and are visually indistinguishable across the sampled couplings, consistent with earlier EMD simulations. For the highly charged binaries with equal-magnitude opposite charges, however, the dominant gravitational strain exhibits visible differences between the EMD cases with $a\neq0$ and the charged Einstein-Maxwell reference at $a=0$. At $a=1$, the coordinate-time interval from the same separation to the waveform-based merger is approximately $62\%$ shorter than for the neutral binary. In this high-charge sequence at fixed input charges, increasing $a$ strengthens the scalar dressing while leaving the individual dilaton-weighted horizon charges nearly unchanged. Within the same sequence, increasing $a$ across the sampled values decreases the dipolar electromagnetic-flux peak, increases the scalar-flux peak, and modestly lengthens the merger interval. The sampled like-charged binaries have corresponding merger intervals approximately $15\%$-$20\%$ longer than the neutral baseline and emit predominantly quadrupolar electromagnetic radiation. At $a=1$ and $a=\sqrt{3}$, the like-charged mergers leave charged remnants with scalar fields consistent with secondary dilaton hair. The opposite-charge remnants instead approach neutral black holes without stationary scalar hair.