Accretion, Jets, and Recoil in a Merging Supermassive Black Hole Binary: A Prompt Electromagnetic Postmerger Counterpart for LISA

Abstract

We report the first three-dimensional general relativistic magnetohydrodynamic simulation to follow, self-consistently in a dynamical spacetime, the magnetized gas around a misaligned-spin supermassive binary black hole, from late inspiral through merger to the gravitational-wave recoil of the remnant. The equal-mass binary, in a $\textit{hang-up kick}$ configuration, is embedded in an equilibrated circumbinary disk (CBD), relaxed for 165 binary orbits before we evolve the final $\sim\!40$ orbits. During the inspiral, each black hole hosts a strongly warped minidisk whose jet follows the local spin axis near the horizon before aligning with the binary's angular momentum farther out. The merger imparts a recoil of $\simeq\!1032$ $km\,s$$^{-1}$ to the remnant, which nonetheless retains its gravitationally bound CBD, and the relaunched jet preserves its pre-merger orientation. The bolometric luminosity brightens by a factor $\sim\!2.2$, powered by merger-driven shocks concentrated within $r \lesssim 15\,M$, and the enhancement persists through the recoil. We identify a distinctive postmerger electromagnetic signature: magnetized structures, generated at coalescence, drive correlated quasi-periodic modulations of the horizon magnetic flux, the Poynting flux, and the thermal output, decoupled from the accretion rate. The postmerger radiative efficiency $L_{\rm EM}/\dot{M}c^2$ rises by a factor $\simeq 2.6$ at nearly constant accretion rate, showing that this emission is powered by the merger rather than accretion. The transient turns on within minutes and lasts at least several hours for a $10^6 \ M_{\odot}$ LISA source, establishing recoiling remnants as prompt postmerger counterparts to massive black hole mergers and a first-principles framework for interpreting candidates such as 3C186.