Introduction
Accreting black holes (BHs) are ubiquitous in the Universe. Black hole masses typically lie between a few solar masses to more than 109$M_{\odot}$ for the most extreme supermassive black holes (SMBHs). On the low-mass end, the typical remnant of binary neutron star or black hole-neutron star mergers is a highly spinning BH surrounded by a torus of gas debris. On the high-mass end, there is strong observational evidence that most galaxies harbor super massive black holes (SMBHs) in their centers. Therefore it is expected that when two galaxies merge, a supermassive black hole binary (SMBHBH) with a separation of less than 100 kpc is formed. Dynamical friction and star ejection bring the binary to pc-scale separation and, simultaneously, the BHs carve out a low-density inner cavity just outside their orbit, beyond which a circumbinary accretion disk forms.
As the accretion takes place, electromagnetic (EM) radiation is produced from tenuous hot plasma in the disk and, in most cases, from the magnetically-dominated outgoing jet launched from the poles of the BHs and its interactions with the environment. This radiation may be detectable by multiple EM instruments, such as FERMI, the Event Horizon Telescope, PanSTARRS, the HST, and JWST. Gravitational waves (GWs) from inspiralling BHBHs or their BH-disk remnants have been, or are expected to be detected either by ground-based or space-based GW observatories. Therefore, the possibility of coincident detection of gravitational radiation with electromagnetic radiation from these systems make them prime sources for multimessenger astronomy. However, multimessenger observations of accreting BHs require a detailed understanding of the environment surrounding them, and simultaneously the identification of the "smoking guns" that can be used to distinguish these systems from other EM sources. Existing theoretical work on accreting BHs has sought to identify characteristic EM features that may accompany GW signals.
Recently, we have explored the evolution of self-gravitating disks around tilted, highly spinning BHs with $M_{disk}$/$M_{BH}$ ~ 0.2. We found that tidal torques from the disk induce a BH spin precession inducing a reorientation of the relativistic jet powered by such systems. Such a jet may be observed by various EM instruments.
Studies of accretion onto BHBHs are inconclusive and leave many open questions. In particular, no consensus has been reached about the nature of the vicinity of merging BHs. Theoretical efforts involving BHBHs in circumbinary disks incorporating some degree of relativistic effects and magnetic fields have been launched. We adopted ideal general relativistic magnetohydrodynamics (GRMHD) to probe EM signatures from magnetized circumbinary accretion disks onto nonspinning BHBHs of unequal masses during the late pre-decoupling and post-decoupling phases.
In this work, we extend our previous studies and consider circumbinary disks around BHBHs with misaligned spins and mass ratios of $q \equiv M_{1,irr}/M_{2,irr} = 1, 2$ and $4$, starting near the end of the binary disk pre-decoupling epoch. Here $M_{i,irr}$, with $i = 1,2$, is the irreducible mass of the $i$th BH. We evolve these systems throughout the late inspiral, merger, and post-merger phases to identify their unique EM emission features and to analyze the impact of the BHs' spins and the binary mass ratio.
