Introduction
Black holes (BHs) immersed in gaseous environments are ubiquitous in the Universe. Black hole-disks (BHDs) appear on a great variety of scales, reflecting their diverse birth channels and sites. From the core collapse of massive stars and the cores of active galactic nuclei, to asymmetric supernova explosions in binary systems, and the merger of compact binaries where at least one of the companions is not a BH, BHDs may be formed and serve as prime candidates for multimessenger astronomy.
The magnitude of the spin of the BH, as well as its orientation relative to the fluid flow, can have large effects, as in the existence and geometry of a relativistic plasma jet. This jet, which can be powered either by magnetic fields threading the event horizon and extracting rotational energy from the BH, or from the accretion flow, can precess when misalignment between the BH and disk angular momentum arises. Such misalignment is expected to be a common phenomenon both in active galactic nuclei as well as in BH X-ray binaries. Even in the recent observation of M87 by the Event Horizon Telescope misalignment could not be excluded.
Tilted BHDs are also the outcome from stellar-mass compact object collisions when their individual spins are not aligned with the orbital angular momentum. Population synthesis studies suggest that in approximately half of the BH-neutron star binaries the angle between the orbital angular momentum and the BH spin is larger than 45°. Such systems will yield misaligned BHDs which in turn will affect the existence and the properties of an electromagnetic counterpart, such as a short gamma-ray burst or a kilonova.
In this work we extend previous studies of self-gravitating BHDs in two ways. For the first time we perform general relativistic simulations of tilted BHDs starting from self-consistent initial values. The tilted BHD models are solutions of the full (i.e. including the conformal metric) general relativistic initial value problem as described in previous work (arXiv:1810.02825). Second, we extend the parameter space by evolving disks around rapidly spinning BHs (aligned, antialigned and tilted with respect to the disk angular momentum) having dimensionless spins up to 0.97. The precessing BHDs are responsible for copious gravitational wave emission in multiple modes, which we compute. Although our simulations do not include magnetic fields, estimation of the effective turbulent magnetic viscous timescale shows that it is much longer than the dynamical timescale of the one-arm instability. Therefore we expect these BHDs to be prominent sources of gravitational waves and Poynting electromagnetic radiation (in the presence of magnetic fields) and thus excellent sources for multimessenger astronomy.
