Variety in the Variability of Accreting Supermassive Binary Black Holes

Accreting supermassive binary black holes are key multi-messenger
sources for LISA, yet are challenging to simulate realistically as
solving the radiation magnetohydrodynamics (MHD) equations over the
full dynamic spatio-temporal range of the problem is computationally
infeasible at present. We will provide a brief summary of the the
progress made in the field to understand these systems theoretically
and what new directions groups are pursuing. We will also report on
our collaboration's progress to simulate these systems using general
relativistic MHD and dynamic GR. In order to cover a larger temporal
range in one set of simulations, we constrain our view to the
circumbinary disk region and measure how the binary mass ratio,
accretion disk size, and black hole spin have on the structure and
variability of the accretion flow. We particularly emphasize how these
parameters influence the overdensity feature, which orbits the binary
near the edge of the cavity, since it is responsible for most of the
electromagnetic emission's variability and variability is a key
signature of a system being a binary. Extending to smaller length
scales, we will report on simulations following accretion all the down
to the event horizons so that we can begin to investigate how black
hole spin affects mini-disk dynamics, accretion rate, and jet
power. The novel computational methods enabling inclusion of the black
holes in the domain, including multi-patch methods, will be described.