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 

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.