Electromagnetic Emission and Orbital Evolution of Eccentric Supermassive Black Hole Binaries in Retrograde Disks

Abstract: Circumbinary disks around supermassive black hole binaries (SMBHBs) are expected to form across a broad range of inclinations, with retrograde configurations potentially a common occurrence. Here we present the first grid-based hydrodynamical simulations of retrograde circumbinary disks around eccentric SMBHBs, solving an energy equation that balances viscous and shock heating against blackbody radiative cooling. We investigate different initial disk Mach numbers $\mathcal{M}<em>a \in {10, 20, 40}$ and consider binary eccentricities $e</em>\mathrm{b}\in[0.0,\,0.8]$, finding that multiple stable states exist for the same binary eccentricity and Mach number. These states differ by the sense of rotation of their minidisks; both retrograde ($\downarrow\downarrow$), both prograde ($\uparrow\uparrow$), or one of each ($\uparrow\downarrow$), a property set by the initial conditions. Our findings indicate that each state produces qualitatively distinct orbital evolution: $\downarrow\downarrow$ and $\uparrow\downarrow$ minidisks drive circular inspirals whereas $\uparrow\uparrow$ minidisks drive eccentric inspirals with potentially observable eccentricity in the LISA band. We measure the electromagnetic emission produced by the disk, finding that a binary of mass $M_\mathrm{b}=8\times10<sup>6\mathrm{M}_\odot$ at redshift $z=1$ would be detectable by both current and upcoming optical and UV instruments. We demonstrate that the time- and azimuthally-averaged disk profiles are well described by 1D models, which naturally set a cavity radius within which angular momentum transport is dominated by Reynolds stresses rather than viscosity.