Massive Binary Black Holes in Gaseous Environments

This thesis explores theoretical astrophysics of massive, sub-parsec black hole binaries

(BHBs) in a gaseous environment with an additional focus on identifying potential

multimessenger signatures.

First, the context of massive and supermassive BHBs in cosmological structure formation

is given and observational techniques both for electromagnetic (EM) and gravitational

waves (GW) are outlined. Then, the numerical methods of (i) Newtonian smoothed

particle hydrodynamics (SPH) and (ii) thermal, optically thick general relativistic radiation

hydrodynamics (GR-RHD) are detailed.

In the case of the GR-RHD formulation, in which stiff source terms arise, a dedicated

numerical treatment in form of an implicit explicit (IMEX) Runge-Kutta time integrator

is shown. This IMEX scheme is implemented, validated and tested in two GR codes with

the aim that, once publicly released, it will be useful for the community. Also to this end,

the physical limitations of the current framework's applicability are explored finding that

the inclusion of radiation gives order of magnitude improvements over evolutions without

radiation even in situations where parts of the underlying assumptions are violated. The

remaining shortcomings are discussed and strategies for future improvement are listed. The

astrophysical accretion scenarios employed are transonic spherical accretion and sub- and

supersonic Bondi-Hoyle-Lyttleton accretion onto a single black hole of galactic size.

The secular dynamics of massive BHBs in self gravitating circumbinary accretion discs is

studied at the evolutionary stage where the binary has already excavated a cavity inside the

disc. Starting with prograde binary-disc systems, the torques coming from the disc-gravity

and from accretion are dissected into components in both space and frequency. Alternative

treatments of the thermodynamics inside the cavity are tested to check the sensitivity of the

numerical results towards such idealised treatments. It is explained why there is a limiting

eccentricity ecrit, which a BHB attains during disc migration, whose existence is independent

of the thermodynamical treatment of the cavity gas and independent of accretion torques.

Ongoing work on BHBs in retrograde, self gravitating discs is presented hinting at the

existence of yet another limiting eccentricity: an instability is observed where the BHB

instead of growing maximally eccentric to e   1, starts tilting with respect to its disc and

the angular momentum vectors of the BHB and the disc subsequently align.

Focussing on the observational implications of the prograde disc scenario, the residual

eccentricity at band-entrance of LISA is calculated and found to be non-negligible.

Furthermore, the expected differences in eccentricity populations produced by stellar and

gaseous environments is estimated finding that eccentricity can be used together with

expected spin populations as a discriminant between the two environmental histories of

BHBs that are detected via GWs only.

For Pulsar Timing sources, analogous calculations are presented and moreover, co-incident

electromagnetic counterparts in the X-ray are identified as realistically detectable with

current and upcoming X-ray facilities. Specifically, periodic variability stemming from the

highly eccentric BHB could be observed by MAXI or eROSITA, whereas a significant

detection of double FeKa lines requires an Athena like X-ray spectrometer.

These results have impact on low frequency GW data analysis, on searches for supermassive

BHBs, on the development of more realistic numerical treatments of relativistic accretion

flows and some of the results on secular evolution of binaries in discs could possibly be applied

to stellar and planetary migration astrophysics.