Suppression of the accretion rate in thin discs around binary black holes

Abstract

We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with H/R 0.1, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs, in this case the mass accretion rate is suppressed, such that only a fraction (linearly dependent on H/R) of the available gas is able to flow within the cavity and accrete on to the binary. Extrapolating this result to the cold and thin accretion discs expected around supermassive black hole binary systems implies that this kind of systems accretes less material than predicted so far, with consequences not only for the electromagnetic and gravitational waves emissions during the late inspiral phase but also for the recoil speed of the black hole formed after binary coalescence, thus influencing also the evolutionary path both of the binary and of the host galaxy. Our results, being scale-free, are also applicable to equal mass, circular binaries of stellar mass black holes, such as the progenitor of the recently discovered gravitational wave source GW150914.

Circumbinary, not transitional: on the spiral arms, cavity, shadows, fast radial flows, streamers, and horseshoe in the HD 142527 disc

“…3) and develops an eccentric cavity similar to those found in previous circumbinary disc simulations (e.g. Farris et al 2014;D'Orazio et al 2016;Ragusa et al 2016) and used by Ragusa et al (2017) to explain dust horseshoes. The spiral arms appear fixed in relation to the binary orbit, but superimposed on this is a precessing overdensity which precesses on a timescale of 2-3 orbital periods, as found by Dunhill, Cuadra & Dougados (2015).…”

“…Recently, Ragusa et al (2017) showed that central binaries can produce both rings and horseshoes in dust emission. Indeed, eccentricities around the cavity edge are a common feature in hydrodynamical simulations of circumbinary discs (Kley & Dirksen 2006;Farris et al 2014;Ragusa et al 2016). Ragusa et al (2017) showed that more massive companions produce progressively more asymmetric structures, with simulated ALMA observations closely matching observed discs.…”

“…Indeed, the average accretion rate in circumbinary discs with H/R ∼ 0.05 is expected to be similar (within a factor of a few) to that in a disc around a single object (e.g. Farris et al 2014;Ragusa et al 2016). In our simulations we measure a mass accretion rate of ≈ 10 −7 M /yr onto the primary, consistent with observational estimates of 2(±1) × 10 −7 M /yr (Mendigutía et al 2014).…”

Viscous hydrodynamics simulations of circumbinary accretion discs: variability, quasi-steady state and angular momentum transfer

“…The ratio of the former to the latter characterizes the extent to which gravitational torques from the binary suppress or enhance accretion. Previous studies have reported values of this ratio both smaller than unity (e.g., MacFadyen & Milosavljević 2008;Ragusa et al 2016), and greater than unity (e.g., Farris et al 2014). In considering these claims, it is necessary to make a distinction between discs whose outer edges spread freely, and those whose outer regions are supplied with mass at a steady rate (e.g., due to an infalling envelope).…”

Accretion and Orbital Inspiral in Gas-Assisted Supermassive Black Hole Binary Mergers

“…Recent results of Ragusa et al (2016) suggest that for colder disks with lower aspect ratio < h r 0.1 the binary accretion rateṀ b actually drops compared to˙¥ M . This would imply that mass accumulates more efficiently near the binary in colder disks, making situation discussed in Section 3.2.1 possible.…”