Massive Perturbers and Transient Pickup Discs in Disc-Crossing Encounters in OJ 287-like Supermassive Black Hole Binaries

Abstract

We study the hydrodynamical response of a massive black hole accretion disc punctured by a lower mass black hole on an inclined, eccentric orbit, as motivated by the quasi-periodic outbursts seen in the blazar OJ 287. Using three-dimensional smoothed particle hydrodynamics simulations, we explore how the secondary black hole's mass and orbital eccentricity, and the disc thickness and viscosity, affect the time variation of mass accretion onto both black holes. We find that disc-crossing events only lead to significant spikes in the primary black hole's accretion rate when the perturber is quite massive ($q \gtrsim 0.1$) and that such spikes are delayed by roughly a free-fall time ($\sim$ months for OJ 287) after the first disc-crossing. The orbital eccentricity, disc thickness, and viscosity can influence the amplitude and temporal structure of the response; in particular, higher eccentricity causes larger and less delayed delivery of gas to the primary, however the condition $q \gtrsim 0.1$ seems to be robust. We also show that when the secondary is this massive, it generally acquires a "pickup" disc which could produce its own luminous signature. For systems like OJ 287, the secondary pickup disc can contain $10^{2-3} M_\odot$, which if accreted over a timescale comparable to the orbital period can power near-Eddington secondary luminosities with thermal emission peaking in the UV/EUV.