Black hole induced spins from hyperbolic encounters in dense clusters

Abstract

The black holes that have been detected via gravitational waves (GW) can have either astrophysical or primordial origin. Some GW events show significant spin for one of the components and have been assumed to be astrophysical, since primordial black holes are generated with very low spins. However, it is worth studying if they can increase their spin throughout the evolution of the universe. Possible mechanisms that have already been explored are via multiple black hole mergers and gas accretion. We propose here a new mechanism that can occur in dense clusters of black holes: the spin up of primordial black holes when they are involved in close hyperbolic encounters. We explore this effect numerically with the Einstein Toolkit for different initial conditions, including variable mass ratios. For equal masses, there is a maximum spin that can be induced on the black holes, χ = a/m ≤ 0.2. We find however that for large mass ratios one can attain spins up to χ 0.8, where the highest spin is induced on the most massive black hole. For small induced spins we provide simple analytical expressions that depend on the relative velocity and impact parameter.

Signatures of primordial black hole dark matter at DUNE and THEIA

“…[57]). A large angular momentum could also be induced in an encounter between two non-rotating black holes [58]. The BH spin parameter is hence a fundamental property, which plays a relevant role in its formation and can affect its characteristic detection signatures [38,[59][60][61].…”

Search for black hole hyperbolic encounters with gravitational wave detectors

“…If in the hyperbolic encounter the two black holes pass sufficiently near each other, the system will emit a large amount of gravitational waves that will be possible to detect in current and future gravitational wave detectors [17][18][19]. Finding these hyperbolic events would be a strong evidence for clustered primordial black holes and could be used to study their mass and spin [20]. Moreover, dense stellar clusters may also accommodate collisions among neutron stars with impact parameters significantly larger than their physical sizes (to prevent tidal disruptions), as well as BH collisions, and could thus emit GW in the LIGO band.…”