Spins of primordial binary black holes before coalescence

Abstract

Primordial stellar-mass black holes, which may contribute to dark matter and to the observed LIGO binary black hole coalescences, are expected to be born with very low spins. Here we show that accretion mass gain by the components of a primordial black hole binary from the surrounding matter could lead to noticeable spins of the components prior to the coalescence provided high initial orbital eccentricities.

Spins of black holes in coalescing compact binaries

“…We have also considered the possible spin-up of primordial BHs due to gas accretion from cold interstellar medium. We have shown (see [58] and Fig. 5) that the accretion spin-up of primordial BH in binary systems can result in the effective spins at several percent level, therefore small positive effective spins of the observed coalescing BH+BH does not contradict the hypothesis of their possible primordial origin.…”

“…However, the estimate of the accretion-induced BH mass growth in a binary system significantly increases is the initial orbit was highly eccentric, e 0 ∼ 1. In this case [58]…”

“…To check the latter statement in paper [58] we studied the possibility of accretion spin-up of BH components of a primordial binary BH in the external medium.…”

“…Maximum effective spin of a coalescing primordial binary BH with the chirp mass M = 30M acquired due to accretion in the medium with the sound velocity c s = 10 −5 = 3 km s −1 and density ρ = 10 −24 g cm −3 as a function of the binary component mass ratio q.Fig. from[58].…”

Primordial black hole merger rates: distributions for multiple LIGO observables

“…There is the possibility that, although the PBH spin is small at formation, they could spin up between formation and merger. However, this is likely to be a small effect [53].…”