GW231123: A Possible Primordial Black Hole Origin

TL;DR

This paper proposes that the GW231123 black hole merger could originate from primordial black holes, with their observed high masses and spins explained by cosmological accretion within a specific parameter space.

Contribution

It demonstrates that the GW231123 event can be naturally explained by a primordial black hole origin, considering cosmological accretion effects, and discusses observational tests for this hypothesis.

Findings

Primordial black holes can account for GW231123's mass and spin.

The parameter space for this explanation is near current observational exclusion limits.

Future observations could confirm or rule out the primordial black hole origin for such events.

Abstract

GW231123, the heaviest binary black hole merger detected by the LIGO-Virgo-KAGRA Collaboration to date, lies in the pair-instability mass gap and exhibits unusually high component spins. In this Letter, we show that both merging black holes may have a primordial origin with smaller initial masses. The observed masses and, crucially, the spins of GW231123 are naturally accommodated within the most vanilla primordial black hole framework, once cosmological accretion is taken into account. Interestingly, the parameter space needed to explain the inferred GW231123 rate is at the edge of the exclusion region from x-ray and CMB observations, suggesting that this interpretation can be either confirmed or ruled out. The upcoming O5 observing run by the collaboration should detect ${\cal O}(20)$ similar events, testing their mass-spin correlation, while next-generation detectors would be capable of observing high redshift events, as predicted in this scenario.