GW231123 Mass Gap Event and the Primordial Black Hole Scenario

TL;DR

This paper explores whether the GW231123 gravitational wave event could be explained by primordial black holes formed in an early matter-dominated era, analyzing their properties, abundance, and associated gravitational waves.

Contribution

It provides a detailed modeling of PBH formation, mass, spin, and merger rates, and assesses their compatibility with observational constraints and gravitational wave data.

Findings

PBH mass function can reproduce GW231123 properties

PBH abundance is near current observational limits

Associated scalar-induced gravitational waves are negligible in the nano-hertz band

Abstract

We investigate the possibility that the recently reported GW231123 event, with component masses $M_1=137^{+22}_{-17}\,M_\odot$, $M_2=103^{+20}_{-52}\,M_\odot$ and a local merger rate $R_{\mathrm{local}}=0.08^{+0.19}_{-0.07}\,\mathrm{Gpc^{-3}\,yr^{-1}}$, originates from primordial black holes (PBHs) formed during an early matter-dominated era. We compute the PBH mass function, abundance, spin distribution and the merger rate density and find a set of choices for the parameters to reproduce the key properties of GW231123. While PBHs formed in such a scenario can acquire large spins through sustained tidal torques, the spin distribution remains uncertain and additional accretion might lead to extreme spin values inferred in GW231123. We also show that the resulting PBH abundance, $f_{\mathrm{pbh}}=1.64^{+5.00}_{-1.59}\times10^{-1}$, lies close to the exclusion bounds from CMB accretion limits and other probes, highlighting a potential tension with current constraints. Finally, we estimate the scalar-induced gravitational waves (SIGWs) that are inevitably generated during PBH formation. PBHs that interpret GW231123 are accompanied by negligible SIGWs in the nano-hertz band, indicating no conflict with current pulsar timing arrays data.