GW231123 Mass Gap Event and the Primordial Black Hole Scenario
Chen Yuan, Zu-Cheng Chen, Lang Liu
TL;DR
GW231123 presents masses $M_1=137^{+22}_{-17}\,M_\odot$ and $M_2=103^{+20}_{-52}\,M_\odot$ with high spins, challenging standard stellar-collapse formation. The paper tests a PBH-merger explanation from PBHs formed during an early matter-dominated era, using a narrow log-normal curvature spectrum to reproduce the observed properties and computing the associated SIGWs. They find a viable parameter subset yielding a present PBH abundance $f_{\mathrm{pbh}}\sim\mathcal{O}(0.1)$ and an average mass $\langle m\rangle\sim\mathcal{O}(400\,M_\odot)$, along with a spin distribution peaked near maximal rotation, while SIGWs remain below current PTA limits ($\Omega_{\rm GW,0}h^2\lesssim 10^{-16}$). The interpretation is sensitive to accretion-model bounds on PBH abundance, with conservative CMB limits allowing it but stronger spherical-halo bounds disfavor portions of parameter space; future SGWB searches and microlensing will be decisive tests.
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.