Spinning into the Gap: Direct-Horizon Collapse as the Origin of GW231123 from End-to-End GRMHD Simulations

TL;DR

This study uses advanced 3D GRMHD simulations to explore how massive, low-metallicity stars with magnetic fields can produce highly spinning black holes within the pair-instability mass gap, explaining GW231123's properties.

Contribution

First self-consistent simulation tracking massive star collapse from helium core burning to black hole formation, incorporating rotation and magnetic fields to explain GW231123's unique features.

Findings

Magnetic fields influence black hole spin and mass during collapse.

Massive stars with moderate magnetic fields produce black holes matching GW231123.

Outflows during collapse can power gamma-ray burst-like jets.

Abstract

GW231123, the most massive binary black hole (BH) merger observed to date, involves component BHs with masses inside the pair-instability mass gap and unusually high spins. This challenges standard formation channels such as classical stellar evolution and hierarchical mergers. However, stellar rotation and magnetic fields, which have not been systematically incorporated in prior models, can strongly influence the BH properties. We present the first self-consistent simulations tracking a massive, low-metallicity helium star from helium core burning through collapse, BH formation, and post-BH formation accretion using 3D general-relativistic magnetohydrodynamic (GRMHD) simulations. Starting from a $250\,M_\odot$ helium core, we show that collapse above the pair-instability mass gap, aided by rotation and magnetic fields, drives mass loss through disk winds and jet launching. This enables the formation of highly spinning BHs within the mass gap and reveals a BH spin-mass correlation. Strong magnetic fields extract angular momentum from the BH through magnetically driven outflows, which in turn suppress accretion, resulting in slowly spinning BHs within the mass gap. In contrast, stars with weak fields permit nearly complete collapse and spin-up of the BH to $ a\approx1$. We show that massive low-metallicity stars with moderate magnetic fields naturally produce BHs whose masses and spins match those inferred for GW231123, and are also consistent with those of GW190521. The outflows may impart a BH kick, which can induce spin-orbit misalignment and widen the post-collapse orbit, delaying the merger. The outflows launched during collapse may power short-lived, high-luminosity jets comparable to the most energetic $\gamma$-ray bursts, offering a potential observational signature of such events in the early universe.