Scientists Solve Puzzle of 'Forbidden' Black Holes: Magnetic Fields Explain an 'Impossible' Merger

Magnetic fields may explain the 'impossible' merger of massive, fast-spinning black holes

Researchers have resolved a long-standing puzzle about an apparently impossible black-hole collision detected as gravitational waves in 2023. The signal, recorded by LIGO, Virgo and KAGRA on Nov. 23, 2023 and catalogued as GW231123, came from a merger roughly 7 billion light-years away between two unusually massive, near–maximally spinning black holes — about 100 and 140 times the mass of the Sun.

Those masses sit in or near the so-called pair-instability mass gap (roughly 70–140 solar masses), a range in which standard stellar-evolution models predict that very massive stars should be completely disrupted in pair-instability supernovae and leave no black-hole remnant. Prior mergers can place objects into this gap, but that explanation was disfavored for GW231123 because merger histories tend to reduce a black hole’s spin, whereas both components in this event were observed to be spinning close to the theoretical maximum.

A team at the Flatiron Institute's Center for Computational Astrophysics (CCA) led by Ore Gottlieb revisited the problem with new, end-to-end simulations that follow very massive stars from their evolution through collapse and the immediate post-supernova phase. Their key addition was explicitly modelling magnetic fields in the stellar debris — a factor previous shortcut approaches often neglected.

"When rotation and magnetic fields are included, the post-collapse evolution can dramatically reduce the final black hole mass compared with the collapsing star's initial mass," Gottlieb said, summarizing the study's core result.

The simulations show that a rapidly rotating progenitor can leave behind a rotating, flattened accretion disk of fallback material around the newborn black hole. Magnetic fields threading this disk can drive powerful, relativistic outflows and jets that eject a substantial fraction of the stellar envelope at near-light speeds. Depending on field strength, up to roughly half of the progenitor's mass can be expelled rather than accreted.

As a result, a very massive star that would otherwise lie above the pair-instability threshold can still produce a black hole with a mass inside the nominal gap. The models also predict a correlation between magnetic-field strength, final black-hole mass and spin: stronger fields tend to produce lighter, slower-spinning remnants (due to stronger mass ejection), while weaker fields allow more mass to accrete and yield heavier, faster-spinning black holes.

Importantly, the simulations suggest an observational test: the mass-ejection process should be accompanied by a burst of high-energy emission such as a gamma-ray flash. A simultaneous detection of gamma rays with a gravitational-wave signal from a mass-gap merger would provide strong multi-messenger evidence for this magnetic-field-driven formation channel.

The team's results were published on Nov. 12 in The Astrophysical Journal Letters.

Why this matters

This work provides a plausible, physically motivated route to produce massive, rapidly spinning black holes from single-star evolution without invoking prior black-hole mergers. It highlights the importance of magnetic fields and rotation in the final stages of stellar collapse and offers a clear observational signature to confirm the scenario.