⚛️ general relativity

Can GW231123 have a stellar origin?

This paper investigates the stellar origin of the massive, rapidly spinning black hole GW231123 by simulating rotating massive stars, finding that differential rotation can shift the pair-instability mass gap to higher values and allow for the direct core-collapse formation of black holes with masses exceeding 150 solar masses and high spins.

Original authors: Djuna Croon, Davide Gerosa, Jeremy Sakstein

Published 2026-02-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Djuna Croon, Davide Gerosa, Jeremy Sakstein

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Mystery of the "Impossible" Black Holes

Imagine the universe as a giant cosmic construction site. For a long time, astronomers had a very specific rulebook for how black holes are built from dying stars. This rulebook said: "You can build black holes up to a certain size, but then there's a 'No-Go Zone' (a gap) where no black holes can exist, and then you can build them again only if they are super massive."

This "No-Go Zone" is called the Pair-Instability Mass Gap. It's like a gap in a staircase where you can't step; if a star is too heavy, it doesn't just collapse into a black hole—it explodes completely, leaving nothing behind.

Then, on November 23, 2023, the LIGO detectors heard a "thud" from deep space: GW231123.

This event was weird. It involved two black holes colliding.

They were huge: One was about 137 times the mass of our Sun, and the other was about 101 times.
They were spinning like tops: They were rotating incredibly fast.

The big problem? The heavier black hole (137 Suns) falls right into that "No-Go Zone." According to the old rulebook, a star that big should have exploded and vanished, not become a black hole. Plus, the fact that it was spinning so fast was a huge mystery.

The Old Theory vs. The New Idea

The Old Theory (The "Hierarchical" Idea):
Some scientists thought, "Maybe this black hole wasn't born from a single star. Maybe it was a 'black hole monster' that ate other black holes." Imagine a game of cosmic Tetris where small black holes merge to make a big one, which merges again to make a giant one.

The Problem: This theory is like trying to stack blocks perfectly to get a specific shape. It requires very specific, unlikely conditions (fine-tuning) to get the spin to be that fast. It's possible, but it feels like cheating.

The New Idea (The "Stellar" Idea):
The authors of this paper asked: "What if this giant black hole was born from a single, massive star, just like the old rulebook says, but the rulebook was missing a page?"

They proposed that the star was spinning incredibly fast while it was alive, and that spin changed the rules of the game.

The Analogy: The Spinning Ice Skater

To understand how spin saves the star, imagine an ice skater.

Non-spinning Star: A star that isn't spinning is like a skater standing still. Gravity pulls everything inward. If the star gets too hot and heavy, it explodes (Pair Instability) and disappears.
Spinning Star: Now, imagine that skater starts spinning their arms out wide. The centrifugal force (the force that pushes you outward on a merry-go-round) fights against gravity.

In this paper, the authors simulated a massive star (a "helium core" of 160 Suns) spinning at different speeds. They found that:

The "No-Go Zone" Moves: Because the star was spinning so fast, the centrifugal force held it together longer. It prevented the star from exploding.
The Gap Shifts: The "No-Go Zone" didn't disappear; it just moved to a higher weight limit. The star could now be heavier than the old limit without blowing up.
The Result: Instead of exploding, the star collapsed directly into a black hole. And because it was spinning so fast to begin with, the resulting black hole kept that spin.

The "Secret Sauce": Nuclear Physics

There was one other variable they had to tweak: a specific nuclear reaction inside the star (turning Carbon into Oxygen). Think of this as the "fuel mixture" in a car engine.

If the fuel burns too fast, the car explodes.
If the fuel burns just right, the car runs smoothly.

The authors found that if this nuclear reaction happens at a slightly faster rate than we usually think, combined with the fast spin, the star can survive the "explosion phase" and collapse into the massive, spinning black hole we saw in GW231123.

The Conclusion: A New Chapter in the Rulebook

The paper concludes that yes, GW231123 can have a stellar origin.

It doesn't need to be a monster that ate other black holes. It can be the direct child of a single, massive, rapidly spinning star.

The Star: It was so big and spun so fast that it cheated death.
The Black Hole: It is the first direct observation of a black hole formed by this specific "photodisintegration" collapse (where the star implodes because it gets too hot to hold itself together, but spin saves it from exploding).

Why does this matter?
It tells us that our "rulebook" for how stars die needs an update. We can't just say "Black holes above 130 Suns don't exist." We have to remember that spin is a powerful force that can push the boundaries of what is possible in the universe. GW231123 is likely the first time we've seen a black hole that broke the old rules by spinning its way to survival.

1. Problem Statement

The gravitational wave event GW231123, detected by LIGO, involves a binary black hole merger with source-frame masses of $m_1 \approx 137^{+23}_{-18} M_\odot$ and $m_2 \approx 101^{+22}_{-50} M_\odot$ . The primary black hole ( $m_1$ ) is the heaviest stellar-mass black hole detected to date and falls within the predicted pair-instability black hole mass gap (BHMG) (typically $60\text{--}130 M_\odot$ ). Furthermore, both black holes exhibit extremely high spins ( $\chi_1 \approx 0.9$ , $\chi_2 \approx 0.8$ ).

The central astrophysical challenge is explaining the origin of such a massive, rapidly spinning object:

Hierarchical Mergers: While often proposed for massive black holes, standard hierarchical merger models in dense stellar environments typically produce spins peaked around $\chi \sim 0.7$ . Achieving $\chi \approx 0.9$ requires extreme fine-tuning of progenitor mass ratios and spin alignments.
Direct Stellar Collapse: Standard stellar evolution theory predicts that stars with helium core masses between $\sim 65$ and $\sim 135 M_\odot$ undergo pair-instability supernovae (PISN) or pulsational PISN, completely disrupting the star or leaving a lighter remnant, thus creating a mass gap. Additionally, standard models with efficient angular momentum transport (e.g., Spruit-Tayler dynamo) predict that massive stars lose most of their core rotation, resulting in non-spinning black holes.

The authors investigate whether GW231123's primary could have formed via direct core collapse of a single rotating massive star, specifically exploring the regime of photodisintegration instability (the upper edge of the mass gap), while accounting for the extreme spin.

2. Methodology

The authors performed a grid of stellar evolution simulations using the MESA (Modules for Experiments in Stellar Astrophysics) code (v15140).

Initial Conditions:
- Progenitor: $160 M_\odot$ helium cores (stripped of hydrogen envelopes), representing the upper edge of the 90% credible interval for $m_1$ in GW231123.
- Metallicity: $Z = 10^{-5}$ (low metallicity).
- Rotation: Initial rotation rates ( $\Omega/\Omega_{crit}$ ) varied from 0 to 1. The study explicitly excluded the Spruit-Tayler (ST) dynamo to model a scenario of weak angular momentum transport (differential rotation), allowing the core to retain substantial spin until collapse.
Nuclear Physics:
- The study varied the $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ reaction rate over a range of $\pm 3\sigma$ from the median value. This rate is the dominant uncertainty in determining the boundaries of the pair-instability mass gap.
Simulation Scope:
- Simulations tracked evolution from the Zero-Age Horizontal Branch (ZAHB) to either core collapse or total disruption (PISN).
- Spin Estimation: A model based on Batta & Ramirez-Ruiz (2019) was used to estimate the final black hole spin, assuming the innermost $3 M_\odot$ collapses promptly to a maximally spinning BH, with the remainder accreting via a disk or direct fall, regulated by feedback to ensure $\chi \leq 1$ .

3. Key Contributions

First Investigation of Rotating Photodisintegration Collapse: This is the first study to simultaneously model the effects of rapid stellar rotation and variations in the $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ reaction rate on the upper edge of the pair-instability mass gap (photodisintegration regime).
Re-evaluation of the Mass Gap: The authors demonstrate that rotation fundamentally alters the stability criteria for massive stars, shifting the mass gap boundaries.
Stellar Origin for GW231123: They provide a viable astrophysical pathway for the formation of the primary black hole in GW231123 via direct stellar collapse, challenging the necessity of hierarchical merger explanations for this specific event.

4. Key Results

Rotation Shifts the Mass Gap:
- Centrifugal support in rapidly rotating stars lowers the core temperature for a given density. This delays the onset of explosive oxygen burning and photodisintegration.
- Consequently, the upper edge of the mass gap shifts to higher masses. Stars that would have undergone PISN in non-rotating models can survive to collapse directly into black holes if they rotate rapidly.
- For the median $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ rate, the threshold for pair-instability disruption ( $\Omega_{PISN}$ ) increases from $\sim 0.1 \Omega_{crit}$ (non-rotating) to $\sim 0.5 \Omega_{crit}$ for the $+3\sigma$ reaction rate.
Formation of High-Spin, High-Mass Black Holes:
- In the weak angular momentum transport regime, the authors found that black holes with masses $\gtrsim 150 M_\odot$ and spins $\chi \approx 0.8\text{--}0.9$ can form from the direct collapse of $160 M_\odot$ helium cores.
- These results align with the 90% credible intervals for the mass and spin of GW231123's primary component.
Dependence on Nuclear Rates:
- The formation of a $\sim 137 M_\odot$ black hole with high spin is possible if the $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ rate is between the median and $+3\sigma$ of its uncertainty range.
- If the reaction rate were significantly lower than the median, the star would likely undergo PISN and not form a black hole in this mass range.
Secondary Component:
- The secondary black hole ( $m_2$ ) is consistent with forming below the mass gap. The authors suggest GW231123 may "straddle" the gap, a scenario supported by the anti-correlation of mass measurements in high-mass events.

5. Significance

Challenging the Mass Gap Cutoff: The results suggest that the pair-instability mass gap is not a hard cutoff but a region whose boundaries are sensitive to stellar rotation and nuclear physics. Future gravitational wave population analyses should not impose a hard cutoff at $\sim 130 M_\odot$ but should allow for "gap-crossing" systems formed via photodisintegration collapse.
Implications for Stellar Evolution: The study highlights the critical need to understand angular momentum transport in very massive stars. If the ST dynamo is inefficient in these regimes (as suggested by GW231123), massive stars can retain enough angular momentum to produce rapidly spinning, massive black holes.
Population Constraints: GW231123, with its extreme mass and spin, serves as a key data point for constraining the $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ reaction rate and the efficiency of internal angular momentum transport in massive stars.
Alternative to Hierarchical Mergers: The paper provides a strong alternative to the hierarchical merger hypothesis for GW231123, offering a single-star formation channel that naturally explains both the high mass and the high spin without requiring fine-tuned merger histories.