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Black Hole Spin-down in Collapsars in 3D Neutrino Transport GRMHD Simulations

Using 3D neutrino-transport GRMHD simulations, this study demonstrates that neutrino-cooled collapsar disks spin down black holes to an equilibrium spin of approximately 0.13, which is significantly higher than in non-radiative models and produces sufficiently powerful jets to successfully escape the progenitor star.

Original authors: Danat Issa, Beverly Lowell, Jonatan Jacquemin-Ide, Matthew Liska, Alexander Tchekhovskoy

Published 2026-04-24

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CC BY 4.0

Original authors: Danat Issa, Beverly Lowell, Jonatan Jacquemin-Ide, Matthew Liska, Alexander Tchekhovskoy

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 Big Picture: The Cosmic Firework Factory

Imagine a massive star, much bigger than our Sun, running out of fuel. It collapses under its own weight, crushing its core into a Black Hole. If this star is spinning fast enough, it doesn't just collapse quietly; it becomes a cosmic firework factory, shooting out two incredibly powerful beams of energy (jets) that we see as Gamma-Ray Bursts (GRBs).

Scientists have been trying to figure out exactly how powerful these beams are and what the "engine" (the black hole) looks like while it's running. The key to the engine's power is two things:

Magnetic Fields: Like a tangled rubber band, they store energy.
Spin: How fast the black hole is rotating.

The Old Theory: The "Brake" Problem

For a while, scientists thought these black hole engines had a major problem. They believed that as the black hole sucked in gas and magnetic fields, the magnetic fields would act like a giant brake.

Think of a spinning top. If you wrap a strong rubber band around it, the top slows down. Previous simulations suggested that this "magnetic brake" would slow the black hole down so much that it would barely spin at all (a spin of about 0.03 to 0.07).

The Problem: A black hole that spins that slowly is like a car engine with the parking brake on. It wouldn't have enough power to blast the jets out of the star. The jets would get stuck inside, and we wouldn't see the spectacular Gamma-Ray Burst.

The New Discovery: The "Cooling" Effect

This new paper says, "Wait a minute! We forgot about the heat."

In the center of a collapsing star, it gets so hot that it creates neutrinos. Neutrinos are ghostly particles that fly away instantly, taking heat with them. Think of them as a cosmic air conditioner or a heat vent.

Without Neutrinos (Old Model): The gas around the black hole is hot and puffy (like a thick, fluffy cloud). It's hard for the magnetic fields to organize, and the "brake" works too well, spinning the black hole down to a crawl.
With Neutrinos (New Model): The neutrinos suck the heat out. The gas cloud cools down and shrinks, becoming thin and flat (like a pizza dough instead of a fluffy cloud).

Why does this matter?
When the gas is thin and flat, the magnetic fields can grip the black hole better, but they don't spin it down as aggressively. It's like the difference between trying to stop a spinning top on a thick, sticky carpet (hard to spin, easy to stop) versus a smooth, thin sheet of ice (harder to stop).

The Result: A "Goldilocks" Spin

The team ran super-complex 3D simulations (using supercomputers to model the physics) and found a new "sweet spot."

Because of the neutrino cooling, the black hole doesn't spin down to a crawl. Instead, it settles at a moderate spin (about 0.13).

Is it fast? No, it's not a record-breaker.
Is it slow? No, it's much faster than the old models predicted.
Is it just right? Yes! This moderate spin is exactly strong enough to power a jet that can punch through the star and create a visible Gamma-Ray Burst.

The "Engine" Analogy

Imagine the black hole is a drill.

The Star: A giant, tough block of wood.
The Jet: The drill bit.
The Spin: How fast you turn the handle.

Old Theory: The magnetic fields were so strong they jammed the drill, slowing the handle down so much that the drill bit couldn't penetrate the wood. The job failed.

New Theory: The neutrinos act like lubricant. They cool the wood and the drill, reducing friction. The magnetic fields still grip the handle, but they don't jam it completely. The drill spins at a steady, moderate speed—fast enough to drill a clean hole through the wood, but not so fast that it breaks.

Why This Matters for the Universe

It Explains What We See: We see Gamma-Ray Bursts in the sky. This new model explains how they happen. If the black holes were spinning as slowly as the old models said, we probably wouldn't see these bursts at all.
It Matches Gravity Wave Data: When black holes crash into each other (detected by LIGO), they usually have low-to-moderate spins. This new model predicts exactly that: black holes born from collapsing stars naturally settle into this moderate spin range.
It Solves the "Too Powerful" Problem: If black holes stayed spinning super fast, they would create jets so powerful they would blow the star apart instantly, creating bursts far brighter than anything we observe. The "neutrino brake" ensures the bursts are strong enough to be seen, but not so strong that they break the laws of physics as we know them.

The Bottom Line

The universe has a built-in thermostat. The neutrinos (ghostly heat-carrying particles) cool down the material around a newborn black hole. This cooling prevents the magnetic fields from spinning the black hole down to a dead stop. Instead, the black hole finds a "Goldilocks" speed—fast enough to launch a cosmic jet, but slow enough to match what we actually observe in the sky.

It turns out, the "ghostly" particles are the secret ingredient that makes these cosmic fireworks possible.

1. Problem Statement

Long-duration gamma-ray bursts (LGRBs) are believed to be powered by "collapsars"—massive stars whose cores collapse into black holes (BHs), forming accretion disks that launch relativistic jets via the Blandford-Znajek (BZ) mechanism. The power of these jets ( $L_{BZ} \propto a^2 \Phi^2$ ) depends critically on the BH spin ( $a$ ) and magnetic flux ( $\Phi$ ).

Previous General Relativistic Magnetohydrodynamic (GRMHD) simulations of non-radiative (adiabatic) disks suggested that magnetically arrested disks (MADs) rapidly spin down BHs to very low equilibrium spins ( $a_{eq} \approx 0.035 - 0.07$ ). Such low spins would produce jets too weak to escape the progenitor star, contradicting observations of powerful LGRBs. Furthermore, gravitational wave (GW) observations of binary BH mergers suggest low spin distributions, but the specific spin evolution of collapsar-born BHs in realistic, cooling environments was unknown.

The central problem addressed is: How does neutrino cooling, which is critical in dense, hot collapsar disks, affect the equilibrium spin and jet power of the central black hole? Previous studies lacked neutrino transport due to computational cost, potentially misrepresenting disk thermodynamics and angular momentum transport.

2. Methodology

The authors performed a suite of 3D GRMHD simulations using the GPU-accelerated code $\nu$ H-AMR. Key methodological features include:

Neutrino Transport: Implemented a two-moment, energy-integrated ("gray") M1 scheme for three neutrino species ( $\nu_e, \bar{\nu}_e, \nu_x$ ). This allows for self-consistent modeling of neutrino cooling, which reduces disk thickness and alters thermodynamics.
Progenitor Models: Two distinct stellar density profiles were simulated to cover a range of mass accretion rates ( $\dot{m}$ $\overset{m}{˙}$ ):
- p# models: Power-law density profiles ( $\rho \propto r^{-1.5}$ ), typical of collapsars, yielding $\dot{m} \sim 0.1 - 1 M_\odot/s$ .
- c# models: Constant-density cores followed by steeper profiles, yielding high early-time accretion rates ( $\dot{m} \gtrsim 10 M_\odot/s$ ).
Initial Conditions: Simulations started with BHs of varying initial spins ( $a = 0.0$ to $0.8$) and magnetic field configurations conducive to MAD formation.
Analysis Framework: The authors computed the spin-up parameter ( $s$ ) and decomposed fluxes into hydrodynamic (HD), electromagnetic (EM), and neutrino (NU) components to derive a semi-analytical spin evolution model. They extrapolated these results over the typical LGRB engine lifetime ( $t_{eng} \sim 40$ s).

3. Key Contributions

First Neutrino-Cooled MAD Simulations: This is the first study to combine full 3D GRMHD with neutrino transport to investigate BH spin evolution in collapsars.
Revised Equilibrium Spin: The study establishes that neutrino cooling significantly alters the equilibrium spin, finding a value of $a_{eq} \approx 0.13$ . This is 2–4 times higher than the values found in non-radiative MAD simulations ( $a_{eq} \approx 0.035 - 0.07$ ).
Spin-Down Mechanism Quantification: The authors quantitatively demonstrate that while neutrino fluxes themselves exert negligible torque on the BH, neutrino cooling is crucial because it thins the disk. Thinner disks reduce the efficiency of the spin-down torque exerted by the magnetic field, allowing the BH to retain a higher spin.
Jet Stability Analysis: The paper links low BH spins to jet instability (kink instability), explaining why low-spin models produce variable or failed jets, particularly in progenitors with shallow density profiles.

4. Key Results

Equilibrium Spin ( $a_{eq}$ ):
- In the MAD state, the BH spin evolves rapidly to an equilibrium where the spin-up parameter $s(a_{eq}) = 0$ .
- The derived equilibrium spin is $a_{eq} \approx 0.13$ .
- This value is consistent with LIGO/Virgo/KAGRA inferred spin distributions for binary BH mergers (centered around $a \lesssim 0.3$ ).
Jet Power Enhancement:
- Because jet power scales with $a^2$ , the higher equilibrium spin ($0.13$ vs. $0.07$) results in 4–16 times more powerful jets compared to non-radiative MAD models.
- These enhanced jets are capable of successfully drilling out of the progenitor star, resolving the "failed jet" problem of previous low-spin models.
Spin Evolution Timescales:
- For typical LGRB durations ( $t \gtrsim 30$ s), BHs in both high- and low-accretion rate progenitors have sufficient time to accrete enough mass to reach $a_{eq} \approx 0.13$ .
- Shorter or lower-accretion events may leave behind BHs with slightly higher spins ( $a > 0.13$ ).
Neutrino Role:
- Direct neutrino torques on the BH are negligible.
- The primary role of neutrinos is cooling, which reduces the disk aspect ratio ( $h/r$ ). Thinner disks are less effective at transporting angular momentum away from the BH via magnetic torques, resulting in a higher equilibrium spin.
Variability and Instability:
- Models with low initial spins ( $a=0.1$ ) and shallow density profiles (p#) exhibit significant variability in magnetic flux and jet efficiency.
- This is caused by the global kink instability in weaker jets, leading to disk misalignment and temporary exits from the MAD state. This mechanism may explain subluminous or failed GRBs.

5. Significance

Resolution of the "Low Spin" Paradox: The study reconciles the need for high jet power (requiring higher spin) with GW observations of low-spin binary BHs. It suggests that collapsar BHs naturally settle at $a \approx 0.13$ , which is low enough to match GW data but high enough to power successful LGRBs.
Implications for GW Progenitors: If LGRBs are progenitors of binary BH mergers, the predicted spin distribution ( $a \sim 0.1 - 0.2$ ) aligns perfectly with current GW population studies, supporting the collapsar model for a subset of BBH mergers.
Upper Limits on GRB Luminosity: The rapid spin-down of initially high-spin BHs ( $a \sim 1$ ) to $a \approx 0.13$ within the first few seconds of the engine activity provides a physical mechanism for the observed upper cutoff in GRB luminosities. Even if a star collapses with a maximally spinning core, the jet power is capped by the rapid spin-down before the jet breaks out of the star.
Future Modeling: The work highlights the necessity of including neutrino transport in collapsar simulations to accurately predict jet energetics and r-process nucleosynthesis conditions.

In conclusion, this paper demonstrates that neutrino-cooled MADs allow collapsar black holes to maintain a higher equilibrium spin ( $a_{eq} \approx 0.13$ ) than previously thought, enabling the production of powerful, successful gamma-ray bursts while remaining consistent with gravitational wave observations of binary black hole spins.