⚛️ general relativity

Relativistic jets from millisecond proto-magnetars

This study uses 3D general-relativistic magnetohydrodynamic simulations to demonstrate that rapidly rotating millisecond proto-magnetars can launch ultra-relativistic jets within seconds of formation, as centrifugal forces create a dense equatorial wind that confines and collimates high-latitude outflows into structured bipolar jets capable of powering gamma-ray bursts.

Original authors: Dhruv K. Desai, Luciano Combi, Daniel M. Siegel, Brian D. Metzger

Published 2026-01-15

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CC BY 4.0

Original authors: Dhruv K. Desai, Luciano Combi, Daniel M. Siegel, Brian D. Metzger

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

Imagine a newborn neutron star as a cosmic newborn baby, but instead of crying, it spins incredibly fast and glows with intense heat. This paper explores what happens when these "millisecond proto-magnetars" are born, specifically asking: Can they launch a super-fast beam of energy (a jet) right from the start, or does the baby's own "sweat" (neutrinos) choke it off?

Here is the story of what the researchers found, using simple analogies.

The Problem: The "Sweaty" Baby

When a massive star collapses or two neutron stars smash together, they create a super-dense, super-hot neutron star.

The Heat: This new star is so hot it sweats out a massive amount of invisible particles called neutrinos.
The Choke: These neutrinos hit the gas right above the star's surface, heating it up and turning it into a thick, heavy wind. Think of this like a baby sweating so much that its clothes get soaked and heavy.
The Fear: Scientists worried that this heavy, "baryon-rich" wind would be too thick to let anything fast escape. They thought the star might only be able to push out a slow, heavy breeze, not the ultra-fast laser beam needed to create a Gamma-Ray Burst (GRB).

The Experiment: A Cosmic Simulation

The researchers built a super-complex 3D computer simulation (like a virtual universe) to watch what happens when these stars are born. They looked at stars that are:

Spinning incredibly fast (once every millisecond, like a spinning top).
Magnetic monsters (with magnetic fields trillions of times stronger than Earth's).

The Discovery: The "Ice Skater" Effect

The simulation revealed something surprising. The star doesn't just blow a uniform wind in all directions. Because it is spinning so fast, the physics changes based on where you are on the star.

1. The Equator: The Heavy Conveyor Belt
At the star's "equator" (the middle, like a belt), the centrifugal force (the same force that pushes you outward on a spinning carousel) is strongest.

What happens: The star flings out a massive amount of heavy, slow-moving gas here. It's like a thick, slow-moving conveyor belt of heavy mud.
The Result: This creates a dense, sub-relativistic wind (moving at about 10% the speed of light). This is the "sweaty" part that was feared.

2. The Poles: The Clean Tunnel
At the "poles" (the top and bottom of the star), the spinning force is much weaker.

What happens: Because the heavy gas is being flung out sideways at the equator, the path straight up and down remains relatively empty and clean.
The Result: The magnetic field lines here are not clogged with heavy gas. They act like a clean, narrow tunnel.

The Magic Trick: Self-Contained Jets

Here is the most important part of the discovery: The heavy wind at the equator actually helps create the fast jet at the poles.

Imagine a garden hose. If you squeeze the hose with your hand, the water shoots out faster and in a straighter line.

In this cosmic scenario, the heavy, slow wind at the equator acts like a pair of hands squeezing the hose.
It physically confines and pushes the magnetic field lines at the poles, forcing them into a tight, narrow beam.
Because this beam is so clean (low density) and so tightly squeezed, the magnetic energy can accelerate the gas to ultra-relativistic speeds (close to the speed of light).

The Result: Two Different Outflows

The star ends up launching two very different things at the same time:

The Jet (The Laser): A narrow, super-fast beam shooting out the top and bottom. This is fast enough to power a Gamma-Ray Burst (the "short" bursts seen in the sky).
The Wind (The Spray): A wide, slower, heavy cloud of gas shooting out the sides. This doesn't make a laser beam, but it carries a lot of energy that can power the explosion of the star (supernova) or the glowing debris cloud (kilonova).

Why This Matters

Before this study, scientists thought the "sweat" (neutrino heating) would stop these stars from making fast jets. This paper proves that multidimensional effects (spinning in 3D) save the day. The spinning creates a natural separation: the heavy stuff goes sideways, leaving a clean path for the fast stuff to shoot up.

In short: A rapidly spinning, super-magnetic newborn star can naturally launch a super-fast jet within seconds of its birth, without needing help from outside. The "heavy" wind at the bottom actually helps squeeze the "fast" jet at the top, solving a long-standing mystery about how these cosmic explosions happen.

Technical Summary: Relativistic Jets from Millisecond Proto-Magnetars

Problem Statement
Millisecond proto-magnetars—rapidly rotating ( $P \sim 1$ ms), strongly magnetized ( $B \gtrsim 10^{15}$ G) neutron stars formed in core-collapse supernovae, binary neutron star mergers, or white dwarf accretion-induced collapse (AIC)—have long been proposed as central engines for gamma-ray bursts (GRBs). A critical theoretical challenge remains: during the first few seconds after birth, these objects emit intense neutrino fluxes that heat the surface and drive baryon-rich winds. This "neutrino wind" loads the outflow with matter, potentially suppressing the magnetization ( $\sigma$ ) and limiting the achievable bulk Lorentz factor ( $\Gamma_\infty$ ). Previous semi-analytic models, calibrated to one-dimensional simulations and assuming angle-averaged split-monopole geometries, predicted $\sigma \lesssim 1$ at early times, suggesting proto-magnetars could not launch the ultra-relativistic jets ( $\Gamma \gtrsim 10-100$ ) required to power short-duration GRBs. The question of whether multidimensional effects could overcome this baryon loading to produce relativistic jets within seconds of formation had not been fully addressed.

Methodology
The authors present the first three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations of neutrino-heated proto-magnetar winds. The study utilizes a modified version of the open-source GRHydro code (part of the Einstein Toolkit) employing a finite-volume scheme with WENO-Z reconstruction and an approximate HLLE Riemann solver. Key numerical features include:

Neutrino Transport: A one-moment ( $M_0$ ) approximation of the general-relativistic Boltzmann equation is used to self-consistently model neutrino heating and cooling, including charged-current $\beta$ -processes, pair annihilation, and scattering.
Magnetic Field Evolution: The magnetic field is evolved via the electromagnetic 4-potential in the generalized Lorenz gauge using an upwind constraint transport scheme to maintain the solenoidal constraint to machine precision.
Initial Conditions: Models assume a $1.4 M_\odot$ neutron star with a central temperature of $20$ MeV. Simulations span a range of initial spin periods ( $P \approx 1 - 18$ ms) and polar surface magnetic field strengths ( $B_P \approx 10^{13} - 3 \times 10^{15}$ G).
Setup: The simulations employ reflection symmetry across the equatorial plane and utilize adaptive mesh refinement (Berger-Oliger) with the innermost grid resolving the proto-magnetar surface. Models are evolved first to a neutrino-driven hydrodynamic steady state before initializing a poloidal dipole magnetic field.

Key Contributions and Results
The simulations reveal that multidimensional effects fundamentally alter the wind structure compared to 1D or angle-averaged models, particularly for rapidly rotating ( $P \approx 1$ ms) and highly magnetized ( $B_P \approx 3 \times 10^{15}$ G) models:

Angular Stratification of the Outflow: The outflow is not uniform but divides into three distinct regions:
- Equatorial Belt ( $\theta \gtrsim 30^\circ$ ): Centrifugal forces significantly enhance mass loss near the rotational equator. This creates a dense, sub-relativistic wind ( $v \sim 0.1c$ ) with high baryon loading.
- Polar Jet ( $\theta \lesssim 10^\circ$ ): The polar field lines experience minimal centrifugal enhancement of mass loading. Consequently, this region remains "clean" (low baryon loading) and highly magnetized.
- Transition Region: Intermediate latitudes where the wind accelerates most efficiently.
Intrinsic Jet Collimation: The dense, sub-relativistic equatorial wind acts as a confining medium, naturally collimating the less baryon-loaded polar outflow. This self-collimation occurs at small radii ( $\lesssim 100$ km) without requiring confinement by external supernova or merger ejecta.
Achievement of Ultra-Relativistic Conditions: In the most rapidly spinning and magnetized models (e.g., model B3P1), the polar jet achieves peak magnetization values of $\sigma \sim 30 - 100$ . This is sufficient to reach bulk Lorentz factors $\Gamma_\infty \sim 100$ on larger scales. The jet half-opening angle is approximately $8^\circ$ , consistent with observational estimates for short and long GRBs.
Energy Partitioning: The total wind energy is partitioned between an ultra-relativistic polar jet and a sub-relativistic, wide-angle equatorial wind. The latter carries the bulk of the kinetic energy and mass, potentially powering the supernova or kilonova ejecta, while the former powers the GRB.

Significance and Claims
The paper claims to demonstrate that millisecond proto-magnetars can launch relativistic jets within seconds of their formation, resolving the long-standing tension between neutrino-driven baryon loading and the requirements for GRB engines. The authors argue that:

Multidimensionality is Essential: Baryon loading does not preclude relativistic jet formation when the latitudinal dependence of centrifugal forces and neutrino heating is accounted for.
Unified Engine Scenario: A single central engine can simultaneously power a short-duration GRB (via the polar jet) and the associated supernova/kilonova (via the equatorial wind), explaining the observed partition between beaming-corrected GRB energies and ejecta kinetic energies.
Temporal Consistency: The mechanism operates within the first few seconds of the proto-magnetar's life, consistent with the prompt emission phase of short GRBs.

The authors note that while their models assume ordered dipolar fields and fixed neutrino luminosities, the results suggest that the emergence of relativistic jets is a generic consequence of extreme rotation and magnetization in the earliest stages of proto-neutron star evolution. They acknowledge that future work must address self-consistent dynamo-generated magnetic fields, time-dependent cooling/spin-down, and the interaction with massive accretion disks surrounding merger remnants.