🔭 astrophysics

Magnetar fraction in Core-Collapse Supernovae

By analyzing the Galactic population of young and nearby isolated neutron stars through comprehensive population synthesis, this study concludes that magnetars constitute approximately 50% of all neutron stars born in core-collapse supernovae, a fraction significantly higher than previously inferred.

Original authors: Celsa Pardo-Araujo, Nanda Rea, Michele Ronchi, Vanessa Graber

Published 2026-01-23

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CC BY 4.0

Original authors: Celsa Pardo-Araujo, Nanda Rea, Michele Ronchi, Vanessa Graber

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 the universe as a giant cosmic factory. When massive stars run out of fuel, they explode in spectacular fireworks called supernovae. Usually, these explosions leave behind a tiny, incredibly dense city block-sized object called a neutron star. Think of a neutron star as a spinning top made of pure atomic matter, so heavy that a teaspoon of it would weigh a billion tons on Earth.

For a long time, scientists thought most of these spinning tops were "ordinary." They spin down slowly, powered by their rotation, like a regular wind-up toy. But there's a special, wilder cousin: the magnetar.

The Cosmic Wild Card

A magnetar is like a neutron star that got hit by a lightning bolt of pure magnetic energy. Its magnetic field is a trillion times stronger than a fridge magnet. Because of this super-power, magnetars don't just spin; they scream. They blast out huge bursts of X-rays and gamma rays, and they are suspected to be the engines behind some of the most energetic explosions in the universe, like Super Luminous Supernovae (stars that shine brighter than entire galaxies) and Fast Radio Bursts (mysterious cosmic radio signals).

The Great Mystery: How Many Are There?

The big question the paper tries to answer is: When a star explodes, how often does it create a wild magnetar instead of a boring ordinary neutron star?

Previously, scientists were guessing. Some thought it was rare (maybe 1 in 10), others thought it was common. The problem was that magnetars are tricky. They are hard to find because they are only "loud" (bright) for a short time before they quiet down. It's like trying to count how many people in a city are professional rock stars by only looking at the ones currently performing on stage. You might miss all the retired ones or the ones who haven't started yet.

The New Detective Work

The authors of this paper decided to play detective using a "population synthesis" simulation. Imagine they built a giant, virtual Milky Way galaxy inside a computer.

They created a virtual universe: They simulated thousands of stars exploding, giving birth to neutron stars with different "personalities" (some with weak magnetic fields, some with super-strong ones).
They tracked time: They let these virtual stars age, spin down, and evolve, just like real stars do.
They compared notes: They took their virtual list of stars and compared it to the real list of young neutron stars we have actually found in our galaxy (specifically, the ones younger than 2,000 years).

The Big Discovery

Here is what they found:

The "Young" Crowd: When they looked at the very young neutron stars (the "babies" of the galaxy), they found something surprising. Out of 24 young stars they could see, only about 10 were the "ordinary" spinning tops. The other 14 were either magnetars or a related type called Central Compact Objects (which are likely magnetars that got their magnetic fields partially buried at birth).
The Ratio: This means that in the "baby" stage, magnetars make up about 50% of the population.
The Birth Rate: To explain why we see so many of them, the paper concludes that for every 100 core-collapse supernovae, about 50 must result in a magnetar.

The "Bimodal" Analogy

The paper suggests that when a star is born, it doesn't get a random magnetic field strength. Instead, it's like a coin toss with two distinct sides:

Side A: A "Normal" neutron star with a moderate magnetic field.
Side B: A "Magnetar" with a super-strong magnetic field.

The authors found that the "Magnetar" side of the coin comes up about half the time.

Why This Matters

If magnetars are born as often as this paper suggests (about half the time), it changes how we understand the universe. It means that the "wild" objects responsible for the most powerful explosions and radio signals in the cosmos are much more common than we thought. It's like realizing that the rare, explosive fireworks we see in the sky aren't a fluke; they are actually a standard part of the show, happening every time a massive star dies.

In short: The paper uses computer simulations to show that magnetars aren't rare oddities. They are likely born in about 50% of all core-collapse supernovae, making them a major player in the life and death of stars in our galaxy.

Technical Summary: Magnetar Fraction in Core-Collapse Supernovae

Problem Statement
Magnetars, neutron stars powered by ultra-strong magnetic fields ( $\sim 10^{14}$ Gauss), are hypothesized to be the central engines for various extragalactic transients, including Super Luminous Supernovae (SL-SNe), Gamma-Ray Bursts (GRBs), and Fast Radio Bursts (FRBs). However, the formation rate of magnetars relative to ordinary neutron stars remains uncertain. Previous attempts to quantify this fraction have faced significant limitations:

Extragalactic Rate Constraints: Methods relying on the volumetric rate of Giant Flares are limited by small sample sizes, difficulty distinguishing events from short GRBs, and assumptions regarding collimation and luminosity functions.
Population Synthesis Biases: Traditional pulsar population synthesis models struggle with magnetars because they are not discovered via uniform surveys. Young magnetars are bright, while older systems are typically identified only during outbursts, creating an unconstrained selection bias. Furthermore, previous models often failed to self-consistently incorporate magneto-thermal evolution and the evolutionary links between magnetars and other neutron star classes (e.g., X-ray Dim Isolated Neutron Stars [XDINSs] and Central Compact Objects [CCOs]).

Methodology
The authors employ a comprehensive population synthesis approach using the ML-Poppyns code to simulate the entire isolated neutron star population in the Milky Way. The study integrates three key components:

Observational Census: The authors compile a sample of 24 young ( $<2$ kyr) neutron stars in the Galaxy, categorized into Rotation-Powered Pulsars (RPPs), Magnetars, and CCOs. They also analyze a volume-limited sample of nearby ( $<500$ pc) XDINSs.
Dynamical and Magneto-Rotational Evolution: Simulations evolve neutron stars from birth up to $3 \times 10^7$ $3 \times 1 0^{7}$ years. The models incorporate:
- Dynamical Evolution: Natal kicks sampled from a log-normal distribution and spatial evolution within the Galactic potential, accounting for spiral arm structures.
- Magneto-Rotational Evolution: Spin-down driven by magnetospheric torques and magnetic field decay driven by Hall and Ohmic effects in the crust.
- X-ray Emission: Thermal emission is computed using magneto-thermal simulations, including resonant cyclotron scattering to model non-thermal tails and interstellar absorption.
Statistical Inference: The study assumes a bimodal initial magnetic field ( $B_0$ ) distribution at birth: one log-normal component for RPPs and a second for magnetars. The normalization of the magnetar component is the primary free parameter. The authors perform 1,000 simulations for various combinations of Core-Collapse Supernova (CC-SN) rates (1, 2, or 3 per century) and initial magnetar field strengths ( $7.5 \times 10^{13}$ to $10^{15}$ G). They compare simulated distributions of period ( $P$ ), period derivative ( $\dot{P}$ ), and absorbed X-ray flux ( $S_{X,abs}$ ) against observed data using Kolmogorov–Smirnov (K-S) tests.

Key Results

CC-SN Rate Constraint: The existence of 24 observed young ( $<2$ kyr) neutron stars implies a Galactic CC-SN rate exceeding one per century. Independent volumetric scaling of supernova remnants (SNRs) within 2 kpc yields a lower limit of $>2$ per century, consistent with recent estimates of $1.63 \pm 0.46$ . The authors find that a rate of 3 CC-SNe per century is required to reproduce the observed population statistics.
Magnetar Fraction: Under the assumption of a CC-SN rate of 3 per century, the observed population of young neutron stars and nearby XDINSs can only be reproduced if magnetars constitute a significant fraction of the birth population.
- The inferred magnetar birth fraction ranges from 40% to 70% for initial fields peaking at $B_0 \sim 1 \times 10^{14}$ G.
- For fields peaking at $B_0 \sim 2.5 \times 10^{14}$ G, the fraction ranges from 30% to 50%.
- Averaging over the plausible initial magnetic field distributions, the authors conclude that magnetars represent $\sim 50\%$ of the entire neutron star population born in core-collapse supernovae.
Initial Magnetic Field: The simulations indicate that the initial magnetic field distribution for magnetars must peak between $1 \times 10^{14}$ G and $2.5 \times 10^{14}$ G to simultaneously match the observed properties of young magnetars (including CCOs) and the older XDINS population.

Significance and Claims
The paper claims that by self-consistently modeling the evolutionary connections between different neutron star classes and incorporating magneto-thermal physics, it derives robust constraints on the magnetar birth fraction. The finding that magnetars may constitute $\sim 50\%$ of all core-collapse remnants challenges previous estimates (which ranged from 6–40%) and suggests that the "magnetar channel" is a dominant evolutionary outcome for massive stars. This high birth fraction provides strong support for models positing magnetars as the central engines for a large percentage of extragalactic high-energy and radio transients. The authors emphasize that their results rely on the assumption that CCOs are magnetically powered objects that experienced fallback accretion, a hypothesis consistent with their evolutionary framework.