Magnetars May Be Born in Half of All Core-Collapse Supernovae

Half of all neutron stars born in core-collapse supernovae may be magnetars, according to a new population study published in Nature Astronomy. The finding, based on a sophisticated synthesis of the young Galactic neutron star population, suggests that these ultra-magnetically endowed stellar corpses are far more common than astronomers had inferred, and it forces a recalibration of how often the most luminous explosions and flashes in the universe should occur.

Magnetars are neutron stars endowed with magnetic fields exceeding roughly 10^14 gauss, a quadrillion times stronger than Earth’s field and hundreds to thousands of times more intense than the fields of ordinary radio pulsars. Their magnetic energy reservoirs power some of the most spectacular transients known: superluminous supernovae, whose light curves are widely modeled as magnetar-driven; long gamma-ray bursts, for which millisecond protomagnetars have been proposed as central engines; and at least some fast radio bursts, the millisecond-duration radio flashes whose repeaters have been firmly linked to a Galactic magnetar. Despite this central role in high-energy astrophysics, the rate at which magnetars actually form relative to ordinary neutron stars has remained stubbornly uncertain, preventing direct, quantitative comparisons between magnetar birth rates and the observed rates of these cosmic beacons.

The new study, led by Celsa Pardo-Araujo of the Institute of Space Sciences (ICE, CSIC) and the Institute of Space Studies of Catalonia, with Nanda Rea, Michele Ronchi and Vanessa Graber, attacks the problem from an unexpected direction: by taking a complete census of the very youngest neutron stars in the Milky Way. The team focused on isolated neutron stars younger than two thousand years, a sample in which observational incompleteness is minimized and each object retains the magnetic and thermal fingerprints of its birth. In this young Galactic population, only nine of the twenty-three detected sources, or about forty percent, are classical rotation-powered pulsars, the lighthouse-like radio beacons that have historically dominated neutron star catalogs. The remaining sixty percent are magnetars or so-called central compact objects, dim X-ray sources embedded in young supernova remnants whose surfaces show puzzling thermal properties.

To translate this snapshot of the youngest population into a birth fraction, the researchers constructed a population synthesis of isolated neutron star classes in the Galaxy using the ML-Poppyns code, a framework previously refined with simulation-based inference techniques for radio pulsar populations. The synthesis tracks each simulated star from birth through its full dynamical trajectory through the Galactic gravitational potential, incorporating natal kicks imparted by the asymmetric supernova explosion, realistic spatial distributions of neutron star birth sites traced by spiral arms and the Galactic disk, and selection effects that determine which sources would actually be detected by X-ray and radio surveys. Crucially, the code also follows each star’s physical evolution: its spin-down under magnetospheric torques computed from force-free magnetosphere models, and its magneto-thermal evolution, in which the coupled decay of the crustal and core magnetic field through Ohmic dissipation, Hall cascade and ambipolar diffusion simultaneously heats the star and reshapes its X-ray luminosity over millions of years.

A key ingredient is the assumed distribution of initial magnetic fields. The team adopted a bimodal model, a double log-normal distribution in which one component, centered near 10^13 gauss, corresponds to the radio pulsar population inferred from earlier pulsar population synthesis, while a second, broader component peaks at initial dipole fields of approximately 1 to 2.5 multiplied by 10^14 gauss and corresponds to magnetars. This bimodality reflects a growing realization that magnetars are not merely the extreme tail of a single continuous field distribution but may form through distinct physical channels, whether from exceptionally strong internal fossil fields inherited from their massive progenitor stars or from convective dynamo action in the first seconds after collapse. The relative normalization of the two components directly encodes the magnetar birth fraction, which the team varied to test which values reproduce the observed census.

The result is striking: magnetars peaking at those super-strong initial fields represent on average about fifty percent of the isolated neutron star population, roughly double the fraction of classical rotation-powered pulsars among the youngest detected sources and substantially larger than magnetar fractions of around ten percent or less previously inferred from earlier studies. When the authors compared simulated population counts of magnetars, rotation-powered neutron stars and X-ray-dim isolated neutron stars against the observed numbers for a grid of magnetar birth fractions, initial magnetar field strengths and supernova rates, only models with high magnetar fractions could simultaneously match all three classes. The best-fitting models also placed constraints on the environment in which these stars are born: reproducing the observed population requires a Galactic core-collapse supernova rate larger than two events per century.

That rate requirement is itself provocative. Traditional estimates of the Milky Way’s core-collapse supernova rate, often derived from observations of extragalactic supernovae scaled to the Galactic star formation rate, hover near one to two per century, and radio and gamma-ray surveys of supernova remnants have historically suggested lower values of a few tenths per century. The new analysis, calibrated against the census of supernova remnants with robust neutron star associations younger than ten thousand years within two kiloparsecs of the Sun, indicates that the true Galactic explosion rate must be higher than many of those inferences. A higher supernova rate simultaneously alleviates the tension of hosting so many very young, strongly magnetized neutron stars and implies that the Galaxy’s stellar graveyards are being restocked faster than conventional counts suggest.

The implications ripple outward to extragalactic transient astronomy. Magnetar-powered models are the leading interpretation for superluminous supernovae, whose peak luminosities can exceed ordinary supernovae by factors of a hundred, and for the extended emission of many gamma-ray bursts. If half of all neutron stars are born as magnetars, the raw supply of potential engines for these events is far larger than previously assumed, sharpening the question of why only a small subset of magnetars apparently powers such extreme displays. The elevated birth fraction also eases rate-budget tensions for fast radio burst populations, where the volumetric rate of bursts must be reconciled with the birth rate of their proposed progenitors. More broadly, recent work has shown that magnetars are evolutionarily intertwined with other neutron star classes: some central compact objects show magnetar-like outbursts, some young pulsars display magnetar-like flares, and transitional objects such as the fast-spinning magnetar Swift J1818.0-1607 blur the line between the classes. A population synthesis that treats all isolated neutron stars within a single magneto-thermal evolutionary framework, as this study does, is precisely the tool needed to capture those connections.

The team drew its observational backbone from publicly archived catalogues, including the ATNF Pulsar Catalog, the McGill Online Magnetar Catalog and comprehensive supernova remnant catalogues, with thermal luminosities for the seven X-ray-dim isolated neutron stars taken from the literature. The authors note that all observational data are publicly available through NASA and ESA archives, and the ML-Poppyns population-synthesis code has been released on GitHub, allowing the community to test and extend the results. As time-domain surveys such as the Vera C. Rubin Observatory begin to catch superluminous supernovae and fast radio bursts in unprecedented numbers, an accurate magnetar birth fraction becomes an essential ingredient for every population-level comparison. This study’s message is unambiguous: magnetars are not exotic outliers but a mainstream product of stellar death, and the Milky Way has been quietly forging them at a pace that our models of stellar collapse are only now catching up to.

The study’s reliance on a bimodal field distribution carries an important caveat: the inferred fraction depends on where the magnetar component peaks in field strength. Models with lower peak fields of roughly 10^14 gauss can accommodate somewhat different normalizations, which is why the authors report the result as an average across the plausible range rather than a single definitive number. This sensitivity mirrors a broader debate in the literature, where formation-rate estimates have historically ranged from a few percent to tens of percent depending on which observational class was used as a tracer.

The treatment of central compact objects deserves particular attention. These sources, found exclusively within young supernova remnants, have long resisted classification, and evidence that at least some of them harbor strong internal fields, including magnetar-like outbursts from otherwise quiescent objects, supports the paper’s decision to model them within the same magneto-thermal framework as magnetars. If a substantial share of central compact objects are hidden or incipient magnetars whose dipole fields have not yet surfaced, the effective magnetar fraction rises accordingly.

Independent constraints may soon emerge from magnetar giant flares, the rare gamma-ray flashes whose extragalactic detection rates provide a complementary census of the magnetar population. Combining flare statistics with the elevated birth fraction reported here will test whether the flaring subset is a small, biased sample or representative of the class. Similarly, the growing catalog of fast radio burst repeaters, now anchored by the Galactic magnetar SGR 1935+2154, offers an empirical cross-check that future population models can exploit.

Methodologically, the work illustrates the value of simulation-based inference in stellar population studies, replacing ad hoc forward modeling with a statistical comparison across parameter space. Extending the framework to binary neutron stars and to extragalactic populations with different metallicities and star formation histories would be a natural next step.

Subject of Research: The birth fraction of magnetars among neutron stars formed in core-collapse supernovae, constrained by population synthesis of the young Galactic neutron star population

Article Title: The magnetar fraction in core-collapse supernovae

Article References: Pardo-Araujo, C., Rea, N., Ronchi, M., & Graber, V. (2026). The magnetar fraction in core-collapse supernovae. Nature Astronomy. https://doi.org/10.1038/s41550-026-02915-5

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02915-5

Keywords: magnetars, neutron stars, core-collapse supernovae, population synthesis, magneto-thermal evolution, fast radio bursts, superluminous supernovae, gamma-ray bursts, Galactic supernova rate, central compact objects, high-energy astrophysics, Nature Astronomy