Half of the neutron stars could be magnetars

Magnetars are neutron stars with extremely strong magnetic fields. Until now, studies suggested they were relatively rare. Now, a study led by researachers from the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) at the Institute of Space Sciences (ICE-CSIC) and published in Nature Astronomy reveals that magnetars could make up around 50% of the neutron star population.

This work presents the first population synthesis study—a computational technique that simulates the evolution of neutron stars from their birth to the current time—that simultaneously models within a single framework the main types of isolated neutron stars: radio pulsars, magnetars, central compact objects and X-ray isolated neutron stars.

In the Milky Way alone, there are approximately one hundred billion stars, and about one hundred million of them (1%) are neutron stars, remnants of supernova explosions. Magnetars are prime candidates for powering some of the most energetic transient events in the Universe, such as superluminous supernovae, gamma-ray bursts, and fast radio bursts. However, the proportion of magnetars that form relative to the total number of neutron stars with regular magnetic fields is still unknown. Determining this fraction would allow us to assess whether the magnetar population is abundant enough to explain these events.

According to recent studies, evolutionary links exist between magnetars and other types of neutron stars. Until now, studies assumed that magnetars were relatively rare, making it difficult to reconcile their formation rate with the observed rates of these transient objects. Furthermore, they did not account for the possible evolutionary relationships between different types of neutron stars. “Therefore, it is essential to model, in a unified way, the different types of isolated neutron stars and their possible evolutionary connections, which allows for a consistent estimation of the fraction and number of magnetars that form,” says Celsa Pardo Araujo, first author of the article and IEEC predoctoral researcher at ICE-CSIC.

The team combines the study of galactic dynamics (how stars are distributed and move within galaxies), the rotational evolution of neutron stars (how they slow down over time), and their magneto-thermal evolution (how their temperature and magnetic field change throughout their lives)—along with biases arising from astronomical observations—to simultaneously infer both the fraction of magnetars and the number of supernova explosions in our galaxy.

These types of supernovae occur when very massive stars, between approximately 8 and 25 solar masses, die. They exhaust the fuel that makes them shine and, consequently, collapse due to gravitational attraction, generating a neutron star. Reproducing these observations requires a core-collapse supernova rate exceeding two per century in the observed population; in other words, the models can only account for the number of detected neutron stars if more than two such explosions occur in the Milky Way per century. Knowing this rate is fundamental to estimate the total number of neutron stars expected to form in the galaxy.

The team obtained this rate using two independent methods: first, from observed supernova remnants, and second, from the number of neutron stars detected in the galaxy. Based on this rate, the study suggests that magnetars would represent, on average, around 50% of the neutron star population. This result suggests that magnetars are much more common than previous studies had indicated so far.

“If magnetars are indeed this common, they could naturally account for a significant fraction of the most energetic transients in the Universe, including Fast Radio Bursts, Gamma-Ray Bursts and superluminous supernovae”, points out Pardo Araujo.  

“A natural extension of this work would be to test these results in an extragalactic context”, says Nanda Rea, IEEC researcher at ICE-CSIC. In particular, the population-synthesis framework developed in this work could be applied to other galaxies in order to compare the expected magnetar population with the observed rates of magnetar-powered transients, such as superluminous supernovae, fast radio bursts, and gamma-ray bursts.

Such comparisons could confirm the strength of the magnetar central-engine hypothesis behind the existence of transient objects. However, this approach requires a detailed characterization of the star formation history of each galaxy in order to accurately estimate its core-collapse supernova rate, which might differ from the Milky Way’s.