Magnetars, long considered rare celestial objects, could account for about half of the isolated neutron stars in our Galaxy.
A neutron star can form after a massive star explodes. Its core collapses, forming an extremely dense sphere about twenty kilometers in diameter. Some have a particularly powerful magnetic field: these are magnetars. Their field can reach around one hundred trillion gauss, billions of times stronger than Earth's.
Artist's impression of a magnetar, a neutron star with an extremely intense magnetic field.
Credit: ESA
But how many magnetars are actually born? It is impossible to simply count those we know about. Some are much easier to detect than others, and their appearance changes with age. Researchers therefore simulated entire populations of neutron stars on computers, then compared the results with observations made in the Milky Way.
During the lives of these objects, their rotation slows, their temperature drops, and their magnetic field evolves. They also move through the Galaxy. The researchers reproduced these changes, testing different proportions of magnetars at birth. They then looked for the scenarios that best matched the neutron stars actually detected.
The youngest objects provide a valuable clue here. The study considers 23 that are less than 2,000 years old. Only nine are classical pulsars, neutron stars that regularly emit signals because of their rotation. The others are magnetars or neutron stars belonging to another category, observed at the centers of the remnants of stellar explosions.
The result indicates that magnetars may be far more common than expected. According to the simulations, about one in every two isolated neutron stars could be a magnetar. The calculations also indicate that more than two massive stars would explode every century in the Milky Way.
This estimate is of interest to astronomers studying some of the Universe's most energetic explosions. Magnetars could be behind certain gamma-ray bursts, fast radio bursts, and exceptionally bright stellar explosions. Knowing their number therefore makes it possible to determine whether they are common enough to explain some of these events.
This 50% proportion nevertheless depends on the assumptions used in the simulations and the small number of known objects. The researchers now want to compare their estimate with the frequency of violent phenomena observed in other galaxies. To do so, they will notably need to determine how many stars have formed in each of these galaxies over time.