Making Magnetars the Universe’s most powerful magnets

Magnetars — neutron stars with the most powerful magnetic field in the Universe — could form from the collision of other stars and a subsequent supernova explosion suggests new research.

The simulation marks the birth of a magnetic star such as Tau Scorpii. The image is a cut through the orbital plane where the colouring indicates the strength of the magnetic field and the light hatching reflects the direction of the magnetic field line. ( Image: Ohlmann/Schneider/Röpke)

How exactly do some neutron stars become the source for the most powerful magnetic fields in the Universe — magnetars?

That’s the question that a team of British and German researchers — from Heidelberg University, the Max Planck Society, the Heidelberg Institute for Theoretical Studies, and the University of Oxford — believe they may have answered.

The team used vast computer simulations to show how the collision of two stars could create massive stars with such strong magnetic fields, and — if they are to explode in a supernova — a magnetar.

Magnetic fields are ubiquitous throughout the Universe. For instance, stars similar to our Sun have envelopes within which, convection generates magnetic fields. But, the situation is different for larger stars.

“Even though massive stars have no such envelopes, we still observe a strong, large-scale magnetic field at the surface of about 10% of them,” explains Dr Fabian Schneider from the Centre for Astronomy of Heidelberg University, who is the first author of the study, published in the journal Nature.

Such fields have been known to exist for seven decades, but what causes them has thus far eluded astronomers and astrophysicists. Despite this uncertainty, scientists have suggested that magnetars with strong magnetic fields could be produced when two stars collide.

The simulation marks the birth of a magnetic star such as Tau Scorpii. The image is a cut through the orbital plane where the colouring indicates the strength of the magnetic field and the light hatching reflects the direction of the magnetic field line. ( Image: Ohlmann/Schneider/Röpke)

“Until now, we weren’t able to test this hypothesis because we didn’t have the necessary computational tools,” says Dr Sebastian Ohlmann from the computing centre of the Max Planck Society.

With the aid of a dynamic simulation known as the AREPO code running on clusters of computers at the Heidelberg Institute for Theoretical Studies (HITS), the team was able to explain the properties of Tau Scorpii — a massive magnetic star 500 light-years from Earth.

As well as being a massive magnetic star, Tau Scorpii is a ‘blue straggler’— a product of merged stars — its status confirmed in 2016 by Fabian Schneider and Philipp Podsiadlowski from the University of Oxford. The latter confirms the origins of its magnetic field: “We assume that Tau Scorpii obtained its strong magnetic field during the merger process.”

The simulation the team ran did indeed confirm that the strong turbulence during the merger of two stars could indeed produce a magnetic field of such magnitude. These mergers are relatively common — with astronomers believing that 10% of stars in the Milky Way are a result of such effects. Numbers that seem to reflect the rate at which magnetic massive stars are observed to occur.

If these massive stars then go supernova — that is when a magnetar is born. This could be the fate that awaits Tau Scorpii at the end of its life. Indeed, the team’s simulations suggest that the magnetic field generated would be sufficient to explain the exceptionally strong magnetic fields in magnetars.