Magnetars are a rare type of neutron star with an extremely strong magnetic field, and astronomers have been stumped on how these objects form. New observations of a magnetar 16,000 light-years from Earth may have solved the mystery, suggesting that magnetars may need to be part of a binary system and influenced by a companion in order to form.

Artist's concept of the magnetar located in Westerlund 1. ESO/L. Calçada

Led by astronomer Simon Clark from the Open University, the team of researchers used the European Southern Observatory’s Very Large Telescope to observe CXOU J164710.2-455216, a magnetar located in the Westerlund 1 star cluster, a tight group of young massive stars, in the constellation Ara.

CXOU J164710.2-455216 is what was left following the supernova of a star with a mass 40 times that of the sun, says Clark. When a star reaches the end of its life and goes supernova, what’s left is a neutron star, an incredibly dense star, or a black hole. According to Clark, the remnant of the supernova of such a massive star should have been a black hole, but that’s not the case with CXOU J164710.2-455216. The next step for scientists was to explain why a magnetar was formed and not a black hole, and they got an answer after discovering a “runaway” star in the Westerlund 1 cluster.

The star, Westerlund 1-5, was traveling at high speed, compared to other stars in the area, away from the cluster. The researchers believe Westerlund 1-5 was traveling so fast as a result of the supernova responsible for the magnetar. They say Westerlund 1-5 began shedding its outer layers as it began to run out of the necessary fuel for nuclear fusion. This material was transferred to its companion star in the binary system, the star that became the magnetar, causing it to spin faster; the fast rotation is a key component responsible for the strong magnetic field. As the star gains more mass it too begins to shed its outer layers before going supernova. The star lost enough mass to prevent it from becoming a black hole, which was the second important component necessary to form a magnetar.

Westerlund 1 Star Cluster
An image of the Westerlund 1 star cluster with the approximate locations of the magnetar and Westerlund 1-5 highlighted. ESO

Francisco Najarro, from the Centro de Astrobiologia in Spain, participated in the research and said in a statement, "It is this process of swapping material that has imparted the unique chemical signature to Westerlund 1-5 and allowed the mass of its companion to shrink to low enough levels that a magnetar was born instead of a black hole — a game of stellar pass-the-parcel with cosmic consequences!"

Additional observations of other magnetars, which would include searching for a possible companion star, could help confirm the solution proposed by Clark and his team. The research was published in the journal Astronomy and Astrophysics.