Researchers have now solved the mystery regarding the formation of magnetars.
The Westerlund 1 star cluster, located 16 000 light-years away in the southern constellation of Ara (the Altar), hosts one of the two dozen magnetars known in the Milky Way. It is called CXOU J164710.2-455216 and it has greatly puzzled astronomers.
Astronomers, including Simon Clark, lead author of the paper, proposed a solution to this mystery.
They suggested that the magnetar formed through the interactions of two very massive stars orbiting one another in a binary system so compact that it would fit within the orbit of the Earth around the Sun. But, up to now, no companion star was detected at the location of the magnetar in Westerlund 1, so astronomers used the VLT to search for it in other parts of the cluster.
They hunted for runaway stars - objects escaping the cluster at high velocities - that might have been kicked out of orbit by the supernova explosion that formed the magnetar. One star, known as Westerlund 1-5, was found to be doing just that.
This discovery allowed the astronomers to reconstruct the stellar life story that permitted the magnetar to form, in place of the expected black hole. In the first stage of this process, the more massive star of the pair begins to run out of fuel, transferring its outer layers to its less massive companion - which is destined to become the magnetar - causing it to rotate more and more quickly.
This rapid rotation appears to be the essential ingredient in the formation of the magnetar's ultra-strong magnetic field.
In the second stage, as a result of this mass transfer, the companion itself becomes so massive that it in turn sheds a large amount of its recently gained mass. Much of this mass is lost but some is passed back to the original star that we still see shining today as Westerlund 1-5.
