Magnetars are extremely dense and highly magnetic neutron stars that can form when a star goes supernova. They are extremely rare, and until now, it has been difficult to determine how and why they form. However, thanks to new data collected by the Very Large Telescope (VLT) at the ESO’s Paranal Observatory in Chile, astronomers believe they have finally solved the great mystery.
A magnetar is a rare type of neutron star, and one that exhibits an extremely powerful magnetic field, the strongest in the known Universe. Not only do they possess strong magnetic fields, but, like all neutron stars, they are both very small and incredibly dense, to the point that a single teaspoon of a neutron star matter would have the mass of around a billion tonnes. They form when massive stars collapse under the weight of their own gravity.
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There are more than two dozen magnetars in the Milky Way, but the one studied by the VLT, is located in the Westerlund 1 star cluster in the southern constellation of Ara, some 16,000 light-years away. The star from which it formed is thought to have been around 40 times larger than our Sun. Stars of that size would be expected to form a black hole when they collapse, a more common final state for a dying star.
The fact that this star, which is catchily known as CXOU J1664710.2-455216, became a magnetar upon its collapse, is something that has puzzled astronomers for years. There was however an idea as to why the unusual event occurred. It was proposed in 2010 that the single star was actually formed through the interaction of two massive stars orbiting one another, so close that they would actually fit within the orbit of the Earth around the Sun.
Until now, astronomers were unable to detect the second star in the proposed binary star model. However, using the VLT, a team of astronomers was able to study the star cluster, looking for objects moving out of the area at high speeds, known as runaway stars. The theory was that the wayward objects may have been thrown out of their orbits by the large supernova explosion that created the rare magnetar. One such star was found, known as Westerlund 1-5.
Ben Ritchie, author of the paper, commented on the significance of the runaway star, stating “Not only does this star have the high velocity expected if it is recoiling from a supernova explosion, but the combination of its low mass, high luminosity and carbon-rich composition appear impossible to replicate in a single star - a smoking gun that shows it must have originally formed with a binary companion.”
Using the data from Westerlund 1-5, the team was able to construct a model for how the magnetar formed, theorizing that the larger binary stars’ fuel supply dwindled before its sibling, causing it to transfer its outer layers to the smaller object. This caused the smaller star to rotate at increasing speeds, something now thought to be a key ingredient in the formation of a magnetar’s strong magnetic field.
The smaller star then grew larger than its binary companion to the point that the process reversed and it began passing its outer layers back to the once larger star. Team member Francisco Najarro describes this process as a "game of stellar pass-the-parcel with cosmic consequences."
The initially larger of the two stars is then ejected when its smaller, then rapidly spinning companion, goes supernova. In this case, the runaway Westerlund 1-5 star is thought to be the second star in the binary pair, hence it carrying the blueprint of the process.
In conclusion, from the data gathered in relation to the magnetar in the Westerlund 1 star cluster, it’s now thought that the rapid rotation and transfer of mass between binary stars is key in the formation of the rare neutron stars known as magnetars. Mystery (provisionally) solved.