Magnetars are an extreme type of neutron star, having the strongest magnetic fields in the Universe — up to 1011 tesla. In particular, the magnetar in the Westerlund 1 star cluster of the Milky Way is a remnant of a progenitor that might have been forty times more massive than the Sun. So, why did it not collapse into a black hole following the supernova?
Simon Clark and colleagues believe the magnetar was part of a binary system, and they have found the companion star: Westerlund 1-5, a runaway star with an unusual chemical composition. This was the more massive star, but it transferred its outer layers to the magnetar progenitor as it ran out of fuel, causing the progenitor to rotate faster.
The spin-up process could be a crucial factor in creating such powerful magnetic fields. At some point, the progenitor became so massive that it started to shed mass, some of which was transferred back to the companion, thus explaining its composition. Having lost that mass, the progenitor avoided becoming a black hole.
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Chiao, M. Runaway star. Nature Phys 10, 408 (2014). https://doi.org/10.1038/nphys2993
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DOI: https://doi.org/10.1038/nphys2993
