Magnetars are a type of neutron star which, as their name suggests, have very powerful magnetic fields. The defining feature of these stars has defied explanation, but a team of astronomers think they have the answer by solving an even older problem, the source of magnetism in some massive main-sequence stars. Collisions between stars create magnetic fields, they argue, which survive the supernova explosion and intensify in the transformation to neutron star status.
Even ordinary neutron stars involve physics of the extreme, denser than an atomic nucleus with a cupful containing as much mass as Mount Everest.
However, from 1979 onwards astronomers started detecting behavior they initially could not explain, such as soft gamma-ray repeaters, which erratically release radiation less energetic than other gamma-ray bursts, and anomalous X-ray pulsars. In 1992 it was suggested that what was being observed would make sense if some neutron stars were both hypermagnetic and rotated particularly fast, leaving open the question of where the magnetism comes from.
Dr Fabian Schneider of Heidelberg University, Germany moved the question back a step. Like other neutron stars, magnetars are a consequence of supernova explosions in stars 10-29 times the mass of the Sun. Schneider said in a statement: “We know that the Sun has a turbulent envelope in which its magnetic field is continuously generated. But more massive stars do not have such an envelope. Still, about 10 percent have a strong, large-scale surface magnetic field whose origin has eluded us since their discovery in 1947.”
In Nature Schneider and colleagues describe advanced computer modeling of the stellar merger thought to have formed Tau Scorpii, a very magnetic star with 15 times the Sun's mass. They showed the turbulence and shear forces produced in the collision lead to powerful magnetic fields.
The majority of the galaxy's stars exist in binary systems, and sometimes their orbits decay until they combine, producing one larger star. Around 10 percent of the galaxy's very massive stars have the unusually youthful appearance such mergers produce, which fits well with the highly magnetic proportion.
The idea has been around for a while, but Schneider and co-authors provided the computational methods needed to model such a complex event, showing the theory is credible.
Pre-supernova, stars like Tau Scorpii are larger than the Sun, afterward, they are the size of a city but retain much of the mass. The shrinkage intensifies the magnetic field, leading to something a hundred million times stronger than anything humans have manufactured.
Magnetars only maintain their intense fields for 10,000s of thousands of years – a blink of an eye in a star's life. Consequently, there are many more ex-magnetars in the universe than current ones, but the immense bursts of gamma and X-rays they release when their magnetic field is briefly interrupted means we notice the few there are. Other neutron stars are magnetic, but dramatically less so.