Have Experts Finally Solved The Mystery Of Magnetars?

May 14, 2014
Image Caption: Artist’s impression of the magnetar in the star cluster Westerlund 1. Credit: ESO/L. Calçada

John P. Millis, Ph.D. for redOrbit.com – Your Universe Online

When a massive star reaches the end of its life, it explodes in a brilliant supernova explosion. The remnant of the stellar core will usually form either a neutron star or a black hole.

Occasionally, however, there emerges a third option: a magnetar. These exotic objects, share a lot in common with neutron stars. They are incredibly dense – second only to black holes in that regard – and spin very rapidly, though not quite as fast as their neutron star brethren. Finally, like neutron stars, they possess incredibly powerful magnetic fields, hundreds of millions of times stronger than anything man-made. In fact, magnetar fields are about a 100 times stronger than even neutron star fields, making them the most powerful fields in the Universe.

While they certainly share a lot in common with neutron stars, the mystery of their formation has remained elusive. There is the obvious link with neutron stars, but what is special about the progenitor stars that leads to the formation of magnetars?

To find the answer, a team of European astronomers turned the ESO’s Very Large Telescope (VLT) to the nearby Westerlund 1 star cluster, some 16,000 light-years away in the constellation Ara. Here lies the magnetar CXOU J164710.2-455216, one of only 21 confirmed magnetars — though some 30 million are estimated to exist in the galaxy. The astronomers, led by Simon Clark from Open University, have studied this object previously, but the results of that work brought more questions than answers.

“In our earlier work we showed that the magnetar in the cluster Westerlund 1 must have been born in the explosive death of a star about 40 times as massive as the Sun. But this presents its own problem, since stars this massive are expected to collapse to form black holes after their deaths, not neutron stars. We did not understand how it could have become a magnetar,” says Clark, lead author of the paper reporting these results.

So scientists proposed a solution. What if two massive stars were closely orbiting each other, closer than the Earth is to the Sun? A second star might prevent the other from collapsing into a black hole during the supernova event.

At first, the more massive star of the two begins to exhaust the nuclear fuel in its core. The outward radiation pressure then ejects some of its outer layers, which is then gravitationally accreted onto the less massive companion — which is destined to become the magnetar. This additional mass causes the star to rotate more rapidly, which astronomers believe is an essential component in the formation of the magnetar’s ultra-strong magnetic field.

Next, the magnetar progenitor itself becomes so massive that the fusion rate is accelerated in its core, leading it to also push its outer layers out into the interstellar medium. Some of this mass is ultimately accreted back on to the original star.

These two stars are now primed to enter the next phase in their evolution, as the pump has been primed for the supernova explosion that will ultimately become a magnetar, thanks to the added angular momentum contributed by the material of its companion star. The problem, of course, is that no companion had been found in one of these systems.

This wasn’t a surprise, as the companion star would have likely been ejected from the system by the incredible power of the supernova, so it would have been difficult to find, and may not be anywhere close to the magnetar anymore. Therefore, the team used the VLT to search for high velocity stars emerging from Westerlund 1 on unusual paths.

After searching various regions of the cluster, the team found Westerlund 1-5. “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,” adds Ben Ritchie also from Open University, a coauthor on the paper.

Moreover, this star sweeping across the constellation has a particular chemical composition that would only arise from passing matter back and forth between itself and its companion. The team could then use this to tie the star back to the magnetar – a kind of astronomical genetics if you will.

“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!” concludes team member Francisco Najarro from the Centro de Astrobiología, Spain.

This is the first such study to tie a star back to its magnetar companion, and will hopefully open doors to further exploring these dynamic objects. The discovery of Westerlund 1-5 will hopefully be the latest step in answering the 35-year old question of how magnetars form.

Source: John P. Millis, Ph.D. for redOrbit.com - Your Universe Online

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