Double trouble: the birth of the magnetar

Magnetars are bizarre and exotic stellar remnants formed by the gravitational collapse of massive stars that are tens of times more massive than our Sun. The strongest magnets in the Universe, these objects are a rare variant of the neutron star, which are the relics of supernova explosions.

Artist’s impression of a magnetar in a rich, young star cluster. The lines represent the very strong magnetic field of the magnetar. (Credit: ESO/L. Calçada)

Depending on its initial mass, a star will end its live as one of three types of stellar remnants. White dwarfs are the remnants of low to intermediate mass stars (less than about 8 times the mass of our Sun) that eject their outer envelopes towards the end of their lives, leaving behind the exposed core of the initial star.  More massive stars have more violet deaths, ending their lives in a supernova explosion. Stars between about 8 and 40 solar masses will leave a neutron star behind after the supernova event, while even more massive stars will produce black holes as their stellar remnants.  The initial stellar mass that separates neutron stars and black holes is not very well constrained.

Like “normal” neutron stars, magnetars are tiny (about 20 km in diameter) and extraordinarily dense, with a teaspoon of magnetar material weighing around a billion tonnes. What sets magnetars apart from other pulsars is their incredibly high magnetic fields, which are of order 1015 G (which is 100 to 1,000 times stronger than radio pulsars). They rotation every few seconds, but it is thought that they must be born with rotation rates of 100 to 1,000 times per second to achieve such strong magnetic fields.

Exactly how magnetars form is unclear.  Simon Clark from the Open University and collaborators determined that one of the Milky Way’s 21 known magnetars must have formed from a star that was initially at least 40 times the mass of the Sun. The magnetar CXOU J164710.2-455216 (aka J1647-45) resides in the stellar cluster Westerlund 1, which is the closest ‘super star cluster’ to us. At a distance of 16,000 light years, Westerlund 1 contains hundreds of massive stars that are at least 30-40 times the mass of our Sun.

The problem, however, is that stars initially 40 times the mass of the Sun or more should end their lives as a black hole rather than a neutron star. Clark and collaborators have recently identified a small, bright, carbon-rich star that is hurtling away from J1647-45. They suggests that this ‘runaway star’, discovered with the Very Large Telescope in Chile’s Atacama Desert, was once locked in a binary system (two gravitationally bound stars closely orbiting each other) with the stellar progenitor of the magnetar. When the supernovae occurred there was sufficient force in the explosion to kick the companion star, Westerlund 1-5, out across the Westerlund 1 cluster.

The European Southern Observatory’s Very Large Telescope (VLT) in Chile. The world’s most advanced optical instrument, consisting of four Unit Telescopes with main mirrors of 8.2m diameter and four movable 1.8m diameter Auxiliary Telescopes. (Credit: ESO/S. Brunier)

This finding allows astronomers to recreate the evolutionary path taken by the binary system that allowed it to form a magnetar in place of a black hole. In the scenario proposed, the larger of the two stars (Westerlund 1-5) begins to shed its outer envelope, transferring some of its mass to the smaller companion star – fated to form the magnetar (J1647-45). As a result the progenitor of J1647-45 rotates faster and faster as it accrete gas shed by Westerlund 1-5, which amplifies its magnetic field to colossal strengths. Once a critical mass is reached, the progenitor of J1647-45 ejects its outer layers, a portion of which is transferred back to Westerlund 1-5. This volatile swapping of material provides the unique mix and enrichment of elements seen in the runaway star Westerlund 1-5. Crucially, it also reduces mass of the progenitor of J1647-45, slimming the star down sufficiently to produce a magnetar instead of a black hole at the moment of its death following the supernova explosion.

The unique combination of properties of the runaway star – it high velocity (expected from a star ejected by a supernova explosion), low mass, high luminosity and carbon-rich composition – is almost impossible to understand in a single star. These properties all support the idea that the runaway star must have originally been part of a binary system.

The new results suggest that being part of a binary star system may be an essential ingredient in forming at least some magnetars. The mass transfer between the stars produces that rapid rotation rate that is needed to produce the strong magnetic field of the magnetar, and the subsequent mass loss allows the progenitor to be in the right mass range to collapse into a magnetar rather than a black hole

For more information, see

[Toby Brown & Sarah Maddison]

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