Wind nebula found around magnetar for first time

Discovery could shed new light on the behaviour of the rare ultra-magnetic pulsars. Bill Condie reports.

The Crab Nebula is the best-known wind nebula. At its centre is a rapidly spinning neutron star that accelerates charged particles to nearly the speed of light, glowing as they whirl around magnetic field lines.
NASA, ESA, J. Hester and A. Loll (Arizona State University)

A wind nebula – a cloud of high-energy particles many light years across – has been discovered around a rare ultra-magnetic neutron star for the first time.

While wind nebulas are often found around the remains of supernovae, this latest find could bring new insights into the behaviour and composition of or magnetars as the highly magnetic pulsars are called.

They are very rare. While scientists know of 2,600 neutron stars, only 29 magnetars have been found so far.

A neutron star is the crushed core of a massive star that, having run out of fuel, collapsed under its own weight and exploded as a supernova.

Each one compresses the equivalent mass of half a million Earths into a ball just 20 kilometres across, or about the length of New York's Manhattan Island.

Usually they take the form of pulsars producing radio, visible light, X-rays and gamma rays at various locations in their surrounding magnetic fields.

These more common pulsars have enormously strong magnetic fields billions of times stronger than Earth's, but magnetar fields can be 1,000 times stronger again.

The newfound nebula surrounds a magnetar known as Swift J1834.9-0846, which was discovered by NASA's Swift satellite in 2011, during an X-ray outburst.

It is about 13,000 light-years away in the constellation Scutum toward the central part of our galaxy.

“Right now, we don't know how J1834.9 developed and continues to maintain a wind nebula, which until now was a structure only seen around young pulsars,” said lead researcher George Younes from George Washington University in Washington.

“If the process here is similar, then about 10% of the magnetar's rotational energy loss is powering the nebula’s glow, which would be the highest efficiency ever measured in such a system.”

An X-ray image shows extended emission around a source known as Swift J1834.9-0846, a rare ultra-magnetic neutron star called a magnetar. The glow comes from a cloud of fast-moving particles.
ESA/XMM-Newton/Younes et al. 2016

A paper describing the analysis of the nebula is to be published in The Astrophysical Journal.

“For me the most interesting question is, why is this the only magnetar with a nebula? Once we know the answer, we might be able to understand what makes a magnetar and what makes an ordinary pulsar,” says co-author Chryssa Kouveliotou, also of George Washington University.

Wind nebulae are usually found around young pulsars, such as the one at the heart of the Crab Nebula.

“Making a wind nebula requires large particle fluxes, as well as some way to bottle up the outflow so it doesn't just stream into space,” explains co-author Alice Harding, of NASA's Goddard Space Flight Center in Maryland.

“We think the expanding shell of the supernova remnant serves as the bottle, confining the outflow for a few thousand years. When the shell has expanded enough, it becomes too weak to hold back the particles, which then leak out and the nebula fades away.”

That could explain why wind nebulae are not found among older pulsars.

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