Scientists have for the first time discovered a wind nebula - a vast cloud of high-energy particles around a rare ultra-magnetic neutron star known as magnetar. The findings offer a unique window into the properties, environment and outburst history of magnetars, which are the strongest magnets in the universe. A neutron star is the crushed core of a massive star that ran 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. Neutron stars are most commonly found as pulsars, which produce radio, visible light, X-rays and gamma rays at various locations in their surrounding magnetic fields. When a pulsar spins these regions in our direction, astronomers detect pulses of emission, hence the name. Typical pulsar magnetic fields can be 100 billion to 10 trillion times stronger than Earth's. Magnetar fields reach strengths a thousand times stronger still, and scientists do not know the details of how they are created. Of about 2,600 neutron stars known, to date only 29 are classified as magnetars. The newfound nebula surrounds a magnetar known as Swift J1834.9-0846 - J1834.9 for short - which was discovered by NASA's Swift satellite in 2011, during a brief X-ray outburst. Astronomers suspect the object is associated with the W41 supernova remnant, located about 13,000 light-years away in the constellation Scutum towards the centre 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 George Younes, a postdoctoral researcher at George Washington University. "If the process here is similar, then about 10 per cent 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," said Younes. The researchers took another look at J1834.9 using the European Space Agency's (ESA) XMM-Newton X-ray observatory, which showed an unusual lopsided glow about 15 light-years across centred on the magnetar. New XMM-Newton observations in March and October 2014, coupled with archival data from XMM-Newton and Swift, confirm this extended glow as the first wind nebula ever identified around a magnetar. The most famous wind nebula, powered by a pulsar less than a thousand years old, lies at the heart of the Crab Nebula supernova remnant in the constellation Taurus. Young pulsars like this one rotate rapidly, often dozens of times a second. The pulsar's fast rotation and strong magnetic field work together to accelerate electrons and other particles to very high energies. This creates an outflow called a pulsar wind that serves as the source of particles making up in a wind nebula. The research appears in The Astrophysical Journal.