A rare, bizarre type of star has left astronomers scratching their heads after unexpectedly slowing down its spin.

The star's strange behaviour "made our jaws drop," said Victoria Kaspi, a McGill University researcher who has been monitoring this type of star, known as a magnetar, for 17 years.

Such a slow-down had never been observed before and seemed contrary to prevailing theories about the anatomy and behaviour of such stars.

The magnetar, located around 10,000 light years away in the constellation Cassiopeia, is a rare, "exotic" type of neutron star that has an extremely high magnetic field. Neutron stars are themselves very  strange, extremely dense, and small – roughly 20 kilometres in radius, but with a mass half a million times that of planet Earth. Despite their small size, magnetars are sometimes detectable from across the Milky Way galaxy because they emit extremely bright beams of x-ray radiation that appear to pulse as they rotate, like the beam of a lighthouse, Kaspi said.

'It's a new window into the interiors of these very extreme stars.' —Victoria Kaspi

"Most of the time, they just sit there," she added. "But every now and again, they have a spectacular outburst."

Because such massive explosions of x-ray radiation only take place every few years to once a decade, Kaspi and her research team have kept a close eye on a small number of magnetars, first using NASA's Rossi X-ray Timing Explorer satellite and more recently with NASA's Swift satellite.

When Rob Archibald, a Master's student working on Kaspi's team, came to her and told her the monitoring showed that the magnetar had slowed down suddenly on April 28, 2012, she was very skeptical at first.

'Go back and check again'

"I said, 'That's not possible — go back and check again," she recalled in an interview with CBCNews.ca.

Further analysis showed it was no mistake — the star really had slowed down and emitted a spectacular burst of x-rays at the same time.

That was puzzling because in every other known case where a magnetar suddenly changed its spin speed, it sped up. That phenomenon is known as a glitch because the first time scientists observed it a few decades ago, they thought there was a problem with the equipment.

Astronomers have theorized that glitches occur when fluid inside the star rotates faster than the crust and suddenly transfers some extra momentum during a disturbance. They think the spectacular outbursts of x-rays occur in the 20 to 30 per cent of glitches where the disturbance is violent enough to crack the crust, Kaspi said.

Because the strange 2012 outburst was accompanied by a slowdown, Kaspi is calling it an anti-glitch.

That kind of behaviour is "not at all what you expect if that whole picture of glitches in these stars is correct. It shouldn't happen that way," Kaspi added. "It's hard to imagine how fluid interior is slower than crust."

Kaspi has since contacted theorists who have helped her come up with a possible explanation — that pockets of fluid rotating slower than the crust could be responsible.

"It's probably telling us something about the structure of the star," she said. "It's a new window into the interiors of these very extreme stars. That's basically why we're so excited."

Kaspi and her colleagues published their results in the journal Nature this week, but she has also been presenting her discovery to other astronomers at scientific meetings in the hopes of generating more ideas and discussion.

"Immediately," she said, "everyone starts arguing."