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Neutron star bites off more than it can chew

Artist's impression of a neutron star partially devouring a massive clump of matter.

Artist's impression of a neutron star partially devouring a massive clump of matter spat out by its companion star.

ASTRONOMERS HAVE SEEN a faint star flare up at X-ray wavelengths to almost 10,000 times its normal brightness…caused, they think, by the star trying to eat a giant clump of matter.

The flare took place on a neutron star, the collapsed heart of a once much larger star, and part of a binary star system. Only 10 kilometres in diameter, the neutron star is so dense that it generates a strong gravitational field.

The clump of matter was much larger than the neutron star and came from its enormous, blue supergiant companion star.

“This was a huge bullet of gas that the star shot out, and it hit the neutron star…,” says Enrico Bozzo, ISDC Data Centre for Astrophysics, University of Geneva, Switzerland, and team leader of the research.

The flare lasted four hours. The X-rays came from the gas in the clump as it was heated to millions of degrees while being pulled into the neutron star’s intense gravity field.

Because the clump was much bigger than the neutron star, only some of it was swallowed.

An artist's impression of XMM-Newton.

An artist's impression of XMM-Newton.

A lucky observation

The European Space Agency’s XMM-Newton space observatory caught the flare during a scheduled 12.5-hour observation of the system, which is known only by its catalogue number IGR J18410-0535.

But the astronomers were not immediately aware of their catch.

The telescope works through a sequence of observations carefully planned to make the best use of its time, then sends the data to Earth.

It was about 10 days after the observation that Dr Bozzo and his colleagues received the data and quickly realised they had something special. Not only was the telescope pointing in the right direction to see the flare, but the observation had lasted long enough for them to see it from beginning to end.

“I don’t know if there is any way to measure luck, but we were extremely lucky,” says Dr Bozzo. He estimates that an X-ray flare of this magnitude can be expected a few times a year at the most for this particular star system.

The duration of the flare allowed them to estimate the size of the gas clump. It was much larger than the star, probably 16 million kilometres across—that’s about 100 billion times the volume of the Moon, yet it had probably only 1/1,000th of the Moon’s mass.

Adapted from information issued by ESA / AOES Medialab / C. Carreau.

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