Star Caught Eating Another Star, X-Ray Flare Shows

A tiny cannibal has been caught in the act, thanks to a superbright flash of x-rays spied by cosmic hunters.

The culprit is what's known as a neutron star, the tiny but very dense corpse of a massive star that died in a supernova blast. Sitting 16,000 light-years away, this particular neutron star is normally among the faintest objects in the x-ray sky.

But during recent observations with the European Space Agency's XMM-Newton space telescope, the star unexpectedly surged to 10,000 times its original brightness.

"A companion blue supergiant star is believed to have thrown off a gigantic clump of superheated gas from its surface, [which] got attracted by the intense gravitational field of the much smaller and denser neutron star orbiting nearby," said study leader Enrico Bozzo, an astronomer with the ISDC Data Centre for Astrophysics in Geneva, Switzerland.

The lump of wayward stellar matter measured an estimated 9.9 million miles (16 million kilometers) across and took up about a hundred billion times the volume of the moon.

As it became part of the neutron star, the material was heated to millions of degrees, generating a brilliant x-ray flare that lasted for four hours.

(Related: "Black Hole Caught Eating a Star, Gamma-Ray Flash Hints.")

Fast Flashes Were Stellar Mysteries

Astronomers previously knew that the neutron star and the blue supergiant are part of a stellar odd couple known as a Supergiant Fast X-ray Transient, or SFXT. These usually faint binary stars are prone to occasional flareups that cause them to rival the brightest x-ray sources in the sky.

(Related: "Ultrabright Gamma-ray Burst "Blinded" NASA Telescope.")

Unfortunately for astronomers, these flares take place randomly only a few times a year, and they last just a couple hours, making them practically impossible to catch from beginning to end.

What makes this event even harder to see is that most space-based observatories with sensitive x-ray detectors can observe only a tiny fraction of the sky at a time, and they can't be swung into action fast enough when these flares go off.

"What usually ends up happening is that these events are detected by instruments that can move very fast or have much larger fields of view, but which suffer from reduced sensitivity, so that they just can't provide a clear understanding of what caused such an event," Bozzo said.

First Proof of Star's Cannibalism

One theory was that the flares are caused by the neutron star devouring matter cast off by its hefty companion.

Most massive stars generate a constant "wind" of charged particles, which pushes large quantities of stellar material in all directions into space.

Instead of a steady outflow of gas, the blue supergiant in an SFXT system may be emitting winds studded with large "bullets" of material, according to the theory. Flares happen when the neutron star gets shot by one of these clumps.

However, existing observations couldn't offer clear proof for this theory—until now.

By chance, XMM-Newton caught such a flare in 2010 during a scheduled 12.5-hour observation of the SFXT system known as IGR J18410-0535.

"I really could not believe this was true that we were so lucky! I didn't sleep for days," Bozzo said. "We are finally able to provide direct evidence for the existence of these clumps of matter."

(Related: "Mysterious Explosion Caused Massive Star to Brighten.")

Bozzo and his team now hope to make observations of other SFXTs with XMM-Newton, to better understand the unusual flares.

"We think it's the right time," he said, "to ask for an unprecedented large observational time with the space observatory and provide a final clear answer to the nature of these sources."