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Black holes redefined by a jet of rays

binary star

A surpise feature of a binary star system has redefined what it means to be a black hole (NASA)

The definition of a black hole may need to change now that an international team of astronomers has discovered one of its defining characteristics elsewhere.

The team was the first to detect a binary star system emitting high-speed jets. Previously, these jets were thought to be a characteristic of black holes.

Dr Rob Fender from the University of Amsterdam led the research team, which included Australian and U.K. researchers. They reported their find in today's issue of the journal Nature.

Black holes have an enormous gravity and suck in any matter than comes close. They sometimes release some of this material in high-speed jets that can be seen by a telescope as a sudden flash of x-rays. So far, these jets have only been found around black holes, which are extremely massive objects.

But the researchers have for the first time seen jets coming from a binary star system with no black hole. This means the presence of these jets can no longer be a determining factor in the definition of black holes.

The system is in the constellation of Circinus and can be seen during the winter in Southern skies. It consists of a neutron star in orbit with a much smaller companion star. The neutron star sucks in matter that falls towards it from its companion star. Some of this matter is then being flung back out as high-speed jets.

Black holes not necessary

Australian team member Dr Helen Johnston, from the University of Sydney, told ABC Science Online the researchers were surprised to see something being accelerated at such a large fraction of the speed of light, without the presence of a black hole.

"It tells us something important: that whatever is doing the accelerating doesn't depend on there being a black hole there," said Johnston.

The discovery reopens the debate about what causes the jets, said Johnston. It indicates all that's needed for their formation is the presence of an accretion disc, or orbiting ring of matter, around a massive object such as a black hole or a neutron star.

Looking at the jets will help astronomers understand what happens during the formation of an accretion disc, said Johnston.

Although the jets are only found around massive objects like black holes and neutron stars, accretion discs also form around normal stars, such as our Sun, eventually coalescing into planets and asteroids.

"We think the entire accretion disc [in this binary system] must be destroyed when the stars get so close there just isn't enough room for the disc to survive. All the material from the disc falls into the star at once, causing an explosion, and that's what causes the massive burst of x-rays.

"[The disc] is forming and being destroyed once every 16 days and then reforming. That's really fast. That's pretty important because accretion discs are ubiquitous in everything from forming planets all the way out to quasars."
