Universe

Rare X-Ray Binary Star is Spewing Out Winds a Quarter of the Speed of Light

April 29, 2016 | Joanne Kennell

Artist's impression depicting a compact object -- either a black hole or a neutron star -- feeding on gas from a companion star in a binary system.
Photo credit: ESA - C. Carreau

Hold onto your hat!

Using data from the European Space Agency’s (ESA) XMM-Newton space observatory, researchers from the University of Cambridge discovered strong winds hurtling at very high speeds from two mysterious sources of x-ray radiation. The discovery, published in the journal Nature, confirms that these sources are hiding a compact object that is pulling in matter at rates that exceed classical limits.

In the known universe, x-rays come from two types of cosmological objects: supermassive black holes found at the center of large galaxies, and binary systems, which consist of a stellar remnant — a white dwarf, neutron star, or black hole — feeding on the gas of a companion star.

This gas forms a swirling disc around the object, and friction in the disc causes the gas to heat up and emit light at different wavelengths, particularly producing a lot of x-rays.

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However, an intermediate class of objects was discovered in the 1980s. These sources are ten to a hundred times brighter than your run-of-the-mill x-ray binaries, but they are too faint to be linked to supermassive black holes, and in most cases are usually found far away from the center of their host galaxy.

"We think these so-called 'ultra-luminous x-ray sources' are special binary systems, sucking up gas at a much higher rate than an ordinary x-ray binary," Ciro Pinto, a research associate from Cambridge's Institute of Astronomy and the paper's lead author, said in a press release.

Pinto and his colleagues collected several days worth of observations of three ultra-luminous x-ray sources located in nearby galaxies less than 22 million light-years away from the Milky Way.

In all three, the scientists spotted x-ray emissions from a gas in the outer regions of the disc slowly gliding towards the central object. However, two of the three sources — known as NGC 1313 X-1 and NGC 5408 X-1 — also showed x-rays being absorbed by gas that is flowing away from the central source at 70,000 kilometres per second (43,496 miles per second) — almost a quarter of the speed of light.

"This is the first time we've seen winds streaming away from ultra-luminous x-ray sources," said Pinto. "And the very high speed of these outflows is telling us something about the nature of the compact objects in these sources, which are frantically devouring matter.”

But there is a theoretical limit to how much matter can be pulled in and to the radiation being emitted out by an object of a certain mass, known as the Eddington limit.

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In the objects studied, the sources are being fed through the disc, and these discs are naturally exceeding the Eddington limit, making these sources appear brighter when astronomers look straight at them. And as the disc moves material further away from the black hole’s gravitational hold, it produces very high-speed winds like the ones observed by the Cambridge scientists.

"By observing x-ray sources that are radiating beyond the Eddington limit, it is possible to study their accretion process in great detail, investigating by how much the limit can be exceeded and what exactly triggers the outflow of such powerful winds," said Norbert Schartel, ESA XMM-Newton Project Scientist.

Based on the x-ray brightness, researchers suspect that these winds are due to the collection of gas onto either neutron stars or black holes. However, the team will continue to analyze XMM-Newton data in order to search for more of these sources.

"With a broader sample of sources and multi-wavelength observations, we hope to finally uncover the physical nature of these powerful, peculiar objects," concluded Pinto.

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