Powerful winds stream from unusual binary systems


Cambridge scientists are edging closer to understanding two mysterious ultra-luminous X-ray sources in nearby galaxies. Bill Condie reports.


An 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. – ESA–C. Carreau

Scientists have spotted two puzzling binary systems in nearby galaxies that are sending out powerful streams of gas at nearly 75,000 kilometres a second – a quarter of the speed of light.

The systems are believed to be "ultra-luminous X-ray sources", an intermediate system that is neither a supermassive black hole devouring material at the centre of its galaxy, nor a binary system consisting of a stellar remnant – a white dwarf, neutron star or black hole – feeding on gas from a companion star.

In all these systems, gas forms a swirling disc around the compact and very dense central object. Friction in the disc causes the gas to heat and emit light at many wavelengths, with a peak in X-rays.

“We think these ‘ultra-luminous X-ray sources’ are somewhat special binary systems, sucking up gas at a much higher rate than an ordinary X-ray binary,” explains Ciro Pinto, from Cambridge University, who led the study published in Nature.

The research relied on data from the European Space Agency's X-ray space observatory, the XMM-Newton, which Pinto says confirms that these sources conceal a compact object gobbling up matter at extraordinarily high rates.

"This is the first time we've seen winds streaming away from ultra-luminous X-ray sources," he said. "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."

The irregular galaxy NGC 5408 viewed by the NASA/ESA Hubble Space Telescope. It is located some 16 million light-years away and hosts the ultra-luminous X-ray source NGC 5408 X-1, which, along with another system, NGC 1313 X-1, is sending out streams of gas at huge speeds. – ESA/Hubble & NASA. Acknowledgement: J. Schmidt (Geckzilla)

The newly identified objects exceed the Eddington limit – a ratio established by astronomer Arthur Eddington between the amount of radiation coming out of a body such as a star or black hole and the inward gravitational force. This puts a theoretical limit on the amount of material the body can consume.

"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," Norbert Schartel, a scientist with the ESA, told reporters.

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

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