NASA's Fermi Gamma-ray Space Telescope has made the first unambiguous detection of high-energy gamma-rays from an enigmatic binary system known as Cygnus X-3. The system pairs a hot, massive star with a compact object — either a neutron star or a black hole — that blasts twin radio-emitting jets of matter into space at more than half the speed of light.

Astronomers call these systems microquasars. Their properties — strong emission across a broad range of wavelengths, rapid brightness changes, and radio jets — resemble miniature versions of distant galaxies (called quasars and blazars) whose emissions are thought to be powered by enormous black holes. "Cygnus X-3 is a genuine microquasar and it's the first for which we can prove high-energy gamma-ray emission," said Stephane Corbel at Paris Diderot University in France.

The system, first detected in 1966 as among the sky's strongest X-ray sources, was also one of the earliest claimed gamma-ray sources. Efforts to confirm those observations helped spur the development of improved gamma-ray detectors, a legacy culminating in the Large Area Telescope (LAT) aboard Fermi. At the center of Cygnus X-3 lies a massive Wolf-Rayet star. With a surface temperature of 180,000 degrees F, or about 17 times hotter than the sun, the star is so hot that its mass bleeds into space in the form of a powerful outflow called a stellar wind. "In just 100,000 years, this fast, dense wind removes as much mass from the Wolf-Rayet star as our sun contains," said Robin Corbet at the University of Maryland, Baltimore County. Every 4.8 hours, a compact companion embedded in a disk of hot gas wheels around the star. "This object is most likely a black hole, but we can't yet rule out a neutron star," Corbet noted.

Between October 11 and December 20, 2008, and again between June 8 and August 2, 2009, Cygnus X-3 was unusually active. The team found that outbursts in the system's gamma-ray emission preceded flaring in the radio jet by roughly five days, strongly suggesting a relationship between the two. The findings, published in the electronic edition of Science, provides new insight into how high-energy particles become accelerated and how they move through the jets.