Announced November 26, 2009, 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. 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 seventeen 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.
Every 4.8 hours, a compact companion embedded in a disk of hot gas wheels around the star. Fermi's LAT detects changes in Cygnus X-3's gamma-ray output related to the companion's 4.8-hour orbital motion. The brightest gamma-ray emission occurs when the disk is on the far side of its orbit. When ultraviolet photons strike particles moving at an appreciable fraction of the speed of light, the photons gain energy and become gamma rays. Through processes not fully understood, some of the gas falling toward Cygnus X-3's compact object instead rushes outward in a pair of narrow, oppositely directed jets. Radio observations clock gas motion within these jets at more than half the speed of light. 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 on November 26, 2009 in the electronic edition of Science, will provide new insight into how high-energy particles become accelerated and how they move through the jets.