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Fermi peers into microquasar
Posted: November 27, 2009

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NASA’s Fermi Gamma-ray Space Telescope has made the first clear detection of high-energy gamma-rays from the enigmatic binary system Cygnus X-3.

In Cygnus X-3, an accretion disc surrounding a black hole or neutron star orbits close to a hot, massive star. Gamma rays (represented in purple) likely arise when fast-moving electrons above and below the disc collide with the star's ultraviolet light. Fermi sees more of this emission when the disc is on the far side of its orbit. Image: NASA's Goddard Space Flight Center.

The system pairs a hot, massive star with a compact object, thought either to be 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. This system, known as a microquasar, has characteristic strong emission across a broad range of wavelengths, rapid brightness changes, and radio jets. It resembles miniature versions of distant 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,” says Stephane Corbel of Paris Diderot University in France.

A Wolf-Rayet star 17 times hotter than our Sun lies at the centre of Cygnus X-3. It is so hot that its mass wastes into space in a strong stellar wind. “In just 100,000 years, this fast, dense wind removes as much mass from the Wolf-Rayet star as our Sun contains,” says Robin Corbet of the University of Maryland.

This star has a compact companion embedded in a disc of hot gas that spins around its sibling every 4.8 hours. “This object is most likely a black hole, but we can’t yet rule out a neutron star,” says Corbet.

This image locates the view around Cygnus X-3 within Fermi's all-sky map. Brighter colours indicate greater numbers of gamma rays detected in this Fermi LAT view of a region centred on the position of Cygnus X-3 (circled). The brightest sources are pulsars. Image: NASA/DOE/Fermi LAT Collaboration.

The new observations were made with the Large Area Telescope (LAT) aboard Fermi, which detected changes in Cygnus X-3’s gamma-ray output that relate to the companion’s 4.8-hour orbital motion, with the brightest gamma-ray emission occurring when the disc is on the far side of its orbit. “This suggests that the gamma rays arise from interactions between rapidly moving electrons above and below the disc and the star’s ultraviolet light,” says Corbel.

When ultraviolet photons strike particles moving at an appreciable fraction of the speed of light, the photons gain energy and become gamma rays. “The process works best when an energetic electron already heading toward Earth suffers a head-on collision with an ultraviolet photon,” explains Guillaume Dubus from the Laboratory for Astrophysics in Grenoble, France. “And this occurs most often when the disc is on the far side of its orbit.”

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 found the gas within these jets to be moving at more than half the speed of light. Between 11 October and 20 December 2008, and between 8 June and 2 August 2009, Cygnus X-3 was found to be unusually active, and the outbursts in the gamma-ray emission was found to precede flaring in the radio jet by roughly five days, strongly suggesting a relationship between the two.

The findings, published today in the online edition of Science, will provide new insight into how high-energy particles become accelerated and how they move through the jets.

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