Telescope Peers Deep into Microquasar
Saturday, November 28, 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.
colors indicate greater numbers of gamma rays detected in
this Fermi LAT view of a region centered on the position of
Cygnus X-3 (circled). The brightest sources are pulsars.
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 Stéphane 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.
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.
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. "This suggests that the gamma rays arise
from interactions between rapidly moving electrons above and
below the disk and the star's ultraviolet light," 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,"
added Guillaume Dubus at the Laboratory for Astrophysics in
Grenoble, France. "And this occurs most often when the disk
is on the far side of its orbit."
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.
Oct. 11 and Dec. 20, 2008, and again between June 8 and Aug. 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
today in the electronic 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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