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Meterors, Asteroids & Comets
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Waiting for Cygnus X-3

One of the brightest x-ray sources in the Milky Way seems 
about to erupt in a dazzling flare. 
By studying the explosion scientists hope 
to unravel an  extragalactic mystery.


February 25, 2000 -- Astronomers are increasingly convinced
  that supermassive black holes lie at the centers of most large
  galaxies. It's a classic case of truth being stranger than fiction.
  Gigantic disks of gas -- called accretion disks -- swirl around
  central black holes that weigh in at millions or even billions of
  solar masses. As gas in the accretion disk falls into the hole it
  heats up and glows so brightly in x-rays that we can detect
  them a billion light years away. The cores of these systems,
  called active galactic nuclei (AGNs), outshine all of the stars in
  the host galaxy by factors of 10 to 1000.

  About 10% of all AGNs are stranger still. They produce
  narrow beams of energetic particles and magnetic fields, and
  eject them outward in opposite directions away from the disk at
  nearly the speed of light. When one of these beams is pointed
  toward Earth, it looks especially bright and astronomers call it a
  blazar. Among all AGNs, blazars can be detected over the
  widest range of frequencies, from radio waves to gamma rays. 

  Many aspects of blazars remain a mystery. What accelerates the material in the jets to relativistic
  speeds? How are the jets collimated? What are they made of? 

  The answers to some of these questions about distant galaxies may lie right here in our own Milky
  Way, in the binary star system Cygnus X-3. 

  "Cygnus X-3 is a black hole or a neutron star that's accreting matter from an companion star,"
  explains Mike McCollough of the NASA/Marshall Space Flight Center. "Because of the deep
  gravity well, a huge amount of energy can be released in x-rays and gamma-rays. It's also a very
  bright radio source that undergoes massive flares from time to time." 

  During an intense flare in 1997, McCollough and colleagues made a high-resolution radio map of
  Cygnus X-3 using the Very Long Baseline Array (VLBA), a continent-sized radio interferometer. 

  "When we looked at the images, lo and behold, there was definitely a one-sided radio jet, about 50
  milliarcseconds long," recalled McCollough. "Two days later it extended to 120 milliarcseconds
  and then it disappeared. This likely makes Cyg X-3 a galactic blazar -- a jet source where we were looking straight down the jet." 

Above: An artist's concept of a high-mass x-ray binary system like Cygnus
                                                                          X-3. Gas from a massive star feeds the accretion disk of an orbiting
                                                                          black hole or neutron star. The accreting gas heats up and shines
                                                                          brightly as an X-ray source. 

                                                                          "Cygnus X-3 may be the first example of a blazar here in our own
                                                                          galaxy," he continued. "It's the only case known of a Wolf-Rayet star
                                                                          with a compact companion. Wolf-Rayet stars are massive stars -- 7 to
                                                                          50 solar masses -- that have blown away their outer envelope of
                                                                          hydrogen. What's left is mostly helium. These types of stars have a
                                                                          very vigorous stellar wind, and that's probably what's driving things in
                                                                          this source." 

                                                                          "We can't see Cygnus X-3 optically because it's in the galactic plane
                                                                          where optical extinction by interstellar dust obscures the source.
                                                                          Fortunately, we can see it at infrared (IR) wavelengths and that's how
                                                                          we know it's a Wolf-Rayet star, from the IR spectral lines. Modulation
                                                                          of the IR and the X-ray emission gives us the orbital period of the
                                                                          binary, only 4.8 hours." 

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