Back in 1996, the Japanese Advanced Satellite for Cosmology and Astrophysics (ASCA), an X-ray satellite, began a survey of the plane of our Galaxy - the Milky part of the Milky Way. While the Roentgen X-ray Satellite (ROSAT) had covered the Galactic plane in its all-sky survey a few years earlier, its mirrors could not focus X-rays with energies above 2 keV. ASCA probed X-rays up to 10 KeV - both more energetic and more penetrating of the gas and dust that lie in the Galactic plane. This combination could potentially reveal new and different X-ray sources to those uncovered by ROSAT.
Over the next three years, 40 deg2 were observed and 163 discrete X-ray sources detected. However, ASCA's mirrors were designed to have a large collecting area rather than produce sharp images, and the great density of stars and other objects in the Galactic plane made unambiguous identification of the optical counterparts of the X-ray sources seen with ASCA's blurry vision challenging at best. Over 100 of them remained unidentified, for nearly twenty years.
University of Sydney graduate student, Gemma Anderson, now at the time of writing a postdoc at Oxford University, lead a major project to find out the nature of these mysterious, but comparatively bright, X-ray sources using the exquisitely sharp imaging of the Chandra X-ray Observatory combined with extensive observations from ground-based telescopes. Even with Chandra's imaging power, the identity of many of the sources remained ambiguous until detailed hands-on sifting of the data could narrow things down. Finally, after nearly twenty years' wait, we now know that the mystery sources are a mixture of magnetically-active stars (see, eg, the 2013 May 7 posting), massive stars with strong stellar winds, some of which are likely in colliding wind binaries, X-ray binaries in which X-rays are produced by material accreting onto a neutron star from a more "normal" star, and magnetars - the young, terrifically magnetized neutron star remnants of fairly recent supernova explosions. A handful of sources still remain unidentified, and are likely to be either very distant background active galactic nuclei shining through the plane of our own Galaxy, or are perhaps still more enigmatic objects lurking in the murky depths of the Milky Way. This work, lead by Dr. Anderson, was published in the 2014 May edition of the Astrophysical Journal Supplement Series.
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