A NASA-funded instrument helped clear up a decades-old mystery about a "fog" of low-energy X-rays covering the entire sky.
The research helped confirmed that much of this glow comes from million-degree interstellar plasma called the "local hot bubble" (LHB). The study also established a limit on the amount of energy X-rays produced within our solar system by solar wind.
"Interactions between the solar wind and neutral atoms in comets, the outer atmospheres of planets, and even interstellar gas produce soft X-rays," explained team member Steve Snowden, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We need to account for these processes because the X-rays they produce complicate our observations of the wider universe."
The findings suggest our solar system is moving through a region that was once cleared by a number of supernova explosions over the past 20 million years.
Scientists have challenged the LHB interpretation, suggesting it is more likely to be caused by charge exchange. To make a final ruling researchers adapted X-ray detectors (originally from the University of Wisconsin) and flew them on sounding rockets in the 1970s. The mission was called DXL, for Diffuse X-ray emission from the Local Galaxy and has been active ever since.
Our solar system is currently passing through a cloud of cold interstellar gas; causing helium and hydrogen atoms to stream through at 56,000 miles per hour. The phenomenon creates a "helium-focusing cone" with a much greater density of neutral and an enhanced exchange rate. Solar wind is accelerated by the Sun's corona, causing atoms to be ionized. When a neutral atom collides with one from the solar wind its electrons often jump to the charged particle. Once they are captured by the ions the electrons remain in an excited state, causing the soft X-ray glow as they settle down.
"We now know that the emission comes from both sources but is dominated by the local hot bubble," said Galeazzi. "This is a significant discovery. Specifically, the existence or nonexistence of the local bubble affects our understanding of the area of the galaxy close to the sun, and can, therefore, be used as a foundation for future models of the galaxy structure."
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