Source of sky's x-ray glow revealed

NASA's funded X-ray Instrument has helped in resolving long-standing puzzle regarding X-rays glow observed over the entire sky.

The mission was named DXL, for Diffuse X-ray emission from the Local Galaxy and the astronomers have confirmed that much of this glow comes from a region of million-degree interstellar plasma called the local hot bubble, or LHB. The study also established upper limits on the amount of low-energy, or soft, X-rays produced within our planetary system by the solar wind, a gusty outflow of charged particles originating from the sun.

Steve Snowden, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, said that interactions between the solar wind and neutral atoms in comets, the outer atmospheres of planets, and even interstellar gas produces soft X-rays.

He further explained that this glow, known as the soft X-ray diffuse background, was surprisingly bright in the gas-rich central plane of the galaxy, where it should be strongly absorbed and this suggested that the background was a local phenomenon, arising from a bubble of hot gas extending out a few hundred light-years from the solar system in all directions.

Improved measurements also made it increasingly clear that the sun resides in a region where interstellar gas was unusually sparse. Taken together, the evidence suggested that the solar system was moving through a region that might have been blasted clear by one or more supernova explosions during the past 20 million years.

The mission was led by Massimiliano Galeazzi, a professor of physics at the University of Miami in Coral Gables, Florida, and the team included scientists from NASA, the University of Wisconsin, Madison, the University of Michigan at Ann Arbor, the University of Kansas at Lawrence, Johns Hopkins University in Baltimore, Maryland, the French National Center for Scientific Research (CNRS), headquartered in Paris, and other institutions, who rebuilt, tested, calibrated, and adapted X-ray detectors originally designed by the University of Wisconsin and flown on sounding rockets in the 1970s.

Galeazzi said that they now know that the emission comes from both sources but was dominated by the local hot bubble and it was a significant discovery as the existence or nonexistence of the local bubble affects their understanding of the area of the galaxy close to the sun, and could, therefore, be used as a foundation for future models of the galaxy structure.

The researchers have already planned the next flight of DXL in 2015, which would include additional instruments to better characterize the emission.

The study is published online in the journal Nature.