Human eyes can’t see x-rays, but if you look up with an x-ray detector, you’ll see that the whole sky glows brightly with light in this part of the spectrum. This soft glow is called the diffuse x-ray background, and its origin has been controversial since its discovery 50 years ago. Does the soft X-ray emission come from outside our solar system, from a hot bubble of gas? Or does the emission come from within our solar system, due to solar wind colliding with gas?
Both! According to a study published in Nature this week, the x-ray background is dominated by what’s called the “local hot bubble” -- about 1 million degrees hot -- with up to 40 percent of the emission originating from within the solar system. “We now know that the emission comes from both sources, but is dominated by the local hot bubble,” Massimiliano Galeazzi from the University of Miami says in a news release.
Interstellar bubbles are likely created by stellar winds and material cast outward by supernova explosions. These form large cavities in the material that fills the space between the galaxy’s stars (remember, space itself is not empty). If a second supernova occurs within the cavity carved out by the material, then hot, x-ray emitting gas could fill the bubble.
However, x-ray emission also occurs when the solar wind (a stream of charged particles released from the sun’s atmosphere) collides with interplanetary neutral gas. When the particles of the solar wind collide with neutral hydrogen or helium, an electron is captured, emitting x-rays. This process is called the solar wind charge exchange.
The team refurbished an X-ray detector from the 1970s, mounted it onto a modern NASA suborbital sounding rocket, and named it the “Diffuse X-ray emission from the Local Galaxy” (DXL). The sounding rocket launched from White Sands Missile Range in New Mexico on December 12, 2012, with the mission to separate and quantify the x-ray emission from the two suspected sources. It reached an altitude of 258 kilometers and stayed above Earth’s atmosphere for five minutes -- enough time to get a good look at the x-ray background.
Specifically, the study measured the diffuse X-ray emission at low energy, what’s known as the 1/4 keV band. “At that low energy, the light gets absorbed by the neutral gas in our galaxy, so the fact that we observe it means that the source must be ‘local,’ possibly within a few hundred light-years from Earth,” Galeazzi explains. Until now, it’s been unclear whether it comes from within few astronomical units from Earth or from hundreds of light-years from Earth. “This is like traveling at night and seeing a light, not knowing if the light comes from 10 yards or 1,000 miles away,” he adds.
The solar wind charge exchange, they found, contributed about 40 percent of the x-rays. “The rest must come from the local bubble," Galeazzi tells LiveScience. "What is important is that we now know that within the galaxy, these bubbles exist, and they contribute to the structure of our local region in the galaxy."