density of interstellar helium
July 29, 2014

X-Ray Instrument Helps Settle Interstellar Debate

April Flowers for - Your Universe Online

For decades, scientists have been debating over a fog of low-energy X-rays that has been observed over the entire sky. An international group of scientists has used a NASA-funded instrument to resolve this debate. The group refurbished detectors that were first flown on a NASA sounding rocket in the 1970s to confirm that much of this glow is generated by a region of million-degree interstellar plasma known as the local hot bubble (LHB).

The findings, published in papers in Nature and The Astrophysical Journal Letters, define the upper limits on the amount of low-energy X-rays, also known as soft X-rays, produced in our planetary system by the 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. "We need to account for these processes because the X-rays they produce complicate our observations of the wider universe," he told NASA's Francis Reddy.

[ Watch: NASA X-Ray Instrument Settles Interstellar Debate ]

Scientists using decades of data from mapping the sky in X-rays with energies around 250 electron volts, which is approximately 100 times the energy of visible light, have found strong X-ray emission exactly where it shouldn't be. The surprisingly bright glow, called the soft X-ray diffuse background, is found in the gas-rich central plane of the Milky Way galaxy.

According to our current understanding, it should be absorbed in this location. The fact that it wasn't suggested that it was a local phenomenon, created by a bubble of hot gas extending out a few hundred light-years from the solar system in every direction. As our ability to measure the phenomenon improved, it became clear that the sun resides in a region of the sky where interstellar gas is unusually sparse. Combined with the earlier mapping, this indicates that our solar system is moving through a region of the galaxy that may have been blasted clear by one or more supernova explosions during the past 20 million years.

ROSAT, the German X-ray observatory, made a six-month all-sky survey in the 1990s. This survey provided improved maps of the diffuse background. Surprisingly, the images also revealed that comets were a source of soft X-rays. As researchers began to investigate this process of solar wind charge exchange, it became clear that it could occur anywhere neutral atoms interacted with solar wind ions.

The LHB interpretation has been coming under challenge for the last ten years, with researchers suggesting that "much of the soft X-ray diffuse background is a result of charge exchange," said F. Scott Porter, a Goddard astrophysicist. "The only way to check is to design an instrument and make measurements."

That was the purpose of the international collaboration led by Massimiliano Galeazzi, a professor of physics at the University of Miami. The team rebuilt, tested, calibrated and adapted X-ray detectors originally designed by the University of Wisconsin. The detectors flew their first mission in the 1970s on NASA sounding rockets. Other components were scavenged and re-purposed from a 1993 Space Shuttle Endeavor mission instrument.

The new mission, Diffuse X-ray emission from the Local Galaxy (DXL), launched on December 12, 2012, from the White Sands Missile Range, flying into space atop a NASA Black Brant IX sounding rocket. The spacecraft reached a peak altitude of 160 miles above the Earth's surface, and spent five minutes outside the atmosphere, allowing it to observe a worst-case scenario involving charge exchange with interstellar gas.

Currently, our solar system is passing through a small cloud of cold interstellar gas. Neutral hydrogen and helium atoms from the cloud are streaming through the planetary system at about 56,000 mph. Hydrogen quickly ionizes and responds to numerous forces. Helium, on the other hand, travel paths mainly governed by the sun's gravity, creating a "helium focusing cone" downstream from the sun. This cone crosses Earth's orbit and is located high in the sky near midnight in early December.

"This helium focusing creates a region with a much greater density of neutral atoms and a correspondingly enhanced charge exchange rate," Snowden said.

The sun's corona is the hottest part of the star's atmosphere. In this environment, the solar wind is accelerated and the atoms have been ionized, or stripped of most of their electrons. When the charged ions of the solar wind collide with the neutral atoms of the interstellar gas, the atom generally loses an electron to the particle. This electron remains in a brief excited state, then emits soft X-ray before settling down to a lower energy—thus completing the solar wind charge exchange.

The research team used the ROSAT data to establish a baseline for the soft X-ray background. ROSAT captured this data looking along, rather than into, the helium focusing cone. They found that approximately 40 percent of the soft X-ray background originates within the solar system.

"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."

DXL is scheduled to fly again with additional instruments in December 2015.