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It's a gas. Most hard x-rays (blue) from the galaxy's disk stream from a smooth plasma, not from point sources (crosses).

The Case of the Missing X-ray Source

X-rays from the plane of our galaxy have exposed a plain of energy sizzling among the stars. But like radiologists puzzling over unusual smears on their films, astrophysicists are mystified by blurs that point to cryptic processes. A new observation published online in Science today has deepened the mystery--it rules out obvious suspects as the source of these x-rays.

Astronomers have known for 2 decades that x-rays stream from the galaxy's ridge, a band less than 1000 light-years thick that bisects the Milky Way like a layer of cream cheese in a sliced bagel. Early satellites couldn't resolve the origin of the most energetic radiation, called "hard" x-rays, but speculation centered on swarms of familiar objects, such as flaring stars.

However, new telescopes trained on the galaxy's ridge haven't spotted any obvious x-ray sources speckling the region, report astronomer Ken Ebisawa of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and colleagues. In February 2000, they aimed the Chandra X-ray Observatory at a patch of sky about half the size of the full moon in the constellation Scutum, smack-dab astride the galactic ridge. There Chandra spied at least 36 pinpricks of x-ray light--about the number the astronomers would have expected to see gleaming in distant galaxies. Ebisawa's team deduced that most of the pinpricks were not stars inside the Milky Way, but galaxies far, far away. Even so, those point sources accounted for the only 10% of the x-rays Chandra measured. The rest seemed to be coming from some hazy, smeared-out source in the ridge, probably hot interstellar gas.

Most researchers agree that the focus will now shift to figuring out why the rarefied matter drifting among the stars glows so brightly in x-rays. "This component of the interstellar medium is not just a little thing," says astrophysicist Richard Mushotzky of NASA Goddard.

Several explanations have been proposed, each with its adherents and its detractors. Some think the galaxy's rotation generates magnetic fields in the interstellar medium that twist, snap, and reconnect in a large-scale, tenuous process akin to flares on the sun. Others suspect the gas gets its energy from frequent supernova explosions or low-energy cosmic rays.

Related sites

Ken Ebisawa's home page
The Chandra X-ray Observatory home page