Over the last few million years, several stars have exploded within the Milky Way, leaving behind bubbles of expanding hot gas that radiate low-energy X-rays. The solar system sits within one of these shells, known as the Local Hot Bubble. A study using XMM-Newton data reveals that a second and more recent supernova debris cloud - known as the Loop 1 Superbubble - is expanding faster than the Local Hot Bubble.

The competing bubbles are compressing an area of cool dense gas, called the Wall, which lies between their two shells. Astronomers have known for some time the LHB has an hourglass shape, but pressure and density measurements from XMM-Newton provide evidence that Loop 1's compression of the Wall is causing the hourglass's waist.

"The X-ray radiation from the bubbles is very faint," said lead researcher Michelle Supper of the University of Leicester. "In order to see them, we've had to remove all the light from stars, nebulae and cosmic rays the images, leaving only the weak X-ray signal. It's the astronomical equivalent of looking at an aquarium, ignoring the fish and looking only at the water."

Reporting at a meeting this week of the Royal Astronomical Society, Supper said her team has "taken long-exposure images of 10 small areas of sky in the direction of the Loop 1 Superbubble, then removed all the bright objects and studied what's left.

Each structure emits a unique X-ray signal, like a fingerprint, that reflects its temperature and chemical composition. This means that, when we come to analyze the images, we can tell which bits of radiation originated from Loop 1, the Wall or the Local Hot Bubble." Supper explained.

Supper's team developed mathematical models to represent each of the structures, then produced a geometrical model to work out the distances to the structure boundaries and the pressure and density of the interstellar plasma within the structures.

Loop 1 is thought to be expanding because it is being inflated by winds originating from a group of stars known as the Scorpius-Centaurus Association. Supper's measurements of physical properties of the Wall showed its density increases fourfold, reaching a peak near the most indented region of the LHB. The pressures also peak around this point, indicating that the Wall is pushing into the bubble at in this region.

Chemical analysis showed the highest concentrations of gases are found at the center of the Loop 1 Superbubble, and levels decrease dramatically in the expanding shell of the bubble.

"We found that X-ray emissions in an area near the galactic plane are much higher in energy than expected," Supper said, "but we don't know yet whether we've discovered a new X-ray source, or whether its an extension of the very high energy radiation coming from the center of the galaxy."

She said she hopes the data will provide insight into the distribution of the Galactic Halo, a mysterious X-ray signal that can be detected faintly above and below the disc of the Milky Way.

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