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Evidence that we live in a (local) bubble.

Astronomers have confirmed that a soft X-ray glow permeating the sky has two sources: one inside the solar system, one outside.

This background emission is an unexpectedly intense glow of soft X-rays blanketing the sky. X-rays at these low energies are easily absorbed by interstellar clouds, so astronomers previously concluded that they must originate within a few hundred light-years of the Sun.

To explain the X-ray background, scientists proposed the local hot bubble--a cavity in the interstellar medium spanning hundreds of light-years that's filled with million-degree, X-ray-emitting gas. Astronomers suspect that a nearby supernova explosion carved the bubble out hundreds of thousands or even millions of years ago.

But the local bubble paradigm was later challenged when astronomers observed X-rays emanating from a comet as it passed through the solar wind. After this discovery, many began to wonder whether the X-ray background might be produced by the solar wind interacting with other solar system material.

Massimiliano Galeazzi (University of Miami) and colleagues investigated the solar wind's X-ray production by examining the helium focusing cone, a "breeze" flowing into the solar system as our system passes through the Local Interstellar Cloud. The Sun's gravity focuses the helium into a cone shape. Neutral gas is relatively abundant in this cone, making it a good place to test the solar wind's X-ray-making prowess.

Galeazzi's team found that the X-ray emission created by the solar wind would only account for about 40% of the X-ray background. This confirms the existence of a local hot bubble in interstellar space that produces the bulk of the X-ray background, although the result suggests that the bubble's hot gas is slightly less dense than scientists earlier estimated, the team concludes July 27th in Nature.

The confirmation of the local hot bubble is a significant development in our understanding of the interstellar medium, which in turn is crucial for understanding star formation, our galaxy's structure, and galaxy evolution.

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Title Annotation:X-RAY ASTRONOMY
Author:Temming, Maria
Publication:Sky & Telescope
Geographic Code:1USA
Date:Nov 1, 2014
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