About 90% of the matter in our Galaxy is in stars. The rest is gas between the stars, known as the interstellar medium. We think that the gas in our part of the Galaxy was most likely heated to a temperature of a few million degrees by a nearby supernova explosion in the (astronomically speaking) recent past.
In January of 1993 the Diffuse X-ray Spectrometer (DXS) instruments flew on the STS-54 mission of the Space Shuttle Endeavour. They collected x-ray spectra of the diffuse soft x-ray background. This dataset will allow us to learn about the region of space for several hundred light years around the solar system.
The DXS project built the instruments at the University of Wisconsin--Madison Space Science and Engineering Center (SSEC) in collaboration with Space Physics, and SSEC people were instrumental in every phase of the design, development, testing, flight operations, and data analysis. Wilt Sanders is the PI, and Bob Paulos is the PM.
If one observes the sky with an x-ray detector, one sees some stars, but unlike what we see with unaided eyes, the sky is not dark in between the stars. This cloudy glow is known as the Diffuse Soft X-ray Background. X-rays can be emitted from several processes. If gas is heated to temperatures of a few million degrees (for example in the solar corona), it will emit x-rays. High energy electrons interacting with magnetic fields or starlight can also cause x-rays to be emitted. This figure is a map of the sky as seen by 1/4 keV x-ray detectors.
The DXS instruments are unique because of their ability to sort the x-rays they detect by their wavelengths. The resulting distribution of how many x-rays are observed at what wavelengths is called a spectrum. Different emission mechanisms produce different spectra. Thus by observing the spectrum, we can learn about the physical processes that give rise to the x-rays.
The DXS data shown in the figure below shows that the spectrum is characterized by emission lines, that is, narrow ranges of wavelengths where the x-rays are very strong. This is a signature of x-ray emission from hot gas. This spectrum is the first direct evidence that the solar system is surrounded by a bubble of million degree gas.
Each kind of atom or ion has its own favorite wavelengths at which it likes to emit radiation. Notable ions which emit in the soft x-ray band include Si+7, S+7, and Fe+15, that is, silicon or sulfur atoms missing 7 electrons, or iron atoms missing 15 electrons. Atoms can be ionized (have electrons knocked off) by collisions with electrons in a high-temperature gas.
We find that the pattern of emission lines observed in the DXS data cannot be simply explained by assuming a gas like that found in the sun has been at a temperature of around a million degrees for long enough to come into equilibrium. Thus we must either assume that some elements are missing from the gas (silicon and iron tied up in dust grains that have not yet fully evaporated), or that the gas is not yet ionized to the extent one would calculate based on its temperature. Work is still in progress to distinguish between these two explanations. Both of these exciting possibilities set limits on how long the gas has been hot, which gives us a clue to the history of our part of the Galaxy.
If elements such as silicon and iron are mostly missing from the gas, it must have been heated within the last million years or so. If the ions are not yet in equilibrium with the temperature of the gas, the gas cannot have been hot for much more than a hundred thousand years. It seems unlikely that the supernova which heated this volume of gas was more recent than about 10,000 years ago, or there would be folklore about the explosion, as it must have been brighter than the moon.
figure caption: The DXS x-ray spectrum from the diffuse background in the constellations of Puppis and Auriga. The presence of narrow peaks in this spectrum is the first direct evidence that there is very hot gas in the interstellar medium near the solar system. The strong feature at a wavelength of 63 Angstroms coincides with emission lines from both S+7 and Fe+16. The simplest models predict (incorrectly) that a Si+7 feature at 61.5 Angstroms should be the strongest line in the spectrum. The pattern of x-ray intensity versus wavelength contains a wealth of information about the physical state of the hot gas within a few hundred light years of the sun.