Wilton T. Sanders III, Principal Investigator
Richard J. Edgar, Project Scientist
DXS, the Diffuse X-ray Spectrometer instruments were flown on the
STS-54 mission of the Space Shuttle Endeavour in January of 1993.
This picture shows astronauts Mario Runco and Greg Harbaugh near
the starboard DXS instrument.
DXS obtained the first-ever spectra of the diffuse soft x-ray background
in the energy band from 0.15 to 0.284 keV (wavelength 42 to 84 Angstroms).
A Summary of the First Results from the Diffuse
X-ray Spectrometer Experiment
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
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 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+15. 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.