European Space Agency Launches XMM
Europe's Latest Telescope, Chandra's Sister
In July 1999, NASA launched an extraordinary X-ray telescope named
Chandra. At a distance one-third of the way to the moon, the telescope's
deep, lonely orbit gives it an unimpeded view of distant exploding stars
and clusters of galaxies.
Now Chandra has company, a European satellite called XMM that is every bit
as impressive. The two satellites are like equally talented sisters who
may at times compete for a share of the spotlight but ultimately work
hand-in-hand in uncovering the most violent and mysterious phenomena in
XMM (short for X-ray Multi-Mirror) will be a prime tool for astronomers
studying black holes,
star formation and much more. Funded and built
largely by the European Space Agency (ESA), XMM was launched on December
10 and, over the next two months, will undergo a series of calibration
tests before opening its hatch to exploration. First light is expected in
A Telescope the Size of a Tennis Court
From its first incarnation, XMM was designed to complement Chandra. For
example, Chandra's sensitive cameras produce sharp
images of newborn stars
and supernova remnants.
XMM's huge collecting
area, in turn, captures enough X-ray photons to reveal the
temperature and velocity of the gas in these objects.
XMM's total X-ray collecting area, divided among three separate
telescopes, is nearly the size of a tennis court, 120 square meters - yet the
telescopes themselves are only 70 centimeters wide. Sound impossible?
The secret is in the design.
Each sleek, barrel-shaped telescope has 58 wafer-thin mirrors curved into
cylinders and nested within each other like Russian dolls. Each mirror
sits 25 microns from its neighbor, about a quarter the width of a human
The X-ray Rainbow
X-ray photons enter the telescopes and bounce off the mirrors at a shallow
angle towards instruments several meters away at the other end of the
satellite. For one of the telescopes, all the photons are directed to a
camera similar to the one installed on Chandra. In this way, XMM will
generate images of the X-ray sources it observes.
The other two X-ray telescopes split the number of photons. Half the
photons head toward the camera and half go directly to a device that
analyzes the "colors" of the X-rays.
Just like the
ranges from red to blue,
the X-ray spectrum is made up of different colors. Each color corresponds
to a different wavelength
or energy of light. This is also true of visible
light - for example, red light has a longer wavelength than blue light.
The analysis of the colors of light is called spectroscopy. The analysis
of the colors of X-ray light is similarly called X-ray spectroscopy.
Different gases at different temperatures produce a unique set of
chemical fingerprints, giving each
element its own distinctive
spectrum. By studying optical spectra of sunlight, we can learn about the
different elements that form the sun. Likewise, the X-ray spectra of
supernova remnants reveal all the elements a star produced before
its final fiery stellar explosion. The X-ray spectra of the remains of a star
also reveal all the elements that were caught in the
path of the shockwave of the explosion. Some of these are the elements the
star produced before its death, but others (those heavier than iron) are
the elements created in the extreme heat and pressure of the explosion.
Telescopes used from spectroscopy do not produce the kind of images that
we see from the Hubble Space Telescope, however, the kind of "images"
they do produce -- charts and graphs -- are just as valuable.
This is why X-ray astronomers need both Chandra and XMM. The
two satellites will work together just as the the telescopes
Hubble and Keck do now. Hubble produces great images, and Keck
(a ground-based telescope in Hawaii) produces corresponding spectra.
Chandra will produce sharper images than XMM; its angular resolution is as
low as 1 arc second, compared to XMM's 6 arc second. XMM, however, will
generate stronger spectra. This is because its larger collection area can
collect more photons than Chandra can, and with more photons (like data
points) the spectra will be more statistically significant.
By the way, even through X-rays can travel billions of miles through the
universe, X-ray photons cannot penetrate the earth's atmosphere. They
come to a cold stop before reaching us on the ground. As a result, both
Chandra and XMM must be place in space, above the atmosphere, to collect
photons and view the X-ray universe.
XMM's Bonus Telescope, A Mini-Hubble
Wait, there's more! XMM has a fourth instrument, an optical-ultraviolet
(UV) telescope called the Optical Monitor. This is like a scaled-down
version of the Hubble Space Telescope. The telescope has a 30-centimeter
aperture, which means the viewing area is 30-cm wide. But because it's
above the atmosphere, the telescope has the power of a 4-meter-wide
This Optical Monitor is handy for viewing the optical and UV counterparts
of X-ray sources. For example, the gas and miscellaneous star-stuff that
spirals into a black hole can glow in X-rays as well as optical and UV
light. XMM can view this simultaneously across wavelengths, learning more
about the bizarre physics behind a black hole.
In total, XMM has one telescope dedicated completely to images (via
the European Photon Imaging Camera), two telescopes that produce X-ray
images and spectra (the latter via the Reflection Grating Spectrometer),
and one optical-UV telescope (the Optical Monitor).
What XMM Will Actually Look At
Astronomers around the world are so excited about XMM because the
satellite observes so many different types of objects and events. They are
listed below - details of each of these topics can be found at