A Young Research Field
In the past three decades, observational astronomy has expanded
from the relatively narrow wavelength band of visible light, which
is one octave in width, to the entire electromagnetic spectrum.
Today, more than sixty octaves between the long-wave radio band and
the range of high-energetic gamma-ray radiation are used.
The mainspring of this development was the awareness that different spectral
ranges allow different and complementary insights into cosmic events.
To the most fruitful of these newly opened spectral ranges belongs
X-ray astronomy, covering a band of photon energies between
0.1 keV and 500 keV.
In particular, the phenomena which occur at the end of the stellar
lifetimes are observable in the X-ray sky:
supernova explosions, neutron stars, and black holes.
Far outside our own Galaxy, the X-ray sky is dominated by active galaxies
(radio galaxies, Seyfert galaxies, and quasars) and by clusters of galaxies,
the largest physical formations of our universe.
Also, normal stars and galaxies, which are comparatively
weak X-ray radiators, can be studied with modern X-ray telescopes.
And even comets, which pass for "dirty snow balls", are seen in the X-ray sky.
X-ray emission results from cosmic objects under extreme conditions.
Hot plasmas, with temperatures from a million to a billion degrees,
emit X-rays as black body radiation or bremsstrahlung ("braking radiation").
X-ray synchrotron radiation is produced by the interaction of highly
relativistic electrons with cosmic magnetic fields; the inverse Compton
effect produces X-rays when highly relativistic electrons interact with
intense photon fields.
Thus, one learns from X-ray observations something about the
hot universe and nuclear energy processes.
These are often associated with explosive processes, which are
important to cosmic development.
The Beginning and Turbulent Developments
X-ray astronomy is a product of the Space Age. The first direct proof
of X-rays from the Sun was produced after World War II with the aid of captured
The first cosmic X-ray source, Scorpius X-1 and the cosmic X-ray
background were discovered simultaneously in 1962 with a rocket experiment
of the National Aeronautics and Space Administration (NASA),
which was equipped with a Geiger-Müller counter with the aim
of detecting X-rays reflected from the Moon.
Subsequently, numerous rocket and ballon experimets and a whole array
of X-ray satellites equipped with large-area X-ray collectors have been
successfully flown. In 1971, the satellite
UHURU performed the
first all-sky X-ray survey, which yielded 339 sources in total.
These experiments included the German
(flown first in 1973) and later
onboard the Soviet space station (1987-94).
Both facilities were used, in collaboration with the
Astronomical Institute of the University of Tübingen,
to observe neutron stars and black holes. One highlight of these
activities was the discovery of a cyclotron resonance line in the hard
X-ray spectrum of the neutron star Hercules X-1. For the
first time, it was possible to spectroscopically determine the pole
field strength of such an object:
500 Million Tesla, the strongest known magnetic field in the Cosmos.
Quite new vistas for exploration have been opened by the introduction
of imaging telescopes, with which X-rays are focussed at grazing incidence.
In 1951, the physicist Hans Wolter at the University of Kiel discovered
a mirror configuration to produce an X-ray telescope.
It consisted of a paraboloid and hyperboloid mirror
mounted confocally and coaxially. Such Wolter telescopes
were used on Skylab to investigate the corona of our Sun. These
were followed in 1978 by NASA's
Einstein Observatory ,
and in 1983 by ESA's EXOSAT ,
both equipped with Wolter telescopes having openings of 56 cm
and 17 cm, respectively.
Our X-ray astronomy group started in 1972/73 with the help of the
Carl Zeiss company, to investigate and systematically develop X-ray mirrors.
During the years 1974-77, we deployed paraboloid mirrors to obtain X-ray
spectra of the old supernova remnants in the constellations of Vela and Cygnus.
The inclusion of our first Wolter telescope with an opening of
32 cm onboard a Skylark rocket in 1979 was a great success.
The image of the supernova
was the first X-ray exposure of
the sky ever taken with a spectral resolving imaging detector, a position
sensitive proportional counter developed at the MPE.
Later, the 32-cm-telescope was used for exposures of the supernova envelope
of Cassiopeia A (1981) and the supernova 1987A.
But our aim has been from the beginning to fly an X-ray telescope onboard
a satellite. This great adventure named ROentgenSATellit (ROSAT)
after the discoverer of X-rays,
Wilhelm Conrad Röntgen,
was first proposed to the
Federal Department of Research and Technology (BMFT)
in 1975, and was finally awarded in 1982, after bringing in international
With the British contribution of a Wolters telescope
covering the XUV domain the spectral range could be expanded
to long wavelengths.
The NASA was prepared to contribute with a
Space Shuttle launch free of charge and a high-resolution imaging
detector for the X-ray telescope.
Construction of the the satellite began in the year of approval.
On June 1, 1990 a Wolter telescope several factors more powerful than
any of its predecessors was launched with ROSAT .
One of its most important aims has been to survey the entire sky
for the first time with an imaging X-ray telescope.
This part of the mission lasted half a year, and was completed in February 1991.
Thereby, sources have been recorded whose intensity is a hundred times
weaker than the weakest sources in earlier X-ray surveys.
The scientific harvest has been accordingly rich.
More than 60,000 X-ray sources have been detected with the
ROSAT all-sky survey ,
larger by almost two orders of magnitude than the 840 sources of the
catalog of the previously largest all-sky survey of the HEAO-I satellite.
Following the all-sky survey, for over six and a half years to the present
time, ROSAT continues to provide detailed observation of selected sources.
The observation time is advertised and distributed world-wide to almost a
thousand guest observers. All together, so far more than 9,000 pointed
observations have been performed.
The ROSAT all-sky survey, together with the detailed pointed
observations, have yielded a rich harvest of almost 150,000 X-ray
sources, outreaching everything in quality and quantity discovered
previously with imaging X-ray telescopes.
Here, we can point to the numerous
the MPE conferences,
and the ROSAT image gallery.
Several satellites operating simultaneously with ROSAT, for example:
the Russian Granat,
the American supported Japanese ASCA,
the Italian BeppoSAX
which is supported by the Netherlands and MPE, and
the American RXTE.
The collaboration between
ASCA and ROSAT is particularly strong
and intense since both satellites have complementary properties:
while ROSAT provides high sensivity and good imaging in the 0.1 - 2.4 keV
band ASCA is characterized by a region extending to higher energies
(0.5 - 10 keV) and a superior spectroscopic performance.
Large X-ray Observatories in Orbit
X-ray astronomy is moving on into the post-ROSAT era. International
programmes, in which our institute also participates, yield large amounts
The two largest X-ray satellite projects rest on American and
European initiatives, which are complementary in their scientific
The American Advanced X-ray Astrophysics Facility, now called
was launched on July 23, 1999.
The large X-ray telescope - with a focal length of 10 m,
an opening of 120 cm, and four nested Wolter mirrors -
reaches a spatial resolving power of 0.5 arcseconds
(ten times better than ROSAT),
using a micro channel detector similar to the HRI onboard ROSAT
or an X-ray CCD camera. Both detectors can be used in connection
with transmission gratings for high resolution spectroscopy.
The Low Energy Transmission Grating (LETG) of Chandra is a contribution
of the Space Research Organisation of The Netherlands (SRON) and MPE.
The European counterpart is the
X-ray Multi-Mirror satellite (XMM-Newton).
It was launched on December 10, 1999 and is equipped with three large
Wolter mirror systems,
each consisting of 58 nested mirror shells with a focal length of
With its large collecting area and two different CCD camera types
(European Photon Imaging Cameras, EPIC)
XMM-Newton, is especially qualified for highly-resolved, detailed
X-ray spectroscopy and time variability studies.
MPE contributed to XMM-Newton in various ways: X-ray optical design
and tests of the mirror system; development, building and tests
of the novel pn CCD detector and participation in the XMM-Newton
ground calibrations. Since launch health monitoring and in-orbit
calibration of the EPIC pn camera is perfomed by the X-ray calibration
group at MPE.
X-ray Astronomy - Technology Driver
Our scientific curiosity has contributed to spin-off developments,
driving the progress of the art:
The Carl Zeiss company has developed and manufactured
for our X-ray mirrors, and used these methods elsewhere already.
For example, this technology has been introduced into modern
fabrication of eyeglasses.
Free-standing micro structures
were developed and built together with the company
Dr. Johannes Heidenhain
for our transmission grating spectrometer.
They are technologically interesting, e.g. for linear measurement
systems used by computer-controlled machine tools.
To produce our
fast high-resolution CCD X-ray imaging converters,
our own semiconductor laboratory has been established
together with the Max-Planck-Institut für Physik.
Future applications of these converters range from
materials research to medical applications.
attitude control for satellites
developed by the DASA and the GSOC for ROSAT is already used
as the standard system for more than 50 communication satellites.
The invisible sky: ROSAT and the age of X-ray astronomy,
Bernd Aschenbach, Hermann-Michael Hahn, Joachim Trumper, translated
by Helmut Jenkner. New York: Copernicus, 1998. QB472 .A83 1998
Brochure, edited by the Bundesministerium für
Forschung und Technologie, Bonn, Germany and Deutsche Forschungsanstalt
für Luft- und Raumfahrt, Köln, Germany
· the electronic version has been updated and modified
O.S. · 08/06/1997
F.H. · 19/06/2006