History of X-ray Astronomy at MPE

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 German V-2 rockets. 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 counterwith 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 Balloon-HEXE (flown first in 1973) and later MIR-HEXE 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 remnant Puppis A obtained with the rocket payload ASTRO 4/2 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 participation:

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.

ROSAT

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 highlights and the ROSAT image gallery.

Contemporary Projects

Several satellites were 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 of data.

The two largest X-ray satellite projects rest on American and European initiatives, which are complementary in their scientific performance figures:

The American Advanced X-ray Astrophysics Facility, now called Chandra 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 7.50 m. 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 ultra-smooth surfaces 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.

An 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.

Further Reading

The invisible sky: ROSAT and the age of X-ray astronomy,
Bernd Aschenbach, Hermann-Michael Hahn, Joachim Trümper,
translated by Helmut Jenkner. New York: Copernicus, 1998. QB472 .A83 1998

ROSAT 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.

Ortwin Schwentker and Frank Haberl