ROSAT – the end of an exceptional satellite
(1.June 1990 – 23. October 2011)
During the early morning hours of 23rd October 2011, at about 4 am CEST, the research satellite ROSAT plunged back to Earth and disappeared – without a trace – in the Indian Ocean. This was the last stage for one of the most successful satellite missions of X-ray astronomy. During its eight years of active live, the X-ray observatory ROSAT detected more than 150 000 mainly unknown X-ray sources; some 4000 scientists from 24 countries used its data for more than 4200 papers in refereed journals, which were cited over 140 000 times.
It all started in the mid-1970s, when scientists from the Max Planck Institute for Extraterrestrial Physics (MPE) proposed a large X-ray telescope on a satellite to the German research ministry. Soon after, the ROSAT project was expanded to include international partners, but the scientific lead remained with the MPE until the end; this project was one of biggest success stories for the institute in its nearly 50-year history.
First complete all-sky map in X-rays
Already the first ROSAT mission, the first complete all-sky map with an imaging X-ray telescope, exceeded all expectations: over the course of half a year, some 100 000 X-ray sources were found. This was over 100 times more sources than from two previous sky surveys: The legendary Uhuru satellite and the High Energy Astronomy Observatory 1 (HEAO-1) found in their sky surveys in the 1970s about 850 sources. In addition, the second ROSAT camera, designed for extreme UV-wavelengths, provided the very first survey in this range and found 477 sources.
During the following eight years – seven more than originally planned - ROSAT was used for detailed observations of selected objects, ranging from near objects in our solar system, to stars and gas in our Milky Way, to distant objects in other galaxies. And again and again ROSAT found surprises.
ROSAT provided the first X-ray image of the moon and detected X-ray emission from comets. Initially this puzzled many astrophysicists as comets were considered to be “dusty snowballs”, i.e. cold. X-ray emission, however, needs either high temperatures (millions of degrees) or high-energy electrons. Eventually the scientists found that the comets do not produce the X-rays themselves but shine due to the interaction with the solar wind.
Stars in X-rays
From the coldest, low-mass siblings of the Sun to big supergiants, ROSAT provided the first complete overview of the X-ray emission of stars, even detecting X-ray emission from newly formed T-Tauri stars for the very first time. A particularly important domain in X-ray astronomy, however, are the compact, final stages of stellar evolution, white dwarfs, neutron stars and black holes, as well as supernova remnants (SNRs).
The ROSAT telescope discovered a special kind of binary star system in our neighbouring galaxies, the Small and Large Magellanic Clouds. In these so called “supersoft X-ray sources”, hydrogen flows from the companion star to the white dwarf, where it drives a fusion reaction on the surface. This provides the unique opportunity to study nuclear fusion directly, while normally this reaction is hidden in the interiors of stars.
With respect to neutron stars, the scientists scored a remarkable number of first discoveries, such as the X-ray emission from millisecond-pulsars or from numerous, previously unknown neutron stars in supernova remnants. ROSAT also discovered neutron stars emitting practically pure thermal radiation, which means that the hot surface of these tiny stars (with a diameter of about 25 km) can directly be seen. With a density of a billion tons per cubic centimetre and magnetic fields of many billion Gauss, neutron stars are the most extreme “stellar corpses” that can be observed directly.
ROSAT detected many supernova remnants. Due to the sky survey, several much extended formations, such as the “Monogem Ring” with more than 20 degrees in diameter or the “North Polar Spur”, which is even larger, could be mapped for the first time. Outside the expanding shock front of the Vela-SNR, the scientists detected structures indicating that large, compact lumps were ejected in the explosion. Moreover, an additional, previously unknown supernova remnant appeared in front of Vela, dubbed “Vela Junior”.
Supernova explosions are not only interesting in themselves; they also heat the interstellar medium. With ROSAT, the scientists gained new insights into the distribution of hot (~1 mio Kelvin) and cold (~100 – 10 000 Kelvin) gas in the Milky Way and near-by galaxies. Also, the density of interstellar dust, scattering X-rays and producing halos for bright X-ray sources, was determined accurately.
The hot centres of galaxies and galaxy clusters
In Andromeda, a near-by galaxy with a distance of 2 million light-years, ROSAT detected 396 X-ray sources, which is more than the Uhuru satellite found in our Milky Way. In a normal, not active galaxy, ROSAT observed for the first time a central flare, which probably originates from the central black hole disrupting and swallowing a star.
Another important topic is galaxy clusters, the largest physical structures in the Universe. In addition to the hundreds or thousands of galaxies, these clusters also contain very hot plasma with about 100 Kelvin, which is the source of the X-ray emission. Observations of the Perseus cluster showed that the activity of the central black hole leads to bubbles in the surrounding hot plasma.
Studies of the hot gas also showed, that the dominant component of matter in the galaxy clusters is dark matter (~70%). About 25% of matter is in hot plasma and only a few per cent in the stars of the cluster galaxies. Further, synoptic studies of galaxy clusters enabled the scientists to determine the cosmologically relevant mean density of normal and dark matter. Both only account for about 30% of the density necessary to stop the expansion of the Universe.
The biggest category of ROSAT sources – about half – is quasars and other active galactic nuclei, which can be observed to large distances because of their enormous luminosity. With long-duration, “deep” ROSAT observations, the scientists were able to prove that these sources account for at least 80 % of the cosmic background radiation in X-rays. This answered the question of the origin for this radiation, which had been discovered in 1972 by Giacconi et al. (Physics Nobel Prize 2002).
The large scientific yield of ROSAT becomes apparent in the number of publications: up to October 2011, about 4600 papers appeared in referred journals with more than 140 000 citations. This means that ROSAT ranks second among X-ray satellites, with the first place going to the NASA Chandra observatory and on par with the XMM-Newton telescope of ESA, which were both started in 1999, shortly after the ROSAT mission ended.
ROSAT development and operation
The main instrument of the ROSAT X-ray observatory was a Wolter telescope with three imaging X-ray detectors, which - mounted on a turret -could be brought mutually into the focus. The largest of that time Wolter mirror system was built by Carl Zeiss according to the specifications from MPE, which also tested the mirrors and calibrated the complete telescope in its "Panter" test facility. In addition, the MPE was responsible for the design and construction of the focal instrumentation for the X-ray telescope with its turret and two imaging proportional counters. The third focal instrument, the high-resolution channel plate detector was provided by the Smithonian Center for Astrophysics at the Harvard University. A second smaller Wolter telescope for the extreme UV - was provided by the University of Leicester. The construction of the satellite was carried out by Dornier and that of the intricate attitude control and measurement system by Messerschmitt-Bölkow-Blohm (today combined in EADS).
The X-ray telescope, designed and built by Carl Zeiss, featured many years in the “Guinness World Records”, because it was the most accurate mirror with a roughness of only 0.4 nanometres. Compared to the size of Lake Constance (60 km vs. 3 m for the X-ray mirrors) this accuracy would mean that waves would be only 0,008 millimetres high.
ROSAT was launched on 1. June 1990 with a Delta rocket of NASA from Cape Canaveral and delivered to an orbit at about 580 km altitude. The German research centre for aeronautics and space DLR was responsible for the project management, the DLR satellite control centre GSOC in Oberpfaffenhofen for control and regulation of the satellite. Already two weeks after launch, the scientists could celebrate “first light”. The optimised observing schedule was provided by MPE, which organised the selection of observation proposals together with partners in the USA and Britain.
During the first half year, ROSAT provided the first full sky-survey with an imaging X-ray telescope and detected some 100 000 X-ray sources. Subsequently it performed detailed observations of selected objects for not only the projected 12 months but rather 8 years, until the failure of a star sensor in 1998 caused the telescope to look directly into the Sun which led to severe damage. The last astronomical observation was made on December 17, 1998. On 12. February 1999 ROSAT was switched off.
On Sunday, 23. October 2011, between 3.45 and 4.15 CEST (1.45 – 2.15 UTC) ROSAT re-entered the Earth’s atmosphere. Parts could have plunged into the Indian Ocean, but this could not be confirmed.
More information, in particular images of the instruments and many X-ray sources can be found on the MPE Websites:
A detailed discussion of the astrophysical results can be found in the book „The Universe in X-rays“ (Editors: J. E. Trümper and Günther Hasinger, Springer 2007).