The man with X-ray vision

From neutron stars to nebulas – X-ray astronomy has revealed so much of our universe. Here X-ray visionary Joachim Trümper tells us about his experiences in the field and future hopes for X-ray astronomy

In the 1960’s I was a scientist in a nuclear physics institute at Kiel University working on cosmic rays at high energies. The discovery of radio pulsars by Bell & Hewish in 1967/68 and their subsequent identification with rapidly rotating magnetised neutron stars fascinated me. Could they be the accelerators of high energy cosmic rays? Soon after the pulsar in the famous Crab nebula was discovered showing 33ms pulsations in the radio and optical bands and in 1969 in X-rays as well. Soon after writing my first paper on neutron stars I began X-ray astronomy in Germany with the goal of finding more pulsars and studying these cosmic ray accelerators. I proposed in 1960 a dedicated X-ray satellite (A-6) for observations with high sensitivity and time resolution for the national German space program.

The A-6 satellite project proposed in 1970 was cancelled after a few years due to financial reasons. During the 1970’s, scientists from the Netherlands, the UK and Germany made a number of proposals for X-ray satellites carrying imaging X-ray telescopes. They all collapsed. We coped with that by changing gears and moving forward. In 1975, I moved to the Max Planck Institute for Extraterrestrial Physics and proposed a powerful X-ray telescope satellite in the German program for large projects of fundamental research. It was accepted and finally named ROSAT.

Subsequently the US joined (NASA with the launch and Harvard-SAO with a focal plane instrument) and the UK as well (University of Leicester with a separate XUV-Telescope extending the coverage to lower energies). After the tragic Challenger explosion the original plan of a space shuttle launch was instead changed to a rocket launch on June 1st 1990. With an active life of 8.5 years ROSAT provided a wealth of discoveries and results for a wide international science community. As is so often the way with a costly project – it took several steps before it began to yield results.

There are many hurdles and critical moments in such an ambitious project as ROSAT – in the proposal phase before approval, during the development of the instruments and the spacecraft and during the testing of all components. The most critical moment is the launch. All further actions depend on the radio communications and the proper functioning of the onboard systems. My happiest moment of the ROSAT mission was two weeks after take-off when we had first light with a view of the Large Magellanic Cloud hosting the famous Supernova 1987A.

During the operations critical problems can arise, for example, the satellite’s orientation gets lost due to a glitch in the altitude control system. We had several such events during the mission which fortunately could be rectified by the joint efforts of the people responsible for the satellite. Finally, on October 23 2011, when ROSAT plunged into the Indian Ocean, I was happy that it did not hit the Reichstag in Berlin, which had been featured in a dramatic way on the front page of Bild.

I think the Advanced Telescope for High Energy Astrophysics (ATHENA) – scheduled to launch in 2028 – is a great project. With the large collecting power of its mirror system and the advanced focal plane instruments – a wide field imager and an imaging spectrometer combining high throughput and spectral resolution – this ESA-led mission is expected to strongly advance our knowledge of the hot and energetic universe in the era after Chandra and XMM-Newton. A disturbing aspect is the late launch date which is foreseen in 2028, more than 30 years after the first discussions about such a mission in Europe.

Author: Joachim Trümper is one of the directors of the Max Planck Institute for Extraterrestrial Physics in Germany. He has been involved in many rocket and balloon experiments such as EXOSAT, ROSAT, XMM and Spektrum-X. He received the 2016 Tycho Brahe prize in recognition of his visionary development of X-ray instrumentation and work on ROSAT.