Galaxy clusters, active galactic nuclei, supernova remnants and X-ray binaries have one thing in common - they are incredibly hot and therefore emit X-rays. This type of radiation can reveal a great deal about the Universe, and it is being observed by the German eROSITA (extended ROentgen Survey with an Imaging Telescope Array) instrument.

Between 17 and 19 October 2019, all seven of eROSITA's 'X-ray eyes' acquired their first combined images of the hot Universe. "A first light event is always an exciting moment. After many years of preparation and development, followed by a perfect launch from the Kazakh Steppe, we can now see that all of our efforts have been worthwhile.

The Max Planck Institute for Extraterrestrial Physics (MPE) has done a fantastic job, and we are delighted that this important German mission can now begin its observations," enthuses Thomas Mernik, eROSITA Project Manager at the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt; DLR) Space Administration.

The development of eROSITA by MPE, which is based in Garching, near Munich, was financed by the DLR Space Administration. The occasion of first light was celebrated in Garching and at the 70th International Astronautical Congress 2019 in Washington D.C., together with the Russian partners. In addition to the German X-ray telescope, the Russian-German Spektr-RG (SRG) space observatory is also equipped with the Russian ART-XC instrument.

'First light', after a short delay
eROSITA consists of seven individual telescope modules that collect incident X-ray radiation emitted by hot sources in the Universe. Since the launch of SRG in July 2019, engineers and astrophysicists at MPE have been working on commissioning the telescope. At first, only individual components were switched on and their functionality was carefully checked out, step by step. However, in the first few weeks after being put into operation, the electronics of some of the telescope modules malfunctioned, resulting in parameters being adjusted for no apparent reason.

"In order to avert possible damage to the instrument, the modules were 'put to sleep' and individual components were carefully 'awakened' so that they could be tested separately. The 'culprit' was suspected to be cosmic radiation, which may have triggered minor errors in some of the modules. Since then, we have come to the conclusion that the risk is acceptable, so after a brief hiatus eROSITA has now resumed full operations," says Mernik. Since 13 October 2019, all seven modules of the X-ray telescope have been simultaneously observing the sky using their 'tailor-made' CCD cameras.

A historic moment in the life of a telescope
"This is a pivotal moment in the life of the telescope, as this is when we see for the first time how good the overall system really is. With its observations of the Large Magellanic Cloud, eROSITA has demonstrated its capabilities to impressive effect. We have obtained razor-sharp images with remarkably little background noise. These first impressions allow us to anticipate great things over the coming years," Mernik is pleased to report. One source of the X-rays is an old acquaintance, which has already been observed with other telescopes such as XMM-Newton.

"We chose the Large Magellanic Cloud because it has often been observed with other X-ray telescopes. The direct comparison with earlier images shows the true performance of eROSITA. XMM-Newton mainly observes specially selected objects and therefore sees them in great detail. eROSITA also provides detailed images - but will study the entire hot Universe. In addition, we selected this region because it is very close to the Solar System - at a distance of 'only' 170,000 light-years," says Mernik.

With the second observation target - the two merging galaxy clusters A3391 and A3395 - the opposite is the case. "Both galaxy clusters are 800 million light-years away from Earth. Here, we wanted to see how far eROSITA can look into the hot universe and how clear the images still are at this unimaginably long distance. We were all surprised by the excellent result," Mernik is pleased to say.

eROSITA will be taking more such images in the coming months in order to check out the overall performance of the German X-ray telescope. After a more than three-month-long journey, SRG has now entered orbit around the second Lagrange Point of the Sun-Earth system (L2), which is 1.5 million kilometres 'behind' Earth, when viewed from the Sun. From here, eROSITA will embark upon a whole-sky survey, to create a map of the hot structures in the Universe.

eROSITA will create a map of the Universe in the X-ray spectrum
An X-ray telescope observes particularly hot matter in the Universe. Typical X-ray sources are binary systems, in which a star similar to the Sun and neutron star or black hole orbit about a common centre of mass.

In this case, gas is drawn from the star due to the strong gravitational field of its compact companion. This gas then falls into an accretion disc surrounding the companion object. While doing so, it reaches extremely high temperatures and emits radiation in the X-ray spectrum. eROSITA was also designed to observe active galactic nuclei (AGN).

These are super-massive black holes with masses of several million times that of the Sun. They are also surrounded by hot accretion discs that are easy to observe in the X-ray spectrum. In addition, one of the mission's particular areas of focus is the discovery and mapping of galaxy clusters. These each consist of several hundred galaxies and draw in gas from their surroundings due to their gravity. The incoming gas heats up, creating intense radiation in the X-ray spectrum. Over the course of the mission, eROSITA will find and map up to 100,000 galaxy clusters.

For these purposes, eROSITA will survey the entire sky every six months and create a deep and detailed map of the Universe over the course of its four-year nominal mission. This enormous cosmic inventory of the hot Universe will help researchers better understand the structure of the cosmos and its development. The expansion of the Universe represents a major problem for astronomers.

For reasons yet unknown, this expansion is accelerating. The mysterious force that is driving the Universe apart is referred to as Dark Energy. "Since eROSITA will help us to better understand the dynamics of the largest structures in the Universe, we could also learn more about cosmic expansion. This means that eROSITA will provide us with the data needed to get closer to solving the puzzle of Dark Energy," explains Mernik.

Spektr-RG - a space mission with numerous partners
Spektr-RG (SRG) is a space mission with numerous partners. On the Russian side, it involves the space agency Roscosmos, the space company NPO Lavochkin and the Institute of Space Research of the Russian Academy of Sciences (IKI). The German eROSITA X-ray telescope was developed and built with funding from DLR Space Administration by the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, near Munich, in collaboration with the Leibniz Institute for Astrophysics in Potsdam (AIP) and the universities of Erlangen-Nuremberg, Hamburg and Tubingen.

The universities of Munich and Bonn are also involved in the scientific evaluation of the eROSITA data. The partner institutes involved in the German telescope have created software for data analysis, mission planning and simulations, as well as parts of the hardware, but the MPE was primarily responsible for the hardware. Many components were developed there, some in partnership with selected companies from industry. The telescope was also assembled in Garching, from where it was transported to Moscow. There, it was integrated into the SRG spacecraft platform together with the Russian ART-XC instrument.

Related Links
eROSITA at MPE
Understanding Time and Space

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