An X-ray image of half the universe

X-ray astronomy has an eventful 60-year history of exploring the extremes of the universe: from exploding stars to active galactic nuclei, which, with their supermassive black holes, are arguably the most efficient sources of energy in the universe. While most X-ray telescopes were built to take a closer look at such phenomena, eRosita looks at the bigger picture. These include the largest structures in the universe, filaments of hot gas that connect powerful clusters of galaxies and could hold the answers to the biggest questions: how did the universe evolve and why is it expanding?

Here is the first eROSITA sky-survey data release, the largest ever catalogue of high-energy cosmic sources. It comprises an X-ray view of half the sky over Earth, encompassing almost a million high-energy cosmic sources, including over 700,000 supermassive black holes. This catalog, is known as the “eROSITA All-Sky Survey Catalogue (eRASS1)” was published on Feb. 1, 2024.

It represents the most extensive compilation to date of the universe’s most potent energy emitters, such as supernovae and active galactic nuclei fueled by black holes that emit intense X-ray radiation. Additionally, the publication elucidates the largest recognized formations in the cosmos — the cosmic web’s tendrils of heated gas linking galaxies within clusters.

These findings indicate that within a mere six months of commencing observations following its launch on July 13, 2019, eROSITA has uncovered a greater number of high-energy X-ray sources than have been identified over six decades of sky scrutiny

According to a statement: “The eRASS1 observations with the eROSITA telescope were carried out from 12 December 2019 to 11 June 2020. The data published here cover half of the entire sky, the data share of the German eROSITA consortium. In the most sensitive energy range of the eROSITA detectors (0.2-2 keV), the telescope detected 170 million X-ray photons – a record number. In X-ray astronomy, it is possible to measure individual particles of light (photons) with their respective energy in the X-ray spectrum and their arrival time in the detector. The catalogue was then constructed – after careful processing and calibration – by detecting concentrations of photons in the sky against a bright, large-scale, diffuse background. After eRASS1, eROSITA has continued scanning the sky and accumulated several additional all-sky surveys. Those data will also be released to the world in the coming years.

(The Story of the Universe in a Picture)

The 900,000 sources include around 710 000 supermassive black holes in distant galaxies (active galactic nuclei), 180.000 X-ray emitting stars in our own Milky Way, 12.000 clusters of galaxies, plus a small number of other exotic classes of sources like X-ray emitting binary stars, supernova remnants, pulsars, and other objects.”

“These are mind-blowing numbers for X-ray astronomy,” says Andrea Merloni, eROSITA principal investigator and first author of the eROSITA catalogue paper. “We’ve detected more sources in 6 months than the big flagship missions XMM-Newton and Chandra have done in nearly 25 years of operation.”

The statement continues: “Co-ordinated with the release, the German eROSITA Consortium has submitted almost 50 new scientific publications to peer-reviewed journals, adding to the more than 200 which had already been published by the team before the data release. Most of the new papers appear today with selected discoveries including: more than 1000 superclusters of galaxies, the giant filament of pristine warm-hot gas extending between two galaxy clusters and two new ‘Quasi-Periodically Erupting’ black holes. Further studies of how X-ray irradiation from a star may affect the atmosphere and water retention of orbiting planets, and statistical analysis of flickering supermassive black holes .

This first eRASS data release (DR1) makes public not only the source catalogue, but images of the X-ray sky at multiple X-ray energies and even lists of the individual photons with their sky positions, energies and precise arrival times. The software needed to analyse the eROSITA data is also included in the release. For many source classes, supplementary data from other wavebands has also been incorporated into so-called “value-added” catalogues that go beyond pure X-ray information.”

“The scientific breadth and impact of the survey is quite overwhelming; it’s hard to put into a few words,” says Mara Salvato, who as spokesperson for the German eROSITA consortium co-ordinates the efforts of about 250 scientists organised into 12 working groups. “But the papers published by the team will speak for themselves.”

“We’ve made a huge effort to release high-quality data and software,” added Miriam Ramos-Ceja, who leads the eROSITA Operations team. “We hope this will broaden the base of scientists worldwide working with high-energy data and help push the frontiers of X-ray astronomy.”

“The eROSITA collaboration has done an outstanding job with the data release and at the same time publishing all of these amazing new results,” says Kirpal Nandra, Director at MPE. “There’s a lot more to come from us, and we’re looking forward to seeing what the rest of the world will do with the public data.”

eROSITA is an X-ray instrument built by the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany. It is part of the Russian–German Spektr-RG space observatory, which also carries the Russian telescope ART-XC. It was launched by Roscosmos on 13 July 2019 from Baikonur, and deployed in a 6-month halo orbit around the second Lagrange point (L2).

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