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X-ray Space Telescope eROSITA Reveals Millions of Newly Discovered Black Holes

The eROSITA space telescope has revealed its first results with tentative hints at a detailed map of the Universe’s black holes and neutron stars. 

X-ray Space Telescope eROSITA Reveals Millions of Newly Discovered Black Holes
Each bright spot in this image taken by eROSITA, is a black hole or a neutron star. (Image credit: Jeremy Sanders/Hermann Brunner/Andrea Merloni/Eugene Churazov/Marat Gilfanov/IKI/eSASS/MPE)

Launched in 2019, the German-built telescope eROSITA has already discovered more than 3 million previously undiscovered objects, the mission’s senior scientist Andrea Merloni has revealed. The telescope was launched as part of the Spectrum-Roentgen-Gamma (Spektr-RG) observatory and sits at a stable orbital point between the Earth and the Sun known as Lagrangian point 2 (L2).

At the 2021 meeting of the European Astronomical Society, held at the end of June, eROSITA’s operators from the Max Planck Institute released the first X-ray data from the telescope to the wider scientific community. The release was accompanied by the publication of 35 papers by the German eROSITA Consortium on the arXiv preprint server and in a special edition of the journal Astronomy and Astrophysics¹.

The fact that eROSITA operates in the X-ray spectrum means that it allows astronomers and astrophysicists to spot objects that don’t emit light in the visible or infrared regions of the electromagnetic spectrum. This includes black holes and neutron stars  —  stellar remnants left over after massive stars run out of fuel to burn in nuclear fusion and can no longer protect themselves against complete gravitational collapse. 

A study released in December in the journal Nature³ documents solid observations of 1 million X-ray sources  —  equal to the number of such objects discovered in the previous fifty years. Of these million sources, the authors  —  including Peter Predehl also hailing from the Max-Planck Institute  —  say that 80% are active galactic nuclei (AGN) with supermassive black holes at their heart, consuming matter and belching out X-rays, whilst the remaining 20% are active stars in the Milky Way.

Now, Merloni tells SPACE.com² that the space-based -ray telescope has catalogued as many as 3 million previously unobserved sources of X-ray radiation. 

The researcher, who is part of the High Energy Group of the Max-Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, says that over three-quarters of these sources are distant black holes that lie outside the Milky Way. Of the remaining X-ray sources, the vast majority are stars, neutron stars, and black holes within our galaxy. 

Both these published details and those disclosed by Merloni, demonstrate the impressive power of eROSITA. While this is by no means the first X-ray telescope, its ability to image the entire sky has delivered 3 times the X-ray sources provided by five decades of X-ray astronomy.

How eROSITA Positioned Itself Above Other Telescopes

Perhaps the most natural predecessor to the eROSITA mission was the ROentgen SATellite (ROSAT), an X-ray observatory that completed an all-sky X-ray survey. The main difference between eROSITA and the earlier observatory that conducted its survey from 1990 onwards and was decommissioned in 1999 is eROSITA’s three new surveys provide a much deeper view of the Universe.

This deeper view comes from the fact that whereas the ROSAT survey was conducted at energies between 0.1–2.4 keV, the eROSITA survey was carried out at energies of 0.2–8 keV. 

More recent X-ray telescopes have exceeded ROSAT in power, of course, and thus provided deeper views of X-ray sources. In particular, the European Space Agency’s (ESA) X-ray Multi-Mirror Mission (XMM-Netwon) with three sophisticated X-ray telescopes launched in 1999 was described at the time as the most powerful X-ray telescope ever placed in orbit. 

Likewise, NASA’s Chandra X-ray Observatory also launched in 1999, was capable of imagining objects like black holes and neutron stars in exceptional detail thanks to the radiation they emit in the X-ray band of the electromagnetic spectrum. 

Where eROSITA improves on both of these undeniably impressive missions, is whereas they were both restricted to making their stunning observations in just small segments of the Universe, eROSITA has a much wider view of the cosmos. This means it is capable of compiling much larger catalogs of black holes, neutron stars, and other X-ray sources that are more evenly spread across the wider Universe, whilst both XMM-Netwon and Chandra excel at viewing small clusters and groupings of such objects.

In fact, Merloni says that this makes eROSITA an X-ray telescope that is viewing the Universe in a way that is actually analogous to visible light telescopes do. And it should come as no surprise that a telescope as unique as eROSITA is already making waves in X-Ray astronomy.

How eROSITA Will Solve Some of the Universe’s Greatest Mysteries

eROSITA is already delving into some of the Milky Way’s lingering secrets in just its first two years of operation. In particular, in 2020, it was revealed that the X-ray telescope had spotted large-scale X-ray bubbles extending from both the top and bottom of our galaxy’s central region.

This could explain where the energy that is being fed into the Milky Way’s halo  —  which has proved to be an excellent laboratory for the study of shock hot gas  —  is coming from. Current suspects including star formation and the Milky Way’s supermassive black hole Sagittarius A* (SgrA*). Models of galactic evolution predict this phenomenon, and thus its intense study could help us understand how galaxies like the Milky Way formed.

The mysteries that eROSITA is capable of tackling extend well beyond the limits of our galaxy, however. In fact, the X-ray telescope could be critical in unraveling one of cosmology’s most pressing puzzles; the nature of dark energy. 

While we know that the Universe is expanding at an accelerating rate, we currently aren’t sure what is driving this acceleration. Currently, this force is given the placeholder name ‘dark energy.’The space-based X-ray telescope could answer questions about dark energy by studying the distribution and number density of clusters of galaxies. 

Additionally, gathering information about how quickly these clusters of galaxies are separating from each other and from us, the eROSITA telescope could help us pin down an accurate rate of expansion. This, in turn, could help us better understand the geometry and structure of the Universe.

With eROSITA set to continue its mission of conducting all-sky X-ray surveys of the Universe until 2023, it’s a safe bet that in terms of its contribution to science and particularly, our understanding of the Universe, we are currently just seeing the tip of the iceberg. 


1. Campana. S., Combes. F., Forveille. T., et al, [2021], ‘First science highlights from SRG/eROSITA,’ Astronomy & Astrophysics, []

2. Pultarova. T., [2021], ‘German X-ray space telescope captures most complete map of black holes ever,’, []

3. Predehl. P., Sunyaev. R. A., Wilms. J., et al, [2020], ‘Detection of large-scale X-ray bubbles in the Milky Way halo,’ Nature, []

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Robert Lea

Written by

Robert Lea

Robert is a Freelance Science Journalist with a STEM BSc. He specializes in Physics, Space, Astronomy, Astrophysics, Quantum Physics, and SciComm. Robert is an ABSW member, and aWCSJ 2019 and IOP Fellow.


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