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German X-ray space telescope captures most complete map of black holes ever

A German-built space telescope is creating the most detailed map of black holes and neutron stars across our universe, revealing more than 3 million newfound objects in less than two years.


The observatory, called eROSITA, launched in 2019 and is the first space-based X-ray telescope capable of imaging the entire sky. It is the main instrument aboard the Russian-German Spectrum-Roentgen-Gamma mission, which sits in a region known as  Lagrange point 2, one of five stable points around the sun-Earth system, where the gravitational forces of the two bodies are in balance. From this vantage point, eROSITA has a clear view of the universe, which it photographs with its powerful X-ray detecting instruments.


Last month, the team behind eROSITA, led by scientists from the Max Planck Institute for Extraterrestrial Physics in Germany, released the first batch of data acquired by the instrument to the wider scientific community for exploration.


"The X-ray telescopes so far have been able to look very deep into the centre to observe the early Universe," Merloni said. "But it has always been very difficult to compile large populations [of black holes, neutron stars and clusters] and create a large catalogue that you could then use to study their cosmological evolution."

"Large survey optical telescopes are now quite commonplace because they are very useful to study cosmology [the evolution of the universe] and things such as dark energy," Merloni said. "But optical telescopes are much easier to design than X-ray telescopes."

"For the first time, we have an X-ray telescope that can be used in very similar ways as the large field optical telescopes that we use today," Merloni said. "With eROSITA, we cover the entire sky very efficiently and can study large-scale structures, such as the entire Milky Way."
Each bright spot in this image, the first all-sky image by eRosita, is black hole or a neutron star. (Image credit: Jeremy Sanders/Hermann Brunner/Andrea Merloni/Eugene Churazov/Marat Gilfanov/IKI/eSASS/MPE)

eRosita detects the bright X-ray glow emitted by hot gas in galaxy clusters, which are the most complex structures in the universe. (Image credit: Esra Bulbul, Jeremy Sanders (MPE))

The map of black holes in the universe 

eROSITA started taking the first images in October 2019. Since then, it has completed three all sky surveys, maps of the sky reflecting the distribution of sources of X-ray radiation in the universe, Merloni said. 

This data has not yet been released to the wider scientific public but Merloni said the catalogues contain information about 3 million sources of X-ray radiation — black holes, neutron stars and galaxy clusters. About 77% of those sources are distant black holes in other galaxies, 20% are neutron stars, stars and black holes in the Milky Way. The remaining 3% are galaxy clusters, he added.

"In the 50 years of X-ray astronomy until eROSITA, in total, if you sum all the [X-ray] sources discovered by all the missions, there were about a million," Merloni said. "We have already discovered three times more than what was known before, although some of it still has to be validated."

Most of the previously known objects were concentrated in pockets, Merloni added, because Chandra and XMM-Newton are very good at diving deep into small sections of the universe. The new black holes, clusters and neutron stars detected by eROSITA are, on the other hand, distributed evenly all over the sky. 

Just as Gaia enabled scientists to move from studying individual stars to visualising the motions and dynamics inside the galaxy (and making leaps in the understanding of its evolution), eROSITA,  too, is expected to open entirely new possibilities. 

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