Our deepest view of the X-ray sky

Over the course of 182 days, the eROSITA X-ray telescope has completed its first full sweep of the sky which it embarked upon about a year ago. This new map of the hot, energetic universe contains more than one million objects, roughly doubling the number of known X-ray sources discovered over the 60-year history of X-ray astronomy. Most of the new sources are active galactic nuclei at cosmological distances, marking the growth of gigantic black holes over cosmic time.

The energetic universe: The first eROSITA all-sky survey with colour-coded energies (red for energies 0.3-0.6 keV, green for 0.6-1 keV, blue for 1-2.3 keV). CREDITS: Jeremy Sanders, Hermann Brunner und das eSASS-Team (MPE); Eugene Churazov, Marat Gilfanov (im Namen von IKI)

A million X-ray sources revealing the nature of the hot universe—this is the impressive harvest of the first scan of the entire sky with the eROSITA telescope onboard SRG. This first complete sky image from eROSITA is about four times deeper than the previous all-sky survey by the ROSAT telescope 30 years ago, and has yielded around 10 times more sources: about as many as have been discovered by all past X-ray telescopes combined.

While most classes of astronomical objects emit in X-rays, the hot and energetic Universe looks quite different to the one seen by optical or radio telescopes. Looking outside the body of our Galaxy, most of the eROSITA sources are active galactic nuclei, accreting supermassive black holes at cosmological distances, interspersed with clusters of galaxies, which appear as extended X-ray haloes shining thanks to the hot gas confined by their huge concentrations of dark matter. The all-sky image reveals in exquisite detail the structure of the hot gas in the Milky Way itself, and the circum-galactic medium, which surrounds it, whose properties are key to understanding the formation history of our Galaxy. The eROSITA X-ray map also reveals stars with strong, magnetically active hot coronae, X-ray binary stars containing neutron stars, black holes or white dwarves, and spectacular supernova remnants in our own and other nearby galaxies such as the Magellanic clouds.

While the team is now busy analysing this first all-sky map and using the images and catalogues to deepen our understanding of cosmology and high-energy astrophysical processes, the telescope continues its sweep of the X-ray sky. “The SRG Observatory is now starting its second all-sky survey, which will be completed by the end of this year”, says Rashid Sunyaev, Lead Scientist of the Russian SRG team. “Overall, during the next 3.5 years, we plan to get seven maps similar to the one seen in this beautiful image. Their combined sensitivity will be a factor of five better and will be used by astrophysicists and cosmologists for decades.”

Kirpal Nandra, head of the high-energy astrophysics group at MPE, adds “With a million sources in just six months, eROSITA has already revolutionized X-ray astronomy, but this is just a taste of what’s to come. This combination of sky area and depth is transformational. We are already sampling a cosmological volume of the hot Universe much larger than has been possible before. Over the next few years, we’ll be able to probe even further, out to where the first giant cosmic structures and supermassive black holes were forming.”

Source: “Our deepest view of the X-ray sky” PhysOrg, 19 June 2020. <https://phys.org/news/2020-06-deepest-view-x-ray-sky.html>

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