The widest X-ray survey of the night sky has revealed some of the most extreme and violent processes in the universe, from forbidden stars to pulsating supermassive black holes, as well as tracing the universe鈥檚 overall web-like structure.
The eROSITA X-ray telescope, which was launched aboard the Russian-German Spektr-RG space observatory in 2019, is designed to capture the broadest and most sensitive view of the universe in X-rays. This light is produced in ultra-high energy events, such as when gas is heated to millions of degrees around black holes or in between galaxies. 鈥淭he power of the instrument is such that it surpasses anything that has been used to do similar surveys in X-rays, by quite some margin,鈥� says at the Max Planck Institute for Extraterrestrial Physics in Germany.
The telescope鈥檚 preliminary data has already led to discoveries, such as spotting colossal bubbles of X-ray emitting plasma in our galaxy, the Milky Way. Its operation was suspended in 2022 after Russia invaded Ukraine, but a full dataset from the first of four passes over the night sky, consisting of more than 700,000 supermassive black holes in distant galaxies and 180,000 X-ray emitting stars from the Milky Way, has now been shared with astronomers. Here are some of the most eye-popping discoveries.
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1) Pulsating supermassive black holes
eROSITA has found two (QPE), extremely powerful X-ray bursts that repeat every few hours and originate from supermassive black holes similar in size to the one at the centre of the Milky Way. Astronomers only knew of four QPEs before this, bringing the total to six.
These events can look brighter to us than the light from all the stars in their galaxy put together. There isn鈥檛 a clear explanation for how they happen, but astronomers think they probably have something to do with massive stars orbiting very close to the black hole and having their matter sucked away. 鈥淏y looking at these periodic eruptions, we can infer a lot of things about the very close stars around supermassive black holes,鈥� says Merloni, who has been involved in many of the discoveries.
2) Mysterious forbidden stars
Young or quickly rotating stars tend to produce lots of X-ray radiation, but as they grow old and become red giants or supergiants, they slow down and produce fewer or almost no observable X-rays. eROSITA has found a and supergiants that appear to be unusually active, displaying behaviour that they shouldn鈥檛. 鈥淲e see it鈥檚 not that many, but because we now have [data on] hundreds of thousands of stars, we start seeing around a dozen of these giants with X-ray emission,鈥� says Merloni.
Astronomers think that these vigorous old stars might be being stimulated by an unseen companion star, which is injecting energy and matter in the stars鈥� old age.
3) The Goat Horn complex
As well as picking out smaller, individual objects, eROSITA has also spotted some much larger structures, like the Milky Way鈥檚 giant X-ray bubbles. These patches of radiation can be very faint, but the telescope鈥檚 sensitivity means it can still pick them up. While scanning a portion of the southern sky, another of these faint areas using eROSITA, which they have dubbed the Goat Horn complex after a bright, twisted shape near its bottom.
It is unclear what is causing the X-ray emissions to create this overall shape in the sky, which covers many star constellations, including Dorado, Hydra and Chamaeleon, but it appears to sit in broadly the same region of the sky as another unexplained patch of radio wave radiation, known as radio loop XII. If the two patches of different energy radiation are linked, they might have a common origin, such as from an ancient supernova explosion, newly formed stars or hot gas around a satellite galaxy of the Milky Way. Whichever it is, understanding the Goat Horn complex鈥檚 origin will help us fill in a little more of our universe鈥檚 history, says Merloni.
4) The longest intergalactic X-ray filament
Astronomers believe that much of the universe鈥檚 matter lies not within stars or galaxies, but in vast pools of gas in between clusters of galaxies. This gas is extremely hot, at around a million degrees, and it can only be seen using X-rays, but is still incredibly faint. Using the eROSITA survey, scientists have now of this hot gas linking two galaxy clusters. It spans more than 40 million light years, a distance three times longer than the clusters themselves and the longest of its kind.
5) The spiderweb universe
The strangely long filament isn鈥檛 the only one out there and is the start of us seeing a much bigger picture. It seems to be just one segment of a vast cosmic web that links clusters of galaxies together. This web has been roughly mapped out by looking at the galaxies themselves, much like you can see a spider鈥檚 web because of the morning dew attached to its spindles, but there is much we still don鈥檛 know about it. The hot gas filaments themselves have been impossible to image until eROSITA. 鈥淲e have not yet got to the point where you can see this entire filamentary structure, but we have started seeing some of these filaments in more favourable individual cases,鈥� says Merloni. Further eROSITA results are expected soon, which should help us map out this spiderweb universe in detail.
