The x-ray image above shows one of the Milky Way’s neighbours, the Large Magellanic Cloud. This image contains fuzzy emissions from gas, intervening stars and distance galactic cores, and the remnants of a supernova first spotted from Earth 32 years ago right at the centre, all orbiting our galaxy 158,200 light years away. But back on Earth, getting to this point took decades of work—and some dramatic failures.
Scientists at the Max Planck Institute for Extraterrestrial Physics have just revealed the gorgeous first light images from the eROSITA telescope, taken from space in the past two weeks. Eventually, they hope to use eROSITA and the ART-XC telescope, both launched as part of the Spectrum-Roentgen-Gamma (SRG) mission over the summer, to map the sky in x-rays to better understand the very structure of the universe. This mission, eROSITA especially, carries the ghosts of ABRIXAS, which met its unfortunate end shortly after launch in 1999.
Scientists have been hungry for a powerful x-ray survey that measures higher-energy, or “hard” x-rays, for decades. Soviet scientists proposed one such mission in 1987, but it died with the collapse of the Soviet Union. X-rays astronomy studies some of the biggest questions astronomers have about our universe by probing some of its most dense, hot regions—those in black holes, galaxy clusters, or supernova remnants. Getting a handle on galaxy clusters are especially important for understanding the large-scale structure and evolution of the universe. Powerful x-ray telescopes like the Chandra X-ray Observatory and XMM-Newton are used to observe smaller regions of the universe and individual objects.
But scientists need a survey telescope, one that maps the entire sky in x-ray wavelengths to answer broader questions about the universe and as a wayfinder to know where to point those other fancy x-ray telescopes. eROSITA is a way to finally realise that dream. While the image of the Large Magellanic Cloud is beautiful, ultimately the telescope will image the entire sky in hard x-rays, revealing never-before-seen information on the night sky—and perhaps 100,000 new galaxy clusters. More galaxy clusters will help scientists better constrain the behaviour of dark matter, the seemingly transparent stuff that seems to make up most of the universe’s mass, and dark energy, the presence of which seems to drive an increase in the universe’s expansion rate.
Individual galaxies collide, merge, and evolve, but they can’t give you as much information on the history of the entire universe itself. But the galaxy clusters eROSITA and ART-XC hope to image “are like a fossil record of the assembly history of the large-scale structure of the universe,” Grant Tremblay, astrophysicist at the Harvard-Smithsonian Centre for Astrophysics, told Gizmodo. “They become nodes of the cosmic web. It’s like trying to map a spider web, and can’t see the actual filaments, but we can see the nodes that connect them. We can still discern at least a large part of the structure of the web itself.”
The first x-ray sky survey ROSAT, short for Röntgensatellit, launched in 1990. That telescope performed the first ever all-sky x-ray survey in the lower-energy “soft” x-ray wavelengths, and made thousands of observations, according to an eROSITA scientific summary on the arXiv physics preprint server. But a wealth of data seemed to hide beyond the reach of ROSAT in those higher energy “hard” wavelengths. German scientists proposed a successor to ROSAT called ABRIXAS (A Broadband Imaging X-ray All-sky Survey).
ABRIXAS would combine seven smaller telescopes into a compact, yet powerful satellite, relying on the mirror and detector technology incorporated into the XMM-Newton telescope. It took just three years to develop on a relatively small, $US20 ($29) million budget, and was poised to open up a universe to which ROSAT was blind. But just three days after its launch, disaster struck; its main battery failed, and it would never make an observation.
The loss was “tragic,” Jonathan Grindlay, Professor of Practical Astronomy at the Harvard College Observatory of Harvard University, told Gizmodo. There hasn’t been a survey of the sky’s hard x-rays since, Tesla Jeltema, associate professor, at the University of California, Santa Cruz, told Gizmodo.
Scientists pined for a replacement to reveal the galaxy clusters and other higher-energy objects that they knew that ABRIXAS would be able to find. To this day, “there are many projects where everyone is still using the ROSAT all-sky survey data,” Jeltema said.
The German institutes behind ABRIXAS proposed a replacement in 2002—a nearly identical mission that would dock on the ISS—but realised that the ISS wouldn’t be an ideal place to fly it, and there wasn’t room. Then, in 2005, ABRIXAS’ (and now eROSITA’s) Peter Predehl took a flight with the then-director of the Max Planck Institute for Theoretical Physics’ high energy group, Günther Hasinger.
The two discussed a telescope with an effective area five times larger than ABRIXAS based on similar technology and ran some quick calculations. When they landed, they asked a colleague to whip up a presentation overnight to present to their Russian counterparts the next day. Thus, the extended ROentgen Survey with an Imaging Telescope Array, or eROSITA was born. It effectively became the first all-sky hard-x-ray imaging campaign, one that would be 25 times more sensitive than ROSAT, looking at the universe as it was as much as nine billion years ago.
Without ABRIXAS’ failure, “we’d never have eROSITA,” Predehl told Gizmodo. “It’s much bigger and much better than ABRIXAS ever could have been.”
ABRIXAS’ failure gave scientists time to incorporate that satellite’s best software as well as hardware like its mirrors, plus more recent advances in detector technology. The Max-Planck Society and DLR secured funding for eROSITA in 2009, and the mission was scheduled to fly alongside the Russian hard x-ray surveying telescope, ART-XC—realising the dream of the Soviet scientists decades before. After some of the more typical delays that come with big projects, Roscosmos launched the mission this past summer.
These surveys have value beyond just wayfinding for other x-ray missions as well, explained Jeltema. If optical or radio telescopes find something weird, these surveys might be hold extra information to further illustrate the other findings.
But eROSITA and ART-XC will most importantly serve as an important tool for the future. Scientists are dreaming up even newer, more powerful x-ray telescopes, such as the Lynx X-ray Observatory, one four new flagship missions that the National Academy of Sciences might propose in their decadal survey.
“Lynx [will be] ready to pursue the immense discovery space that eROSITA will crack open,” said Tremblay.
Predehl seems to have mostly moved past the ABRIXAS days—he scoffed at me when I asked how it felt to see the eROSITA first light image after a 20 year delay. “It’s fantastic,” he said.