The German eROSITA Consortium (eROSITA-DE), led by the Max Planck Institute for Extraterrestrial Physics (MPE), has completed the most comprehensive census of the high-energy universe ever assembled.
Released as eROSITA Data Release 2 (DR2), the new high-energy universe catalog includes nearly two million X-ray sources, essentially doubling the number cataloged in DR1. The team behind the achievement said the new catalog, which has been made available to the wider scientific community, marks a “major step forward” in mapping the entire high-energy universe.
“DR2 is the best inventory of the X-ray sky we have to date and opens the door to robust statistical studies of cosmic populations,” explained Miriam E. Ramos-Ceja, Ground Segment Manager of the eROSITA instrument and lead author of the DR2 publication.
High-Energy Universe Catalog Nearly Doubles Previous Sources
DR2 was built from the first three scans captured by the eROSITA telescope during the Spectrum-Roentgen-Gamma (SRG) mission. According to a statement emailed to The Debrief, the new high-energy map combined data from multiple passes over the sky, significantly increasing the instrument’s survey depth.
After beginning its mission in December 2019, the observatory surveyed the entire sky every six months. This mission length increased the scans’ depth and sensitivity. The new DR2 includes data captured during the mission’s first 556 days. MPE scientists said the final catalog comprises three “full sky surveys.”
“By stacking these observations, the survey reaches significantly fainter fluxes than the first release, enabling the large increase in detected sources,” they explained.
The approach also revealed a large population of previously uncatalogued systems.
For example, the DR2 catalog includes nearly two million X-ray sources detected in the 0.2–2.3 keV band. The survey also revealed over 1.9 million “point-like sources.” The team said these detections are “primarily stars and actively accreting supermassive black holes.” The survey also added around 64,000 extended sources, including newly cataloged galaxy clusters, nearby galaxies, and supernova remnants.
The Full Diversity of the X-ray Sky
Along with the primary catalog, the new release includes a complementary “hard-band” catalog that includes 15,000 sources detected at higher energies (2.3–5.0 keV). The team said these detections revealed “heavily obscured and intrinsically energetic systems” that scans performed at ‘softer’ X-ray energies typically overlook.
“Together, these catalogues capture the full diversity of the X-ray sky, from nearby stellar coronae to distant supermassive black holes and massive galaxy clusters,” the team explained, adding that many of the cataloged objects are newly identified in X-rays, while others “can now be studied with substantially improved precision.”
Although more analysis is needed to accurately characterize every high-energy source discovered by the new survey, the team said the DR2 includes “multi-wavelength information” that associates the newly cataloged X-ray detections with their most likely optical and infrared counterparts.
“X-ray detection is only the first step,” explains Mara Salvato, eROSITA spokesperson and chair of the follow-up working group. “By linking X-ray sources to their counterparts at other wavelengths, we can work out what these objects are, where they sit on the cosmic distance ladder, and build clean, well-defined samples on an unprecedented scale.”
This data suggests that approximately 88% of the newly discovered sources come from outside the Milky Way Galaxy. The team said the new sources are “dominated by accreting supermassive black holes.”
Data Suggests Unexpected Abundance of Rapidly Growing Black Holes
When discussing the significance of the new DR2 high-energy catalog, the team highlighted its coincidental release with the latest Sloan Digital Sky Survey (SDSS).
“By combining SDSS spectroscopy with eROSITA’s X-ray data, researchers can build three-dimensional maps of active black holes across the sky, revealing how these rapidly growing objects are distributed and evolve across cosmic time,” they explained.
For example, combining eROSITA’s X-ray catalog with the SDSS spectroscopic and photometric redshift data has already resulted in what the team described as “one of the largest and most detailed studies of accreting supermassive black holes to date.” The researchers said these “elusive objects” formed surprisingly early in the history of the Universe, so capturing new data of their growth at high redshift is invaluable.
“The most luminous black holes at high redshift are like needles in a haystack,” explained the corresponding study’s lead author, William Roster. “Thanks to DR2 we found more needles than expected, suggesting that rapidly growing black holes were more abundant in the early Universe than previously thought”
New World Record of X-ray Sources Won’t Be the Last
Due to a pre-existing agreement between the German and Russian eROSITA consortia, the new DR2 covers the western Galactic hemisphere. However, the researchers note, within this region of the sky, the new catalog “represents the largest and most homogeneous public X-ray dataset currently available.”
eROSITA Principal Investigator Andrea Merloni said the new dataset of unprecedented scale and completeness has been made available to the global astronomical community. MPE Director Kirpal Nandra agreed, adding that the MPE team hopes the most complete catalog of high-energy sources ever assembled will provide “a unique foundation for discoveries ranging from rare objects to large-scale population studies.”
“eROSITA just set another world record in terms of X-ray source numbers – and it won’t be the last,” Merloni concluded.
The eROSITA-DE DR2 catalogs, upper-flux-limit server, and full documentation are publicly accessible via the eROSITA-DE Science Data Archive.
Christopher Plain is a Science Fiction and Fantasy novelist and has spent the last six years as Associate News Editor and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him at christopher@thedebrief.org