Second data release nearly doubles the previously known eROSITA X-ray sources to two million
The colour image shows X-ray sources in the western galactic hemisphere of the X-ray sky. The Galactic plane lies horizontally through the centre of the image. Sources in the eROSITA catalogue are plotted with their red, green and blue brightness showing their count rate in soft (0.5-1.0 keV), medium (0.5-1.0 keV) and hard (1.0-2.0 keV), respectively. The sky is plotted using an azimuthal equal area projection.
© Jeremy Sanders / MPE
To the Point
- Record for X-ray Sources: DR2 lists over 1.9 million pointlike sources such as stars and supermassive black holes, plus about 64,000 extended sources including galaxy clusters and supernova remnants.
- Survey depth, sensitivity: Combining data from three fullsky scans, DR2 detects fainter X-ray fluxes and reveals many previously unknown sources.
- Multiwavelength identification: DR2 links X-ray detections to optical and infrared counterparts using six new catalogues, improving the understanding and classification of cosmic objects.
- Collaboration with SDSS: The release coincides with the Sloan Digital Sky Survey's twentieth data release, enabling 3D mapping of active black holes and studies on their growth across cosmic time.
The German eROSITA Consortium (eROSITA-DE), led by the Max Planck Institute for Extraterrestrial Physics (MPE), has released its second major public dataset, eROSITA Data Release 2 (DR2). The new catalogue comprises close to two million X-ray sources-approximately doubling the number of previously released eROSITA sources in the X-ray sky and marking a major step forward in mapping the high-energy Universe.
Built from the first three all-sky scans of the eROSITA telescope aboard the Spectrum-Roentgen-Gamma (SRG) mission, DR2 provides the most comprehensive catalogue of the X-ray Universe currently available to the scientific community. By combining multiple passes over the sky, the release significantly increases the survey depth and reveals large populations of previously undetected sources.
The main DR2 catalogue contains nearly two million X-ray sources detected in the 0.2-2.3 keV band and includes more than 1.9 million point-like sources, primarily stars and actively accreting supermassive black holes, as well as around 64,000 extended sources such as galaxy clusters, nearby galaxies, and supernova remnants. Compared to the first data release, the number of detected sources has roughly doubled.
A complementary hard-band catalogue adds nearly 15,000 sources detected at higher energies (2.3-5.0 keV), tracing heavily obscured and intrinsically energetic systems that are often missed at softer X-ray energies. 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. Many of these objects are newly identified in X-rays, while others can now be studied with substantially improved precision.
After the start of operations in December 2019, eROSITA surveyed the entire sky every six months, progressively increasing depth and sensitivity. DR2 combines data collected over the mission's first 556 days, spanning three full sky surveys (eRASS1-3). By stacking these observations, the survey reaches significantly fainter fluxes than the first release, enabling the large increase in detected sources.
"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," says Miriam E. Ramos-Ceja, Ground Segment Manager of the eROSITA instrument and lead author of the DR2 publication.
Linking X-rays to the broader Universe
To enable physical interpretation, DR2 includes multi-wavelength information that associates the X-ray detections with their most likely optical and infrared counterparts. Based on this information, roughly 88% are extragalactic, dominated by accreting supermassive black holes.
"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."
A joint milestone with SDSS
The release coincides with the twentieth data release of the Sloan Digital Sky Survey (SDSS), which includes extensive optical spectroscopy of eROSITA-DE sources. Together, these datasets represent the culmination of nearly a decade of collaboration between the German eROSITA Consortium and the SDSS collaboration. 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.
Combining eROSITA's X-ray catalogue with spectroscopic and photometric redshifts enabled one of the largest and most detailed studies of accreting supermassive black holes to date. These elusive objects formed surprisingly early in the history of the Universe, and eROSITA has provided a new census of their growth at high redshift. "The most luminous black holes at high redshift are like needles in a haystack. 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" says William Roster, lead author of the corresponding study.
In contrast to the first public data release, DR2 is a catalogue-focused release. It provides rigorously validated source lists derived from the combined eRASS:3 observations, along with an updated upper-flux-limit service that allows researchers to quantify non-detections across the sky.
The data cover the western Galactic hemisphere, reflecting the agreed data-sharing arrangement between the German and Russian eROSITA consortia. Within this region, DR2 represents the largest and most homogeneous public X-ray dataset currently available.
Enabling the next wave of discoveries
"This is a dataset of unprecedented scale and completeness, now in the hands of the global astronomical community," says eROSITA Principal Investigator Andrea Merloni. "With nearly 2 million sources, DR2 provides a unique foundation for discoveries ranging from rare objects to large-scale population studies." MPE Director Kirpal Nandra adds: "eROSITA just set another world record in terms of X-ray source numbers - and it won't be the last."
The eROSITA-DE DR2 catalogues, upper-flux-limit server, and full documentation are publicly accessible via the eROSITA-DE Science Data Archive.
