The eROSITA consortium has released its second public catalogue of the X-ray sky. Built from three complete surveys, Data Release 2 contains nearly two million sources in its main catalogue, roughly twice the number in the first release.
Most are point-like detections associated with stars or actively feeding supermassive black holes. About 64,000 appear extended, a category that includes galaxy clusters, nearby galaxies and supernova remnants. A separate catalogue adds nearly 15,000 sources detected at higher X-ray energies.
Those numbers are the end of a long conversion process. eROSITA does not take a conventional photograph and label the objects it sees. It records individual X-ray events, combines repeated passes, models the instrument and the background, then assigns probabilities to possible sources.
eROSITA is an array of seven aligned X-ray telescopes aboard the Spectrum-Roentgen-Gamma spacecraft. Each telescope module uses mirrors designed to guide incoming X-rays onto a CCD detector. Together they provide a field of view about one degree wide.
During survey operations, the spacecraft rotated once every four hours around an axis pointed near the Sun. That axis shifted by about one degree per day as Earth moved around the Sun. The geometry drew great circles across the sky and produced full coverage roughly every six months.
A typical sky position passed through the field of view several times during each survey, often in exposures lasting up to 40 seconds. Data Release 2 stacks the first three surveys, covering 556 days of operations. Repeated passes add more photons from persistent sources, which improves the chance of finding objects too faint to clear the detection threshold in a single survey.
The public release covers the western Galactic hemisphere. That is not an instrumental limit. Survey data rights were divided between the German and Russian eROSITA consortia, and this release comes from the German side.
Raw detections first need cleaning and calibration. The pipeline reconstructs each event, estimates its energy and determines where the telescope was pointing. It rejects damaged frames, invalid detector patterns and periods affected by background flares.
The sky is then divided into 4,700 overlapping tiles. Overlap reduces the chance that a source near the edge of one tile is measured poorly or missed. For each tile, software builds images, exposure maps and models of the background expected without an astronomical source.
The detection stage compares the recorded pattern with the telescope’s point spread function, which describes how a point source is blurred by the optics. It also tests extended models for objects that cover a larger patch of sky. The resulting fit estimates position, brightness and whether the source is point-like or extended.
Every candidate receives a detection likelihood. This is related to the probability that random background fluctuations could have produced the signal. The main public catalogue requires a likelihood of at least six. Raising that threshold creates a cleaner sample but discards more real, faint objects. Lowering it improves completeness while admitting more false detections.
That trade-off is why the catalogue includes warning flags. Bright diffuse regions, crowded areas and fragments of large extended objects can confuse the detection algorithm. Very bright optical stars can also generate false X-ray-like events in the CCDs, a problem called optical loading.
An X-ray position alone usually does not reveal what produced it. The team therefore compared point sources with optical and infrared catalogues from the DESI Legacy Imaging Surveys, Gaia and CatWISE.
The matching is probabilistic. The software considers the separation between positions, their uncertainties, the local density of possible matches and whether an optical or infrared object has properties typical of an X-ray emitter. It also repeats the calculation after shifting X-ray positions to estimate how often convincing matches occur by chance.
Within the DESI imaging footprint, the team identified counterparts for about 1.4 million sources and estimates that roughly 88 percent are outside the Milky Way. Many are active galactic nuclei, the bright regions powered by matter falling toward supermassive black holes.
This cross-matching turns a detection list into something researchers can filter by likely object type, distance and reliability. It also exposes uncertainty rather than erasing it. A faint source can be genuine but variable, a background fluctuation, or associated with the wrong visible object.
Data Release 2 is catalogue-focused. Unlike the first release, it does not publish the full event files, spectra and sky maps for every detection. It provides validated source tables, counterpart catalogues, documentation and an updated service for calculating upper flux limits where no source was detected.
That last tool matters because absence is also a measurement. Researchers comparing another survey with eROSITA need to know whether an object was truly quiet or simply too faint for the local exposure and background.
eROSITA has been in safe mode since February 2022, so the release does not represent new observing. Its advance comes from combining existing scans with improved calibration and processing. A third German data release is planned for the second half of 2028.
The practical lesson is that a sky catalogue is not a finished picture of the Universe. It is a structured set of measurements, models and confidence levels. The value of nearly two million entries depends on preserving that chain from photon to probability.