Ultraluminous X-ray Sources: The First eROSITA Catalog ✛
Context
Ultraluminous X-ray sources (ULXs) are points in other galaxies that shine in X-rays brighter than a stellar-mass black hole can stably emit during accretion. Their nature is not fully understood: some may be neutron stars or stellar-mass black holes accreting in a supercritical regime, while others may be intermediate-mass black hole candidates. Until now, ULX catalogs were built from pointed observations targeting galaxies selected for other scientific reasons, introducing strong biases. This work uses the first all-sky X-ray survey by eROSITA, which uniformly covers the entire western Galactic hemisphere, thus providing the first truly unbiased sample of ULXs in the local Universe. Alongside this work on the archive are studies on axions, dark matter, pulsars, and dark photons, but thematically it is closest to the ULX catalogs previously compiled from ROSAT, XMM-Newton, Chandra, and Swift data.
Methods
The researchers took the main source catalog from the first eROSITA survey (eRASS1), containing 930,203 objects in the 0.2–2.3 keV band, and matched it against the HECATE galaxy catalog, which includes 204,733 galaxies with known distances, shapes, and star formation rates. For each galaxy, they defined an elliptical D25 region (the 25 mag/arcsec^2 isophote in the B band) and searched for X-ray sources within it, excluding a central zone of radius 12 arcseconds to remove active galactic nuclei. For the remaining sources, they extracted spectra and, assuming a power-law spectrum with photon index 1.7 and absorption of $10^{21}$ cm^-2, computed luminosities in the 0.2–5.0 keV band. Sources with luminosities above $10^{39}$ erg/s were considered candidates. Each candidate was then manually checked by at least two astronomers using X-ray and optical images, and cross-matched with catalogs of supernovae, stars, and active galactic nuclei to remove known contaminants. To assess completeness and sensitivity, they ran simulations using the SIXTE package and the actual eROSITA pointing, and to estimate contamination by background active galactic nuclei, they integrated their logN-logS distribution over the galaxy areas.
Results
The main sample contains 90 ULX candidates in 85 galaxies, of which 53 are newly identified. The extended sample includes 260 less reliable candidates, of which 245 are new. The catalog is complete out to about 7 Mpc for luminosities of $10^{39}$ erg/s, and for hyperluminous sources (above $10^{41}$ erg/s) out to 70 Mpc. Expected contamination by background active galactic nuclei does not exceed 29% in the main sample, and source confusion becomes noticeable only at distances greater than 13 Mpc. Comparison with previous catalogs shows that 37 of the 90 main candidates were already known, but 13 new sources lie in regions previously observed by other instruments without ULX identification. At the same time, many known ULXs with fluxes sufficient for eROSITA detection are absent from the new sample, indicating strong variability of these objects on timescales of months to years.
Implications
This work differs from neighboring catalogs in that it builds a ULX sample from a continuous all-sky survey rather than accumulated pointed observations. Previous catalogs, such as those by Walton et al. (2022) or Tranin et al. (2024), relied on XMM-Newton, Chandra, and Swift archives, where galaxies were observed for various reasons often unrelated to ULXs, creating strong biases. Here, galaxy selection is determined only by their inclusion in the eROSITA survey footprint and the availability of optical data in HECATE, making the sample representative of the local Universe. This allows, for the first time, a proper study of ULX demographics, their relation to host galaxy properties, and the luminosity function without needing complex corrections for unknown selection effects.
Future development
The catalog will serve as a basis for population studies of ULXs: in a companion paper, the authors use it to construct the luminosity function and analyze dependencies on host galaxy properties. In the future, data from the five completed eROSITA surveys (eRASS1–eRASS5) will allow increasing the sample and achieving completeness out to about 14 Mpc, while the six-month cadence of observations will enable systematic studies of ULX variability and searches for periodicities related to orbital motion or accretion disk precession.
Impact
The results will impact research on accretion in extreme regimes, the evolution of X-ray binaries, the search for intermediate-mass black holes, and understanding the role of ULXs in the early Universe.
Next steps
The immediate next step is a population analysis of the main sample, accounting for incompleteness and contamination, as well as optical spectroscopy of new candidates to confirm their association with galaxies and rule out background active galactic nuclei.
Key open problems
In high-energy astrophysics, the nature of ULXs remains an open question: are they neutron stars or stellar-mass black holes in a supercritical accretion regime, or do some harbor intermediate-mass black holes? This unbiased catalog will allow testing whether there is a break in the luminosity function that could indicate a distinct population of more massive accretors, thereby bringing us closer to solving this puzzle.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors have produced the first catalog of ultraluminous X-ray source candidates from the eROSITA survey, matching sources against galaxies in the HECATE catalog and manually vetting the candidates. The main sample comprises 90 sources (53 of them new), and the extended sample contains 260 sources, complete out to 7 Mpc.
Where its strength is
The strength of this work lies in the unbiased nature of the sample: for the first time, a continuous all-sky survey is used rather than pointed observations, allowing the ULX population to be studied without systematic biases.
Where it could go next
- It might be interesting to cross-match the catalog with studies of X-ray source variability: the archive contains an analysis of transients that flare and fade, which could help identify ULX subclasses based on their temporal behavior.based on: Cosmic Beacons That Flash and Fade arXiv:2601.08047
- There is also an analysis of a catalog of red points built from multicolor photometry; perhaps a similar approach, selecting by optical properties of host galaxies, could refine the connection between ULXs and galaxy types and star formation history.based on: Archipelago in the Ocean of Data: Red Points Without Color Boundaries arXiv:2607.22835v1
Worth trying
- Compare the distribution of ULXs across galaxy morphological types with predictions from supercritical accretion models.
- Test for a correlation between ULX luminosity and the star formation rate of the host galaxy, using HECATE data.
- Search among the new candidates for objects with extreme luminosities that might qualify as intermediate-mass black hole candidates.
Why this may matter
The catalog opens the door to systematic population studies of ULXs and the search for rare objects such as intermediate-mass black holes, all within an unprecedentedly clean sample.
Nearby works in our archive
- Sky Hotspots: The Senders of Ultra-Particles Identifiedsimilarity 0.53
- Winds from Black Holessimilarity 0.52
- Solar Neutrinos in a Dark Matter Trap: An Unexpected Tool for Helioseismologysimilarity 0.51
- Cosmic Beacons That Flash and Fadesimilarity 0.50
- Archipelago in the Ocean of Data: Red Points Without Color Boundariessimilarity 0.50
The archive is sparse around this work: the closest review is only 0.53, against a usual 0.64. It looks like few have drilled nearby.
Despite the name 'ultraluminous,' a typical ULX emits only about 20 photons detected by eROSITA over the entire observation period, enough to estimate luminosity but not to study the spectrum in detail.
In Arthur C. Clarke's novel 'Rendezvous with Rama,' X-ray sources in other galaxies are mentioned as possible beacons of extraterrestrial civilizations; ULXs, with their extreme luminosity and variability, could serve as such natural beacons, although their nature is most likely related to accretion onto compact objects.
Key numbers
- main sample: 90 candidates
- extended sample: 260 candidates
- catalog completeness: 7 Mpc for 1039 erg/s
- active galactic nuclei contamination: no more than 29%
- source confusion threshold: 13 Mpc