The SRG/eROSITA All-Sky Survey DR2: Cumulative X-ray catalogues from the first three surveys and multi-wavelength counterparts in the western Galactic hemisphere
M. E. Ramos-Ceja, G. Lamer, M. Salvato, A. Merloni, J. S. Sanders, A. Georgakakis, T. Liu, E. Bulbul
A foundational, community-wide X-ray catalogue resource doubling eRASS1 depth with meticulous validation and multi-wavelength counterparts, though methodologically incremental and catalogue-only.
Abstract
The eROSITA telescope array on board the Spektrum-Roentgen-Gamma (SRG) mission began its all-sky survey program in December 2019, scanning the sky at an approximately six-month cadence. Here, we present new catalogues of point-like and extended X-ray sources derived from the first three completed eROSITA all-sky surveys (eRASS:3), covering the western Galactic hemisphere, which Germany's eROSITA consortium holds proprietary rights for. We describe the observational strategy, data processing, and analysis pipelines. We also characterise the resulting X-ray source populations. The eRASS:3 main catalogue contains nearly two million sources, including 1,911,744 point-like and 63,796 extended sources, detected in the 0.2-2.3 keV energy band, eROSITA's most sensitive energy window. This volume has doubled the X-ray source content of eRASS1 and provides a comprehensive census of X-ray-emitting objects across diverse astrophysical classes. We also released a secondary hard catalogue of nearly 15,000 sources detected in the harder 2.3-5.0 keV energy band. In addition, we released six catalogues, three for the main sample and three for the hard sample, in which we identify and classify the optical and infrared counterparts of eRASS:3 point-like sources. This approach delivers a homogeneous, high-quality identification of mostly extragalactic X-ray emitters; for example, we estimate that approximately 88% of the 1.4 million counterparts identified within the footprints of the Legacy Survey Imaging for DESI are extragalactic sources. This work has enabled the generation of samples optimised for completeness and purity, while expanding the discovery space for rare populations. The second data release (DR2) of the SRG/eROSITA all-sky survey is a catalogue-only release, comprising the catalogues presented in this work together with an updated version of the eROSITA upper flux limit server.
AI Impact Assessments
(1 models)Scientific Impact Assessment
Paper type: Dataset/catalogue release (data + methods) paper.
1. Core Contribution
This paper presents the second data release (DR2) of the SRG/eROSITA all-sky survey for the German-consortium western Galactic hemisphere, based on the cumulative first three all-sky scans (eRASS:3). The central deliverable is a set of X-ray source catalogues containing nearly two million sources (1,911,744 point-like and 63,796 extended in the 0.2–2.3 keV band), plus a ~15,000-source hard-band catalogue and six accompanying optical/IR counterpart catalogues (via LS10, Gaia DR3, CatWISE2020). This roughly doubles the source content of eRASS1 (DR1). The scientific value lies not in a new algorithm but in the depth, homogeneity, and multi-wavelength linkage of a half-sky X-ray census — ~88% of the 1.4M LS10 counterparts are extragalactic (mostly AGN), with additional Galactic/extragalactic classification and probabilistic association metrics that let users tune purity/completeness. The eROSITA survey is the most sensitive soft X-ray all-sky survey since ROSAT (~25× deeper), so these catalogues are a foundational reference resource for X-ray astrophysics.
2. Methodological Rigor
The processing and validation are thorough and professional. The paper documents pipeline improvements (energy calibration to ±1–3 eV, time-dependent boresight astrometry reducing systematic positional error from 0.9″ to 0.4″, recovery of a ~3.8% sensitivity loss), and honestly catalogues known issues (bad-pixel calibration propagation, source confusion causing ~200 missed bright sources, SWCX foreground). Validation is strong: cross-matching against 2RXS, 5XMM-DR15, CSC 2.1, and eRASS1, with flux-consistency comparisons and spurious-source flagging schemes. A notable analytical highlight is the independent assessment of eRASS1 spurious contamination (~12.6% via cross-match + multi-wavelength p_any), which corroborates the simulation-based estimate of Seppi et al. (2022, ~14%) through a genuinely different methodology. This is careful, well-controlled work.
3. Potential Impact
High and broad within astronomy. The eRASS catalogues are already load-bearing community resources — the DR1 paper (Merloni et al. 2024) is heavily cited, and DR2's doubled depth extends this. The catalogues will feed AGN luminosity-function and variability studies, galaxy-cluster cosmology, stellar X-ray activity, compact-object and transient science, and multi-wavelength SED/photo-z work. The tight synergy with SDSS-V spectroscopy (~200,000 sources with spectra forthcoming) further amplifies legacy value. Impact is concentrated in astrophysics with no direct industrial/commercial pathway.
4. Timeliness & Relevance
Very timely. eROSITA is a flagship active mission, and coordinated deep optical/IR surveys (Legacy Survey, Gaia, WISE, SDSS-V, and soon LSST) make this exactly the moment when a homogeneous half-sky X-ray catalogue with probabilistic counterparts is most valuable. It addresses a real bottleneck: turning raw X-ray detections into science-ready, classified, multi-wavelength samples.
5. Strengths & Limitations
Strengths: Enormous, high-quality data volume; meticulous calibration and quality-control documentation; user-focused design (per-tile purity/completeness thresholds, detailed column dictionaries, best-practice use cases); the independent contamination cross-check; strong external cross-validation.
Limitations: (a) This is a catalogue-only release — raw event data and the eSASS v030 software are explicitly *not* released, so the catalogue itself cannot be independently reproduced from scratch. (b) Methodologically incremental relative to DR1: the pipeline, counterpart algorithm (NWAY), and classification schemes are inherited with refinements, so conceptual novelty is modest. (c) The results are unsurprising (deeper survey → more sources; contamination matches predictions). (d) Coverage is restricted to the western Galactic hemisphere due to consortium data rights. (e) Known unresolved issues (source confusion, hard-band spurious fractions, GDR3 shallowness) constrain some use cases.
Additional Observations
Overall, this is a high-impact enabling resource whose value derives from data scale, homogeneity, and community readiness rather than methodological innovation.
Generated Jul 31, 2026
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A foundational, community-wide X-ray catalogue resource doubling eRASS1 depth with meticulous validation and multi-wavelength counterparts, though methodologically incremental and catalogue-only.