BlazEr1: The eROSITA Blazar Catalog. Blazars and Blazar Candidates in the First eROSITA Survey

TL;DR

This work presents BlazEr1, the first large catalog of eROSITA-detected blazars and blazar candidates in the western Galactic hemisphere. It builds a master BLAZE catalog from literature, cross-matches it with eRASS1, and delivers X-ray spectral properties for sources with sufficient counts alongside rich multiwavelength context. The results include 5865 BlazEr1 entries (2106 confirmed blazars), with a contamination level below ~11%, first X-ray data for thousands of new sources, and robust log N–log S and luminosity distributions that align with theoretical expectations while revealing blazar subclass differences. The catalog provides a practical foundation for follow-up campaigns, SED modeling, and variability studies, and serves as a baseline for exploiting future eROSITA surveys like eRASS:5 to study the blazar population in greater depth.

Abstract

Aims. eROSITA on board of the Spectrum Roentgen Gamma (SRG) spacecraft performed its first X-ray all-sky survey (eRASS1) between December 2019 and June 2020. It detected about 930000 sources, providing us with an unprecedented opportunity for a detailed blazar census. We present the properties of blazars and blazar candidates in eRASS1 and the compilation of the eROSITA blazar catalog. Methods. We compile a list of blazar and blazar candidates from the literature and match it with the eRASS1 catalog, creating the Blazars in eRASS1 (BlazEr1) catalog. For sources with more than 50 counts we obtain their X-ray spectral properties. We compile multiwavelength data from the radio to the gamma-ray regimes for all sources, including multiwavelength spectral indices and redshifts. The full catalog is available online. Results. We present the BlazEr1 catalog, containing 5865 sources, of which 2106 are associated with confirmed blazars. For 3668 sources, eROSITA provides the first X-ray data. The contamination from non-blazar sources of the entire sample is less than 11%. Most candidates exhibit properties typical for blazars. We present properties of the entire X-ray detected blazar population, including the distributions of X-ray luminosities and photon indices, multiwavelength properties, and the blazar log N-log S distribution. Our catalog provides follow up targets, such as potential MeV and TeV blazars. Conclusions. The BlazEr1 catalog provides a compilation of X-ray detected blazars and blazar candidates. The catalog serves as a starting point for exploiting further eROSITA surveys using the same methodology, enabling us to study the X-ray variability and a large number of spectral energy distributions of blazars in the future.

Figures (28)

• Figure 1: Histograms of angular separation between BLAZE catalog and eROSITA positions (green) with the distribution of the matched eROSITA source positional uncertainty overlaid (yellow). The vertical line at $8"$ indicates the distance threshold for the final sample. The cumulative distribution of the angular separation is shown in blue. a Sample of eROSITA observed blazars with 93% of the matches found within $8"$, and 55% and 79% within $3"$ and $5"$, respectively. beROSITA observed blazar candidates. Only 81% of matches are within $8"$, and 31%/58% within $3"$/$5"$. We show Rayleigh distributions (black) for illustrative purposes, calculated following merloni:2024.
• Figure 2: Positions of the BlazEr1 sources in Galactic coordinates. The different colors and symbols indicate the blazar types in the catalog. Candidates, especially BCUCs, dominate the sample. The overdensity along $l\sim 240^\circ$ coincides with the footprint of BROS. Confirmed blazars are displayed by filled symbols, candidates are shown in the same color without filling. All following figures will adapt the same color and symbol scheme.
• Figure 3: Sky area in which 95% of all sources above a minimum flux will be detected with DET_LIKE_0 $\geq10$. The dashed line indicates the total area of the western Galactic hemisphere. The red line displays the sensitivity over the entire western Galactic hemisphere, while the blue line shows the extragalactic sky excluding an area of 15$^\circ$ below and above the Galactic plane and regions with radii of $5.5^\circ$ around the Large Magellanic cloud (LMC) and $3.0^\circ$ around the Small Magellanic cloud (SMC).
• Figure 4: Spectral fits of an absorbed power law model for three confirmed blazars with different fluxes. For 5BZQ J0547$-$6728, no prior X-ray observations are available. The three lower panels display the fit residuals for each source. Data from TM5 and 7 are displayed in a lighter color.
• Figure 5: Distribution of source counts with its cumulative distribution shown by the solid line curve. The dashed vertical line marks the threshold of 50 counts, above which we deem a spectral analysis meaningful. Only 20% of all sources are above this limit, while about half have fewer than 20 counts, as shown by the cumulative distribution (blue).