Work

Ultraluminous X-ray sources in the first eROSITA survey

Year: 2026

Type: article

Abstract: Context . Ultraluminous X-ray sources (ULXs) are luminous non-nuclear X-ray point sources embedded in galaxies. Their luminosities exceeding 10 39 erg s −1 potentially require sub-Eddington accretion by objects with masses from ∼10 2 M ⊙ to ∼10 4 M ⊙ or supercritical accretion beyond the Eddington limit by stellar mass black holes or neutron stars. As such, ULXs are excellent laboratories for extreme accretion flows in X-ray binaries and, in some cases, potential candidates for intermediate-mass black holes. The first catalog by the eROSITA instrument on board the Spectrum-Roentgen-Gamma mission provides the most complete X-ray view of the whole of the western Galactic hemisphere and is therefore an excellent resource in which to find new ULXs. Aims . We identified ULXs by searching for X-ray counterparts in the ∼10 5 local galaxies of the HECATE catalog in the footprint of the first eROSITA all-sky survey. The catalog is unaffected by X-ray selection biases. We characterized the completeness of the sample and its contamination by unidentified active galactic nuclei (AGN) and determined the influence of source confusion. The catalog provides the basis for ULX population studies and follow-up observations that can be used to perform a more detailed investigation of individual sources and variability. Methods . We searched for X-ray point sources exceeding the ULX threshold in HECATE galaxies while excluding the nuclear regions of the galaxies to avoid the selection of AGN. We removed known contaminants such as supernovae, stars, and AGN, and we manually vetted all candidates to further remove potential contaminants. We derived the X-ray selection function and fraction of unidentified AGN interlopers using a simulation with subsequent source detection. By comparing our sample to other ULX catalogs, we identified previously unknown candidates and those not detected although expected from their flux, indicating source intrinsic variability. Results . We present two samples of ULX candidates. The main sample consists of 90 sources with highly confident X-ray detections, 53 of which are identified for the first time. This sample is complete to a distance of ∼7 Mpc, contains at most 29% of unknown background AGN, and is mostly unaffected by source confusion. Several candidates are newly identified even though their locations have been observed by other instruments before, and some previously reported candidates we expected to identify are absent from our sample, demonstrating the transient behavior of ULXs. Based on our list of less confident detections, we also provide an extended catalog of 260 sources identified as potential candidates for further ULX identifications. Of these, 245 are identified as potential candidates for the first time.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202659942