eROSITA all-sky survey - stars and optical loading
Year: 2026
Type: article
Abstract: The extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) spacecraft performed the eROSITA All-Sky Survey (eRASS) at X-ray energies. We studied the brighter stars to assess the impact of optical loading in eRASS, here specifically the eRASS:3. Furthermore, we used thermal plasma models to investigate the general properties of stellar eRASS sources and compare stars affected by optical loading with coronal sources. We compared data from Gaia DR3, Tycho-2, 2MASS, and the SIMBAD database with the eRASS count rates of brighter stars. We analyzed X-ray images, light curves, and spectra for sources suspected to be affected by optical loading and determine their properties. We compared eROSITA source properties with those of genuine coronal sources using sets of generated APEC model spectra, for which we determined rate conversion factors and hardness ratios for stellar X-ray emitters. We find a clear correlation between the optical brightness and the observed minimum count rate for magnitudes of about 2,--,5 mag (V/G band); the count rate then saturates for very bright sources. Although magnitude and color are the main determinants, exposure depth and specific scanning paths also influence the observed properties. For intrinsic X-ray emitters, we likewise observe optical contamination. We characterize spurious X-ray sources and address pseudo-variability and spectral properties. To interpret stellar eRASS data, we provide hardness ratios and count rate conversions for thermal plasma models. The eROSITA data are prone to optical loading, which we studied in great detail. The optical loading flag for the DR2 is set if the sources are brighter than 5.0 mag in the B, V, or G bands, or 3.5 mag in the J band. The flag depends somewhat on the exposure depth, but we expect only minor changes for future data releases.
Source: Astronomy and Astrophysics
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W7171808303