The SRG/eROSITA All-Sky Survey: X-ray Selection Function Models for the Erass1 Galaxy Cluster Cosmology

Characterising galaxy cluster populations from catalog of sources selected inastronomical surveys requires knowledge of sample incompleteness, known asselection function. The first All-Sky Survey (eRASS1) by eROSITA onboardSpectrum Roentgen Gamma (SRG) has enabled the collection of large samples ofgalaxy clusters detected in the soft X-ray band over the Western Galactichemisphere. The driving goal consists in constraining cosmological parameters,which puts stringent requirements on accuracy, flexibility and explainabilityof the selection function models. We use a large set of mock observations ofthe eRASS1 survey and we process simulated data identically to the real eRASS1events. We match detected sources to simulated clusters and we associatedetections to intrinsic cluster properties. We train a series of models tobuild selection functions depending only on observable surface brightness data.We develop a second series of models relying on global cluster characteristicssuch as X-ray luminosity, flux, and expected instrumental count-rate as well ason morphological properties. We validate our models using our simulations andwe rank them according to selected performance metrics. We validate the modelswith datasets of clusters detected in X-rays and via the Sunyaev-Zeldovicheffect. We present the complete Bayesian population modelling frameworkdeveloped for this purpose. Our results reveal the surface brightnesscharacteristics most relevant to cluster selection in the eRASS1 sample, inparticular the ambiguous role of central surface brightness at the scale of theinstrument resolution. We have produced a series of user-friendly selectionfunction models and demonstrated their validity and their limitations. Ourselection function for bright sources reproduces well the catalog matches withexternal datasets. (abridged)