Example for the computation of the expected X-ray flux for a specific γ-ray source, 4FGL J0159.8–2234. The top panel indicates the normalized X-ray flux distributions of field sources (black), and for the assumption of a young pulsar, MSP, or AGN nature. The bottom panel depicts the corresponding ratio of probabilities. The markers indicate the fluxes of the detected X-ray sources around the Fermi source. For reference, the γ-ray flux of the source is log F_γ = −12.00, and the limiting X-ray flux is λ = −13.95.