X-ray source counts

X-ray images suffer complex instrumental effects (e.g. variable point spread function, variable sensitivity across the field of view), which need to be quantified in order to understand the type of sources a particular observation is (or is not) sensitive to. Uncertainties in the determination of the point source selection function propagate into the analysis and severely compromise any statistical study using the X-ray data.

A Bayesian method has been developed for quantifying the selection function of X-ray imaging observations that correctly and accurately accounts for instrumental effects (e.g. variable PSF size, vignetting). We have applied our method to the problem of determining the X-ray source counts (Georgakakis et al. 2008). Unlike previous studies, we factor into our analysis observational selection effects, such as the Eddington bias, uncertainties in the determination of the source flux and the fraction spurious sources in the X-ray catalogues. A key feature of our approach is that it allows us to extend previous determinations of the logN-logS to fluxes that are 1.5-2 times fainter. This is because the method correctly accounts for the completeness and flux bias corrections, particularly for sources with few photons close to the detection limit of a given survey.

The figure on the left shows our Bayesian determination of the differential X-ray source counts (black points and red curve) in comparison with previous estimates (blue points).