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Charge-coupled devices (CCDs) are widely used in astronomy to carry out avariety of measurements, such as for flux or shape of astrophysical objects.The data reduction procedures almost always assume that ther esponse of a givenpixel to illumination is independent of the content of the neighboring pixels.We show evidence that this simple picture is not exact for several CCD sensors.Namely, we provide evidence that localized distributions of charges (resultingfrom star illumination or laboratory luminous spots) tend to broaden linearlywith increasing brightness by up to a few percent over the whole dynamic range.We propose a physical explanation for this "brighter-fatter" effect, whichimplies that flatfields do not exactly follow Poisson statistics: the varianceof flatfields grows less rapidly than their average, and neighboring pixelsshow covariances, which increase similarly to the square of the flatfieldaverage. These covariances decay rapidly with pixel separation. We observe theexpected departure from Poisson statistics of flatfields on CCD devices andshow that the observed effects are compatible with Coulomb forces induced bystored charges that deflect forthcoming charges. We extract the strength of thedeflections from the correlations of flatfield images and derive the evolutionof star shapes with increasing flux. We show for three types of sensors thatwithin statistical uncertainties,our proposed method properly bridgesstatistical properties of flatfields and the brighter-fatter effect.

Evidence for self-interaction of charge distribution in charge-coupled devices