Probability density function (PDF) obtained from the long-term light curve of the Crab Nebula. The histogram is normalised in a way such that the integration of the probability density over the log10(Flux) is 1. The double and triple log-normal models fit to the PDF are overlaid as green-dashed and purple-solid curves respectively. Their lowest-flux components model the shot-noise limited distribution of the extrapolated IC flux. The three components of the triple log-normal model are overlaid as purple-dotted curves. The blue and red vertical lines indicate the threshold of the “low” and “high” states respectively. The brown-dashed vertical line indicates the flux sensitivity corresponding to a detection significance of ∼3σ and a photon count of ∼20 in a five-day interval.
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