Threshold significance σ as a function of the interval for BeppoSAX, BATSE, INTEGRAL, and WFM. The red, horizontal lines represent the σ₀ threshold as calculated from 1 (see also Band et al. 1993). In the case of BeppoSax, BATSE, and WFM, we report two horizontal lines because the value of the threshold σ₀ depends on the angle between the direction perpendicular to the plane of the detector and the direction of the source. We use the lower value throughout the whole analysis.