NH(z)−z relation, see Fig. 4 for the general description. Left panels: the solid grey curves represent the extrapolation of the last-detection limit of each blazar, providing an overall sensitivity range of the instrument for excess absorption detections. Right panels: the same last-detection limits are shown for each blazar with red horizontal dashes. The underlying grey area is the “upper-limit” region for each blazar. Observing a blazar with a specific XMM-Newton response, exposure time, absorbed by its and a putative excess absorption column density, one would be sensitive for detections of the latter only above its red dash, while below it one would fit an upper limit. Bottom panels are analogous, with the exclusion of any blazar with a Galactic column density greater than 1021 cm−2.
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