Reasoning
- Different declination bands see different celestial regions; a spectral feature produced by a handful of localized sources (e.g. the Centaurus-region overdensity) would appear preferentially in the bands containing them, distorting those bands’ spectra relative to the rest.
- The five band spectra are statistically consistent with the all-sky reference (departure probabilities 13-71%), and targeted checks around Centaurus, NGC 253, and NGC 1068 show no significant feature attributable to those regions; the northernmost band also matches Telescope Array’s independent measurement of the same sky.
- Hence whatever produces the instep operates identically across the sky - consistent with a propagation/composition effect of a broadly distributed source population (narrow rigidity maximum, He/CNO interplay) and inconsistent with a few-foreground-source origin, up to the statistical power available (fluctuations still prevent conclusions inside the 24-degree Centaurus window itself).
Validity verdict (step 6)
Reconstruction: premise = a handful of localized sources with distinctive spectra would imprint declination-dependent band spectra; observation = five bands statistically consistent with the all-sky spectrum (departure probabilities 13-71%) plus targeted null checks. Conclusion is properly hedged (“disfavors”, “favors”). Traced the load-bearing modus-tollens-shaped evidential step. Undercutting probe: a source configuration arranged to mimic isotropy across bands, or sources below the statistical power - both are covered by the explicit “up to the statistical power available” hedge in the body. Valid as stated.