Top panel: column density per velocity interval distribution of o-H₂O⁺ (solid purple) and p-H₂O⁺ (dashed blue) over the entire velocity interval covering absorption towards NGC 253. Middle panel: OPR H₂O⁺ distribution (left) and corresponding histogram (right). Bottom panel: nuclear spin temperature distribution (left) and corresponding histogram (right). The median and 1 σ levels of the OPR and nuclear spin temperature are marked by solid and dotted black lines, respectively.