a) Spectrum obtained toward Sgr B2(M) at the frequency of the SH⁺ 1₀–0₁ line with the APEX telescope in main beam brightness temperature. The red curve is the synthetic spectrum described in Sect. 4.1, the horizontal dotted line shows the continuum level, and the vertical dotted line marks the systemic velocity of the source (62 km s^-1). The strong absorption lines are spiral arm components of CO 3–2. The lower axis gives the LSR velocity for SH⁺, while the upper axis refers to CO, with the same labeling as in panel b). b) CO(1–0) spectrum obtained toward Sgr B2 as part of our line survey with the IRAM 30 m telescope. The horizontal dotted line indicates the zero level (after baseline removal because the observations were performed in position-switching mode with a far OFF position, which yields a very uncertain baseline level). In both panels, the dashed line marks the systemic velocity of the source in CO 3–2 and the dot-dashed line indicates the CO velocity +172.6 km s^-1 at which an extra absorption component appears in the ¹³CH⁺ spectrum (see discussion in Sect. 4.1.3.)