Odin observations of the Galactic centre in the 118-GHz band

Aims. The Odin satellite has been used to search for the 118.75-GHz line of molecular oxygen (O2) in the Galactic centre. Methods. Odin observations were performed towards the Sgr $\rm {A}^{*}$ circumnuclear disk (CND), and the Sgr A +20 km s-1 and +50 km s-1 molecular clouds using the position-switching mode. Supplementary ground-based observations were carried out in the 2-mm band using the ARO Kitt Peak 12-m telescope to examine suspected SiC features. Results. A strong emission line was found at 118.27 GHz, attributable to the $J = 13{-}12$ HC3N line. Upper limits are presented for the 118.75-GHz O2 $(1_{1}{-}1_{0})$ ground transition line and for the 118.11-GHz $^{3}\Pi_{2}$, $J = 3{-}2$ ground state SiC line at the Galactic centre. Upper limits are also presented for the 487-GHz O2 line in the Sgr A +50 km s-1 cloud and for the 157-GHz, $J = 4{-}3$, SiC line in the Sgr A +20 and +50 km s-1 clouds, as well as the CND. The CH3OH line complex at 157.2-157.3 GHz has been detected in the +20 and +50 km s-1 clouds but not towards Sgr $\rm {A}^{*}$/CND. Conclusions. A $3\sigma$ upper limit for the fractional abundance ratio of [ O2] /[ H2] is found to be X(O2) ≤ 1.2 $\times$ 10-7 towards the Sgr A molecular belt region