Observed transitions of HSO toward B1-b with the Yebes 40 m (upper panels) and the IRAM 30 m (bottom panels) telescopes. The red line shows the LTE synthetic spectrum obtained from a fit to the observed line profiles (see text). Note: the 2_0, 2 − 1_1, 0, J = 5/2–3/2, F = 2–1 component, shown in the LTE model to the right of the bottom panels, is blended with a negative artifact resulting from the frequency switching observing technique. The observed position in B1-b is α_J2000 = 03^h33^m20.8^s and δ_J2000 = 31° 07′34.0″.