Methanol masers have been empirically categorised into two different classes depending on the pumping mechanism that creates them; class I methanol masers are collisionally pumped while class II methanol masers are radiatively pumped. The collisionally pumped class I methanol masers are primarily associated with outflows and expanding HII regions, whereas the radiatively pumped class II methanol masers are closely associated with bursts of activity around young massive stellar objects. Class II masers therefore play an important role in the study of high-mass star formation.

Johnson et al. report the first observations in a rare family of class II methanol maser transitions in both CH₃OH and ¹³CH₃OH along with the first maser detected in the ¹³CH₃OH line. Methanol transitions were observed with the ATCA at 28.9 GHz (well outside the nominal 30 to 50 GHz frequency range of the 7mm receiver!) and 41.9 GHz toward three star formation regions with recent maser flaring events. The newly detected maser transitions are associated with the primary millimetre continuum sources (MM1).

The image above is a comparison of two spectra for the source G358.93-0.03. The signal in black shows the 28.9 GHz line detected on 2019 April 30 with a peak of 10.5 Jy, while the magenta line shows the 41.9 GHz line detected on 2019 April 26 with a peak of 0.4 Jy. Note the difference in the scale of the respective axes; the 28.9 GHz line is ∼26 times stronger than the 41.9 GHz line. The similarity in signal-to-noise despite the different scales is due to the longer integration time and the better bandpass in the 41.9 GHz observations. The first detections of these rare masers toward these sources presents more information for maser modelling, which can be used to constrain their known attributes.