2023MNRAS.522.4728M

We report the detection of ammonia masers in the non-metastable (6, 3), (7, 5), and (6, 5) transitions; the latter being the first unambiguous maser detection of that transition ever made. Our observations include the first very long baseline interferometry detection of ammonia maser emission, which allowed effective constraining of the (6, 5) maser brightness temperature. The masers were detected towards G 358.931-0.030, a site of 6.7-GHz class II methanol maser emission that was recently reported to be undergoing a period of flaring activity. These ammonia masers appear to be flaring contemporaneously with the class II methanol masers during the accretion burst event of G 358.931-0.030. This newly detected site of ammonia maser emission is only the 12th such site discovered in the Milky Way. We also report the results of an investigation into the maser pumping conditions, for all three detected masing transitions, through radiative transfer calculations constrained by our observational data. These calculations support the hypothesis that the ammonia (6, 5) maser transition is excited through high colour temperature infrared emission, with the (6, 5) and (7, 5) transition line ratio implying dust temperatures >400 K. Additionally, we detect significant linearly polarized emission from the ammonia (6, 3) maser line. Alongside our observational and radiative transfer calculation results, we also report newly derived rest frequencies for the ammonia (6, 3) and (6, 5) transitions.