Discovery of six new class II methanol maser transitions, including the unambiguous detection of three torsionally excited lines toward G358.931-0.030

Abstract

We present the unambiguous discovery of six new class II methanol maser transitions, three of which are torsionally excited (vt=1). The newly discovered 6.18-GHz 17_-2 -> 18_-3 E (vt=1), 7.68-GHz 12_4 -> 13_3 A- (vt=0), 7.83-GHz 12_4 -> 13_3 A+ (vt = 0), 20.9-GHz 10_1 -> 11_2 A+ (vt=1), 44.9-GHz 2_0 -> 3_1 E (vt=1) and 45.8-GHz 9_3 -> 10_2 E (vt=0) methanol masers were detected towards G358.931-0.030, where the known 6.68-GHz maser has recently been reported to be undergoing a period flaring. The detection of the vt=1 torsionally excited lines corroborates one of the missing puzzle pieces in class II maser pumping, but the intensity of the detected emission provides an additional challenge, especially in the case of the very highly excited 6.18-GHz line. Together with the newly detected vt=0 lines, these observations provide significant new information which can be utilised to improve class II methanol maser modelling. We additionally present detections of 6.68-, 19.9-, 23.1- and 37.7-GHz class II masers, as well as 36.2- and 44.1-GHz class I methanol masers, and provide upper limits for the 38.3- and 38.5-GHz class II lines. Near simultaneous Australia Telescope Compact Array (ATCA) observations confirm that all 10 of the class II methanol maser detections are co-spatial to ~0.2 arcsec, which is within the uncertainty of the observations. We find significant levels of linearly polarised emission in the 6.18-, 6.67-, 7.68-, 7.83-, 20.9-, 37.7-, 44.9- and 45.8-GHz transitions, and low levels of circular polarisation in the 6.68-, 37.7- and 45.8-GHz transitions.