Five years of 6.7-GHz methanol maser monitoring with Irbene radio telescopes

Abstract

We present the results of a 5-yr monitoring program of 42 sources targeted at 6.7-GHz methanol masers, conducted from 2017 March to 2022 October using the Irbene 32- and 16-m radio telescopes. Sources were observed with irregular intervals where time between two consecutive observations ranged from 24 h to 35 d. We found that more than 55 per cent of the sources showed significant variability, but often only one or a few spectral features were varying significantly. Numerous type of variability were found in our sample: low-variable, periodic, irregular, synchronized, and anticorrelated between features and steadily raising or falling flux. Our analysis techniques also uncover new variability trends for several sources. The maser monitoring program is one of the first single-dish science initiatives at the Irbene radio telescope complex, initiated shortly after the instrument’s reconstruction and upgrades. Our findings unequivocally demonstrate its suitability for maser research purposes.