Discovery of recurrent flares of 6.7 GHz methanol maser emission in Cepheus A HW2

Abstract

Context.Class II methanol masers at 6.7 GHz originate close to high-mass young stellar objects (HMYSOs). Their high sensitivity to local condition variations makes them a useful marker of the activity of the emerging massive stars.Aims.We aim to closely examine the variability of the 6.7 GHz CH3OH masers in Cep A HW2 using the new and archival single-dish and high-resolution Very-Long-Baseline Interferometry (VLBI) datasets.Methods.We monitored 6.7 GHz methanol masers towards the target between 2009 and 2021 using the Torun 32m radio telescope, and analysed nine datasets of VLBI observations taken over 16 yr.Results.Faint, extremely redshifted maser emission located close to the presumed central star position and coincident with the radio jet shows flaring activity with a period of ~5 yr. Flares have an asymmetric profile with a rise-to-decay time ratio of 0.18 and relative amplitude higher than 10. The velocity of the flaring cloudlets drifts at a rate of 3–4 × 10−5km s−1d−1for about 11.5 yr of the monitoring. The time-lag between the peaks of the two flaring features implies a propagation speed of the exciting factor of ~1000 km s−1. Synchronised and anticorrelated variations of the flux density of blue- and redshifted features begin ~2.5 yr after flares of the extremely redshifted emission.Conclusions.Our observations suggest that the recurrent flares are the response of low-gain unsaturated maser regions to a relatively low increase in luminosity, which has little effect on the behaviour of most maser regions of higher gain.