Fast variability and circular polarization of the 6.7 GHz methanol maser in G33.641$-$0.228

TL;DR

This study investigates rapid flux variability and circular polarization in the 6.7 GHz methanol maser in G33.641$-$0.228, revealing short-term changes and proposing a model involving explosive events behind the maser cloud.

Contribution

It introduces a new model explaining burst-like variability and polarization changes through explosive events causing polarized continuum emission amplified by the maser.

Findings

Flux density varies on 0.3-day timescales during bursts.

Circular polarization reaches up to 20% and varies over days.

A proposed model involves explosive events behind the maser cloud.

Abstract

The 6.7 GHz methanol maser in a high-mass star-forming region G33.641$-$0.228 is known to exhibit burst-like flux variability due to an unknown mechanism. To investigate the burst mechanism, we conducted high-cadence flux and circular polarization monitoring observations, simultaneously using left- and right-hand circular polarizations. We found that the flux density increased and decreased on a short timescale of 0.3 d during a burst. We also found strong circular polarization, reaching up to 20\% in the component exhibiting the bursts. Circular polarization of 0--20\% was continuously observed from 2009 to 2016, even in the quiescent period. The polarization also varied on timescales of less than one day. When a burst occurred and the flux density increased, the circular polarization decreased to zero. To explain the observational properties of the flux variability and circular polarization, we propose a model in which an explosive event similar to a solar radio burst occurs on the line of sight behind the maser cloud, producing circularly polarized continuum emission due to gyro-synchrotron or gyro-resonance radiation, which is then amplified by the maser.