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

TL;DR

The study investigates the burst mechanism of the 6.7 GHz methanol maser in G33.641−0.228 by conducting high‑cadence, dual‑polarization monitoring over 2009–2016. Component II alone exhibits bursts, reaching up to ~$350$ Jy with sub‑day timescales of variability, while circular polarization remains in the $0.0$–$0.2$ range and often anti‑correlates with flux during bursts; quiescent periods also show persistent CP. The authors propose a solar‑like explosive event behind the maser that generates circularly polarized continuum radiation, which is then amplified by the maser cloud, offering a plausible mechanism for the observed CP and rapid flux changes. They also discuss a complementary magnetospheric reconnection scenario and emphasize VLBI follow‑ups during bursts to localize the background source and directly measure the continuum region size, potentially < $50$ AU, thereby testing the proposed model.

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.

Figures (11)

• Figure 1: Spectra of the 6.7 GHz methanol maser in G33.641$-$0.228. RHCP is shown as a solid black line and LHCP as a dashed red line. (a) MJD 56893.443 (2014-8-24), before the burst, (b) MJD 56898.429 (2014-8-29), at the burst. Alt text: Two line graphs showing the 6.7 GHz methanol maser spectra of G33.641$-$0.228 before and at the burst.
• Figure 2: The light curves of component II over the entire observation period. Black and red points represent RHCP and LHCP, respectively. Alt text: Graph of flux variation of component II of the 6.7 GHz methanol maser of G33.641$-$0.228. The horizontal axis represents the observation period (2009 to 2016), and the vertical axis represents the flux density.
• Figure 3: Temporal variation of circular polarization for components I, II, and IV, for panel (a), (b), and (c), respectively. Only component II shows significant positive circular polarization. Alt text: Three graphs. The horizontal axis represents the observation period (2009 to 2016), and the vertical axis represents the magnitude of the circular polarization.
• Figure 4: The light curves of RHCP (black) and LHCP (red) (a) and circular polarization (b) of component II observed during 2014-2015. Alt text: Two graphs. The upper panel shows the flux variation of component II of the 6.7 GHz methanol maser of G33.641$-$0.228. The lower panel shows the circular polarization of component II. The horizontal axis is common to both the upper and lower panels and represents the observation dates from 2014 to 2015.
• Figure 5: The light curves of RHCP (black) and LHCP (red) (a) and circular polarization (b) of component II observed during the large burst in 2014. Alt text: Two graphs. The upper panel shows the flux variation of component II of the 6.7 GHz methanol maser of G33.641$-$0.228. The lower panel shows the circular polarization of component II. The horizontal axis is common to both the upper and lower panels and represents the 12-day observing window during the 2014 burst.