Determine the origin of excited-OH maser flux variability

Determine the physical origin of flux changes in excited hydroxyl masers, including whether the flaring events observed in Mon R2 are caused by accretion events in the protostar.

Background

The paper notes that excited-OH masers at 4.7 GHz vary on timescales from months to years, with particularly strong flaring observed in the 4.766 GHz maser toward Mon R2. Although variability in excited-OH masers has been associated with a possible accretion event in NGC 6334 I, the physical mechanism responsible for the flux changes remains unresolved.

The authors specifically highlight the lack of sufficient evidence to determine whether the Mon R2 flares are caused by protostellar accretion events. Long-term monitoring of a sample of excited-OH masers is suggested as a way to clarify their behavior and possible connection to transient accretion activity.

References

Without a multi-epoch interferometric observation programme, it is impossible to determine which source harbours the varying 1.665\,GHz OH masers.

— Ten years since the NGC 6334I-MM1 accretion outburst: the impact on its environment  (2609.20014 - MacLeod et al., 17 Sep 2026) in Section 4.3.2, “Accretion induced impact on the OH masers”

It is not clear what the energy source is for the OH masers prior to the pre-accretion event, if this is not a coincidence.

— Ten years since the NGC 6334I-MM1 accretion outburst: the impact on its environment  (2609.20014 - MacLeod et al., 17 Sep 2026) in Section 4.3.2, “Scenario 2: Pre-heating in MM1”

There are arguments for and against each of these scenarios and there is no way at this time to determine which, if any, is the correct explanation.

— Ten years since the NGC 6334I-MM1 accretion outburst: the impact on its environment  (2609.20014 - MacLeod et al., 17 Sep 2026) in Section 4.3.2, end of “Scenario 3: The impact of MM1 on MM3”

The origin of these flux changes has not been established. However, some variation of excited OH (exOH) masers has been seen in NGC6334 I in association with a supposed accretion event \citep{MSG18}. It appears that methanol masers can be an indicator of these accretion events, but clearly other species of masers are also affected by the injection of energy associated with an accretion event. There is not enough evidence yet to prove that the flaring events seen in Mon R2 are due to accretion events in the protostar, but if they are, they have very different characteristics to those seen when 6.7\,GHz methanol masers flare.

— Polarization Observations of a sample of Excited OH masers  (2609.19980 - Smits et al., 17 Sep 2026) in Section 1, Introduction