Fast Population Leakage in Astronomical Masers: Maser Amplification and Transient Superradiance

Abstract

We use a $Λ$-type three-level Maxwell--Bloch model to test whether an inverted molecular transition in an astronomical maser source can produce maser amplification or superradiance when its upper level also decays through a second radiative pathway with a much larger spontaneous decay rate. Such shared-upper-level configurations occur in multilevel, radiatively pumped molecules, including Class II methanol masers and several OH maser transitions. The model follows the coupled evolution of level populations, molecular coherences, radiation fields, and phenomenological relaxation and dephasing, separating population leakage from coherence loss. We focus on the mixed configuration in which the observed transition is inverted while the faster pathway is non-inverted and acts as a leakage channel. We find that rapid spontaneous decay through the competing pathway does not, by itself, suppress maser amplification or superradiant emission from the inverted transition. The response is controlled by the shared upper-level population reservoir, the available initial coherence, and the relaxation and dephasing timescales. For small effective coherence in the leakage pathway, its radiative output remains weak, while the inverted transition either amplifies a seed field in the quasi-steady maser regime or develops macroscopic coherence and produces a transient superradiant burst. A larger inversion does not necessarily produce a stronger burst if it is accompanied by a weaker initial coherence seed. As a benchmark, we apply the model to the 6.7 GHz methanol flare in S255IR-NIRS3, whose upper level also decays through the 239.7 GHz transition at a spontaneous rate more than four orders of magnitude larger. The calculated flare remains compatible with a transient-superradiance interpretation when this fast leakage pathway is included explicitly.