New evidence for Dicke’s superradiance in the 6.7 GHz methanol spectral line in the interstellar medium

Abstract

We present new evidence for superradiance in the methanol 6.7 GHz spectral line for three different star-forming regions: S255IR-NIRS3, G24.329+0.144, and Cepheus A. Our analysis shows that some of the flux-density flares exhibiting fast rise times and asymmetric light curves reported in these sources can naturally be explained within the context of superradiance. When a threshold for the inverted population column density is exceeded in a maser-hosting region, the radiation mode switches from one regulated by stimulated emission (maser) to superradiance. Superradiance, as a more efficient energy release mechanism, manifests itself through strong bursts of radiation emanating from spatially compact regions. Elevated inverted population densities and the initiation of superradiance can be due to a change in radiative pumping. Here, we show that an increase in the pump rate and the inverted population density of only a factor of a few results in a significant increase in radiation. While the changes in the pump rate can take place over a few hundred days, the rise in radiation flux density when superradiance is initiated is drastic and happens over a much shorter time-scale.

Detection of new methanol maser transitions associated with G358.93−0.03

“…The possibility of a rapid increase in the maser pumping efficiency that is accompanied by only small changes in continuum emission should be explored akin to the arguments put forward in Sobolev et al (1997). Another possibility might be superradiance as described by Rajabi et al (2019) pertaining to the flaring methanol masers associated with S255IR-NIRS3. It is not clear what best describes the conditions of the maser environment nor this rapid variation presented here.…”

Generalisation of the Menegozzi & Lamb Maser Algorithm to the Transient Superradiance Regime

“…We simulate the cylindrical sample of methanol molecules ( 0 = 2 × 6.7 GHz, = 0.7 D) described in over a duration = 10 8 s, having length = 2 × 10 15 cm, radius = 5.4 × 10 7 cm, population inversion relaxation time constant 1 = 1.64 × 10 7 s, and polarisation dephasing time constant 2 = 1.55 × 10 6 s. Despite such time scales being orders of magnitude longer than those typically used in maser models of star formation regions, there is in theory no physical barrier to their occurrence. In Rajabi et al (2019) it is shown that 2 = 1.55 × 10 6 s corresponds to a gas density of approximately 10 5 cm −3 . Rather than precluding such a large value of 2 on account of an assumption of high gas density, the very natural fit of the SR model in Rajabi et al (2019) under such a value of 2 may be considered evidence of low gas density in the star formation region which it models.…”

“…The initial preparation of equation ( 16) would be realised by a pumping flare of finite duration shorter than that of the SR transient process, but of sufficient amplitude to raise the inverted population column density above the SR threshold. In Rajabi et al (2019), for example, the 6.7 GHz methanol emission is believed to be pumped by infrared radiation from dust (Sobolev et al 1997) We assume that the relevant photon wavelengths are much larger than the size of a molecule, and we neglect the Hilbert space associated with the motions of the molecules' centres of masses. We also assume that the molecules are sufficiently separated so as not to require wavefunction symmetrisation.…”

“…These entangled states couple strongly to the radia-tion field and produce enhanced emission rates as compared to those rates predicted for independently radiating molecules. Recent work (Rajabi & Houde 2016b;Rajabi & Houde 2017;Houde et al 2018;Houde et al 2019;Rajabi et al 2019; has demonstrated that SR is a strong candidate for describing transient astrophysical processes demonstrating sharp flux rises in maser-harbouring regions, which possess conditions similar to those prerequisite to the development of SR. In Rajabi & Houde (2017), for example, it was argued that the relative delay (∼20 days) and differing duration (∼7 days and ∼20 days) of similarly periodic (∼34.4 days) methanol 6.7-GHz and water 22-GHz flares (respectively), observed in the intermediate-mass young stellar object G107.298+5.639 (Szymczak et al 2016), is difficult to explain by the quasi-steady state dynamics of a maser model with periodic pumping source.…”

“…The recent applications of SR to astrophysics (Rajabi & Houde 2016b;Rajabi & Houde 2017;Houde et al 2018;Houde et al 2019;Rajabi et al 2019; have been restricted to slices of velocity coherent (on resonance) populations, where the relevant Maxwell-Bloch (MB) equations describe a gas of molecules travelling at only a single shared velocity. It is the objective of the present work to extend modelling of SR to realistic velocity distributions.…”