Discovery of Terahertz Water Masers with SOFIA/GREAT

We report the discovery of water maser emission at frequencies above 1THz. Using the GREAT instrument on SOFIA, we have detected emission in the 1.296411 THz 8(27)-7(34) transition of water toward three oxygen-rich evolved stars: W Hya, U Her, and VY CMa. An upper limit on the 1.296 THz line flux was obtained toward R Aql. Near-simultaneous observations of the 22.23508 GHz 6(16)-5(23) water maser transition were carried out towards all four sources, providing quantitative determinations of the 22GHz / 1.296 THz photon luminosity ratio in the range 0.012 to 0.138. These values confirm the 22 GHz maser transition to be unsaturated. We also detected the 1.884888 THz 8(45)-7(52) transition toward W Hya and VY CMa, and the 1.278266 THz 7(43)-6(52) transition toward VY CMa. Based upon a model for the circumstellar envelope of W Hya, we estimate that stimulated emission is responsible for ~ 85% of the observed 1.296 THz line emission, and thus that this transition may be properly described as a terahertz-frequency maser. In the case of the 1.885 THz transition, by contrast, our W Hya model indicates that the observed emission is dominated by spontaneous radiative decay, even though a population inversion exists.