  We report the discovery of water maser emission at frequencies above 1 THz.
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 using the
Effelsberg 100m telescope. The measured line fluxes imply 22 GHz / 1.296 THz
photon luminosity ratios of 0.012, 0.12, and 0.83 respectively for W Hya, U
Her, and VY CMa, values that confirm the 22 GHz maser transition to be
unsaturated in W Hya and U Her. We also detected the 1.884888 THz 8(45)-7(53)
transition toward W Hya and VY CMa, and the 1.278266 THz 7(43)-6(52) transition
toward VY CMa. Like the 22 GHz maser transition, all three of the THz emission
lines detected here originate from the ortho-H2O spin isomer. 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.
