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Journal Articles The Astrophysical Journal Year : 2017

SOFIA/GREAT Discovery of Terahertz Water Masers

David A. Neufeld
  • Function : Author
Gary J. Melnick
  • Function : Author
Michael J. Kaufman
  • Function : Author
Helmut Wiesemeyer
Rolf Güsten
  • Function : Author
Alex Kraus
  • Function : Author
Karl M. Menten
  • Function : Author
Oliver Ricken
  • Function : Author


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 toward all four sources using the Effelsberg 100 m 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}52 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.

GREAT is a development by the MPI für Radioastronomie and the KOSMA/Universität zu Köln, in cooperation with the MPI für Sonnensystemforschung and the DLR Institut für Planetenforschung.

Dates and versions

insu-03692492 , version 1 (09-06-2022)



David A. Neufeld, Gary J. Melnick, Michael J. Kaufman, Helmut Wiesemeyer, Rolf Güsten, et al.. SOFIA/GREAT Discovery of Terahertz Water Masers. The Astrophysical Journal, 2017, 843, ⟨10.3847/1538-4357/aa7568⟩. ⟨insu-03692492⟩
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