The λ0 = 1.35 cm H2O maser line: The hyperfine structure and profile asymmetry

The spectra of several H₂O maser sources exhibit single λ₀ = 1.35 cm maser lines with narrow asymmetric profiles. We consider the hyperfine structure of the line that corresponds to the transition between the rotational 6₁₆ → 5₂₃ levels of ortho-H₂O molecules to account for the line asymmetry. Our numerical simulations of the maser line profile agree well with the observations if the hyperfine structure is taken into account.