High-frequency component of the HRS spectrum of the H₂O 1₁₀–1₀₁line at 556.936 GHz observed on UT 8.78 August with overplotted synthetic spectrum of the 3-σ upper limit shown as the dashed line. The vertical axis is the calibrated main beam brightness temperature. The lower horizontal axis is the upper sideband frequency, while the upper axis shows the velocity with respect to the nucleus’s rest frame.