2025ApJ...993..134K

In this study, we analyzed millimeter-wave data toward the quasar PKS1830-211 observed with the Atacama Large Millimeter/submillimeter Array to obtain absorption spectra of HCN J = 2-1, J = 3-2, J = 4-3, and J = 5-4 lines at the cosmological redshift of z = 0.89. We confirmed multiple absorption components in each spectrum, and the two lower-J lines are highly saturated at the velocity centers of the two most prominent components. The effect of uncertainty in the continuum covering factor was carefully assessed using a Monte Carlo approach. We also accounted for systematic uncertainties in the HCN J = 4-3 and J = 5-4 absorption due to time variability during an intense flaring event of the quasar. Assuming local thermodynamic equilibrium and neglecting molecular collisions, we derived the excitation temperature profiles and their uncertainties in the optically thin regime. We determined the cosmic microwave background radiation temperature at z = 0.89 to be 5.13 ± 0.06 K by taking a weighted average of calculated excitation temperatures; this is highly consistent with that expected from the standard model of the Universe.