Rapid variability of H2O masers in Cepheus A

The 22-GHz H₂O maser emission from Cepheus A has been monitored in three separate experiments in 1982–83 using single telescopes at Chilbolton and Darnhall, and the MERLIN array. Spectacular variations in the H₂O maser lines are reported. Time-scales for spectral features to change in intensity by a factor of two were as short as 2.4 day, making these some of the fastest variations in H₂O masers yet reported. A number of features showed correlated variations which are interpreted as being due to variations in a common radiatively-coupled maser pump. The intensity variations were frequently accompanied by small (∼0.5 km s⁻¹) shifts in velocity. In most cases these could not be explained in terms of blending, but were real effects intrinsic to the masers. They may be due to the hyperfine structure of the H₂O line. Line-narrowing was also observed in several features. The variations in linewidth followed a power law with intensity $$\Delta V_{1/2}\propto I^{-1/2}$$ which indicates saturated masers, although some of the fastest variations occurred in features which showed negligible line-narrowing and were presumably unsaturated. A maser flare observed in 1983 March showed both types of behaviour: a rapid unsaturated rise with no line-narrowing, followed by saturation and a slow fall during which the line broadened. The observed variability puts constraints on possible pumping schemes for the masers, and provides strong support for the hot dust-cold gas scheme.