2002A&A...390..281Z

We report the results of our short time scale monitoring observations of W49N. Three water maser features were found to show obvious variations in flux density, line width and center velocity. Over a period of about 22 days, the -88.3, -56.9 and -12.8km/s features declined linearly in flux density at rates of 57, 113, 98.8Jy per day, respectively. The line widths of the -56.9 and -12.8km/s features increased at rates of ∼0.022 and ∼0.046km/s per day, respectively, while the line width of the -88.3km/s feature increased at a rate of 0.01km/s per day during the first 17 days and decreased at a rate of 0.017km/s per day during the remainder of the days. All three features show a systematic shift of ∼1km/s in center velocity. We propose a model based on the superposition of the spectra of two maser clouds to explain the variations of these maser features: two rigidly rotating, unsaturated spherical maser clouds during the declining phase reach the same line-of-sight velocity, then they depart from each other because of their relative acceleration.