Long-term observations of outbursts and variability of the W49N water masers

Abstract

A long-term monitoring of the 22 GHz water masers in W49N has been carried out with the 14-m Metsähovi radio telescope (operated by Helsinki University of Technology) during November 1980 – June 1983 and February – April 1985 (Liljeström et al., 1989). Here some new results are presented : (1) some basic statistics of the H₂O outbursts, (2) the, locations of the correlated outbursts on the VLBI maps (Gwinn et al., 1992) obtained from the same time period as our observations, (3) the long-term total flux variability, and (4) the observed line narrowing relation ΔV_masex ∝ F_o ^−0.5 of saturated masers has been interpreted according to the suggestion of Strelnitskij (1986) that it may result from changes in the kinetic temperature Tk. Using this approach upper limits to the kinetic temperature can be derived from 3 outbursts of the flare feature (V_LSR = −0.9), and are some 940 K at F_o(max) and 1160 K at ΔF(outburst)/2. If a 75% turbulence is assumed in ΔV_D the above temperatures are 408 K (at F_o(max)) and 509 K, respectively, yielding a cooling rate of some 4.8 K/day during a time period of 3 weeks. The big majority of the outbursts does, however, not show any line narrowing suggesting that these masers are in the constant temperature plateau of post-shocked gas.