Water masers diagnosing postshocked conditions in W49N

We present spectroscopic results of 146 water maser outbursts in W49N, obta ined with the Metsahovi radio telescope at 22 GHz. We found the following c haracteristics: (1) Inside (outside) the velocity range of the dense ambien t medium, the increase in flux density during an outburst is typically 10(4 ) Jy (10(3) Jy) and covers I order (2.5 orders) of magnitude. (2) The outbu rst durations closely trace space velocities of maser features. (3) Nonther mal velocity fluctuations produce variations in the line velocity of the ma ser features, which are comparable to the variations measured in the line w idth during outbursts. (4) There is no correlation between flux density and line width. Combining these data with Gwinn's VLBI results, notably obtain ed during the same time period and with the same velocity resolution, we we re able to fix the free parameters in the shock model of Hollenbach & McKee and the maser model of Elitzur, Hollenbach, & McKee. This enabled a straig htforward determination of some 20 shock and maser parameters including, am ong others, the following typical values: kinetic temperature 350 K, postsh ock density (3.6-8.7) x 10(8) cm(-3) water abundance (1-5) x 10(-4) water d ensity (0.9-1.9) x 10(5) cm(-3) water column density (2.2-7.9) x 10(19) cm( -2), preshock held strength 0.8-1.6 mG, and total postshock held strength 8 0-160 mG. A step-by-step presentation of our diagnostic method is given, an d the relation between observations and model parameters is discussed. One uniquely powerful outburst feature during 1981-1983, hereafter referred to as the "big flare feature," showed also the narrowest line width (0.5 km s( -1)). Observations indicate that the velocity of this feature lies in the p lane of the sky, whereas preshock and postshock magnetic fields are directe d nearly along the line of sight. Consequently, Alfvenic wave fluctuations along the line of sight, and line width, are minimal, and a very high aspec t ratio is achieved. Furthermore, the big hare feature stands out through i ts low space velocity, higher temperature (480 K), and larger preshock magn etic field strength (8.2 mG). These are naturally explained, if the big fla re feature was located closer to the shock front than the other masers.