The 22 GHz water maser is the brightest spectral line in the radio universe and highlights shocked star forming regions, dense circumstellar shells around evolved stars as well as circumnuclear disks around massive black holes of active galactic nuclei.
The Metsähovi radio telescope has been used for long-term water maser observations of the most powerful maser source in our Galaxy, W49N. Water maser observations commonly show dramatic variations in flux density as a function of time. Such short-lived (typically some 2 months), dramatic increases in flux density are termed as ``outbursts''. Combining the Metsähovi 22 GHz database of some 150 maser outbursts of W49N (Liljeström 2000) with simultaneous VLBI data of Gwinn (University of California Santa Barbara), notably obtained with the same velocity resolution, Liljeström and Gwinn (2000) were able to fix the free parameters in the shock model of Hollenbach and McKee (1979, 1989) and the maser model of Elitzur, Hollenbach and McKee (1989). This enabled a straightforward determination of some 20 physical parameters of W49N. We checked the validity of these models with independent measurements and found a very good agreement between predictions and observations.
The most important characteristic of the novel diagnostic method of Liljeström and Gwinn (2000) is its capability to determine both preshock and postshock magnetic field strengths as well as the inclination angle of the mean magnetic field with respect to the line of sight. The preshock field strenght is determined from the jump condition of strong shocks and is observationally based on the observed preshock density and rms value of the observed line velocity fluctuations of maser features during outbursts. In order to determine the postshock field strength, the preshock density and space/shock velocities of the maser features (measured by VLBI) must be observationally known. The observed nonthermal line velocity fluctuations of maser features are caused by Alfvenic wave pressure, which is substantially increased in the sudden compressions associated with shocks. The fact that Alfven wave fluctuations are oriented perpendicular to the field lines enabled us to estimate the inclination angle of the mean field from the observed dispersion of the line velocity fluctuations.
One uniquely powerful outburst feature in our W49N data, referred to as the "big flare feature", showed also the narrowest linewidth. Our data indicated that the space velocity of this feature was directed along the plane of sky, whereas preshock and postshock magnetic fields were directed nearly along the line of sight. Consequently, Alfvenic wave fluctuations along the line of sight, and linewidth, were minimal, and a very high aspect ratio was achieved. This big flare feature of W49N stood out through its low space velocity, higher kinetic temperature (480 K), and larger preshock magnetic field strength (8.2 mG). These are naturally explained, if the big flare feature was located closer to the shock front than the other masers in W49N.