| We investigate the H2O maser flare that occurred in the source NGC 7538 S during 1998 2005 in the radial-velocity range from -57 to -52 km s-1. We have found a large number of emission features, suggesting that the medium where the flare occurred is highly fragmented. We have identified four spectral groups of emission features. All groups are most likely associated with the cluster of maser spots located in the center of an elongated structure and related to a massive rotating disk. The observed pattern of variations in the fluxes and radial velocities of the features can be explained by the presence of inhomogeneities in the medium, which can form elongated structures like filaments or chains. The mean extent of this structure is estimated to be 6 8 AU. Two cycles of maser activity have been observed, 1998 2002 and 2003 2005, which may be determined by the cyclic activity of the central object, a massive O-type protostar. |
