ИСТИНА

In recent years the water maser source in the massive star-forming region G25.65+1.05 (IRAS 18316-0602, Mol 62, RAFGL 7009S) has undergone several powerful flares. During regular monitoring of the source with the 22-m radio telescope in Pushchino (Moscow Region) a strong burst was detected with the peak flux density of the strongest maser line increasing from 70 Jy (on 16.06.2016) to 50,000 Jy (on 12.12.2016), i.e., a more than 500-fold increase. A second super-burst was observed with the RT-22 in Simeiz (Crimea), detecting a flux density of 20,500 Jy. The maximum flux density of 65 kJy was detected on 16.09.2017. The maser source was observed several times with the space-ground interferometer RadioAstron (RA) and also with the VLBA and the EVN groundbased interferometers. Strong fringes were obtained during the 29.09.2017 session on the space-ground baselines RA-Torun and RA-HartRAO, with baseline lengths from 8.6 to 9.3 Earth Diameters. The angular resolution of 24 μas corresponds to a linear size for the emitting region of about 0.05 AU (assuming a source distance of 2.08 kpc). This is the first time that such compact structures have been detected in water masers. The extraordinarily high intensity of the observed flare provides a large dynamic rangefor the analysis of the compact components found in this source. The differing responses on the space-ground and ground-ground baselines indicates that a range of size scales are present. Using the precise spatial distribution of individual maser components, it is possible to identify hierarchical structures within the masers. These observations serve to clarify the nature of the masing source, which in turn offers additional insights into the high mass star formation process.