RadioAstron reveals super-compact structures in the bursting H2O maser source G25.65+1.05

Water masers are well-known to be variable on a variety of time scales, but only three Galactic H₂O masers are known to flare to the level of 10⁵–10⁶ Jy (${\text{T}}_{\text{B}}～{10}^{17}$ K): Orion KL, W49N, and the recently discovered G25.65+1.05. Recently detected flaring activity of H₂O maser in the massive star-forming region G25.65+1.05 gave us a unique opportunity to study the fine structure of H₂O maser emission in the bursting state with extremely high space VLBI angular resolution. Observation of the source was carried out with ～9 Earth diameter space-ground baseline within the framework of the RadioAstron project. H₂O maser emission from two spectral features, including the bursting one, was detected in the experiment. Only ～1% of the bursting H₂O maser emission was detected on the space-ground baselines: it indicates the presence of a very compact spatial structure with a size of ～25 $\mathrm{\mu }$as, which corresponds to 0.05 AU or ～5 solar diameters at the distance to the source of 2.08 kpc, and the brightness temperature of ～3 × 10¹⁶ K. Analysis of the flux density as a function of the baseline length for the bursting H₂O maser feature in the source shows that most of the emission comes from an extended “halo” structure, while the core of emission is very compact and has an extreme brightness temperature. These results are in agreement with the model of interacting maser clouds considered as the likely explanation of the nature of the burst in the source. Under the assumption of such a model, the beam size of maser emission is reduced while the brightness temperatures similar to the highest observed values are produced.