Long-Term Variability of Water Maser Emission in S128

Abstract

The evolution of the H₂O maser emission at the S128 source during 19812024 is analyzed, based on long-term regular observations. The emission from ‑73 to ‑70 km/s corresponds mainly to source B, while that of velocities < ‑76 km/s to source A. It was identified that in the time interval 2004-2006, the maser activity gradually shifted from source B to source A. The active phases of masers B and A lasted approximately 23 and 16 years, respectively. It is assumed that the common cause of the variability of the maser emission at both sources (A and B) can be shock waves that occur near the ionization front. When clouds collide, a disturbance propagates along the resulting ionization front, which leads to the appearance of shock waves. It is shown that the disturbance is spreading in the south-north direction. The star formation process occurs in the same direction.