Detection of Periodicity in SiO Maser Intensity and Velocity Shift of the Symbiotic Star CH Cyg

We performed simultaneous monitoring observations of H2O 616–523 and SiO v = 1, 2, $J=1\to $ 0, SiO v = 1, $J=2\to $ 1, $3\to 2$ maser lines toward the symbiotic star CH Cyg from 2012 December to 2019 November using the Korean very long baseline interferometry Network single-dish telescope. SiO v = 1, 2, $J=1\to 0$ and v = 1, $J=2\to 1$ maser lines were detected for the first time. A periodicity of about 2.5 yr in the SiO maser intensity was found, showing its association with the pulsation of the M giant in CH Cyg. Only the SiO v = 2, $J=1\to 0$ maser, without detections of the v = 1, $J=1\to 0$ maser, was detected in 6 epochs out of 15 v = 2, J = 1–0 maser detected epochs. In addition, the SiO v = 1, $J=2\to 1$ maser intensity was always stronger than that of the v = 1, $J=1\to 0$ maser except for one epoch, providing a critical constraint on the different masing conditions by different transitions, according to changes of the CH Cyg symbiotic system. We also found the gradual redshifted peak velocities of the SiO masers had about a 2.5 yr periodicity corresponding to the period of peak intensities (redshifted up to about $+40\,\mathrm{km}\,{{\rm{s}}}^{-1}$ with respect to the stellar velocity of the M giant). Two possible explanations can be suggested for the 2.5 yr period of the SiO maser intensity and velocity behaviors, although their actual causes remain uncertain.