Peak locations of spiral arm S1 in polar coordinates after correction for viewing geometry. The solid curves represent the best-fit model spiral for the peak locations (dot points) between 2015 and 2019, assuming the companion-driven scenario. The derived angular velocities of spiral pattern motion are yr⁻¹. The uncertainty estimation is presented in Appendix D. The 2019 observations significantly deviate from the gravitational instability prediction (dashed curve), adopting a central star of 1.7 M_⊙.