Further evidence of shocks in the first-overtone RR Lyrae pulsators: first detection of shock-triggered magnesium emissions

The behavior of the shock wave in the atmosphere of the non-fundamental mode RR Lyrae pulsator remains a mystery. In this work, we firstly report a blueshifted Mg triplet emission in continuous spectroscopic observations for a non-Blazhko RRc pulsator (Catalina-1104058050978) with LAMOST medium resolution spectra. We analyse the photometric observations from Catalina Sky Survey of this RRc pulsator with pre-whitening sequence method and provide the ephemeris and phases. An additional frequency signal with $P_1/P_x = 0.69841$ is detected and discussed. The redshift and radial velocity of the spectra are provided by fitting process with $Sacute{e}rsic$ functions and cross-correlation method. Moreover, we plot the variation of H$alpha$ and Mg lines in a system comoving with the pulsation. Clear evolution of comoving blueshifted hydrogen and Mg emission is observed, which further confirms the existence of shock waves in RRc pulsators. The shock-triggered emission lasts over $15%$ of the pulsation cycle, which is much longer than the previous observations.