SN 2019ehk: A Double-Peaked Ca-rich Transient with Luminous X-ray Emission and Shock-Ionized Spectral Features

We present panchromatic observations and modeling of the Calcium-rich supernova 2019ehk in the star-forming galaxy M100 (d\(\approx\)16.2 Mpc) starting 10 hours after explosion and continuing for ~300 days. SN 2019ehk shows a double-peaked optical light curve peaking at \(t = 3\) and \(15\) days. The first peak is coincident with luminous, rapidly decaying \(\textit{Swift}\)-XRT discovered X-ray emission (\(L_x\approx10^{41}~\rm{erg~s^{-1}}\) at 3 days; \(L_x \propto t^{-3}\)), and a Shane/Kast spectral detection of narrow H\(\alpha\) and He II emission lines (\(v \approx 500\) km/s) originating from pre-existent circumstellar material. We attribute this phenomenology to radiation from shock interaction with extended, dense material surrounding the progenitor star at \(r