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 (SN) 2019ehk in the star-forming galaxy M100 (d ≍ 16.2 Mpc) starting 10 hr 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 Swift-XRT-discovered X-ray emission ( ${L}_{{\rm{x}}}\approx {10}^{41}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ at 3 days; L<SUB>x</SUB> ∝ t<SUP>-3</SUP>), and a Shane/Kast spectral detection of narrow Hα and He II emission lines (v ≍ 500 $\mathrm{km}\,{{\rm{s}}}^{-1}$ ) originating from pre-existent circumstellar material (CSM). We attribute this phenomenology to radiation from shock interaction with extended, dense material surrounding the progenitor star at r < 10<SUP>15</SUP> cm and the resulting cooling emission. We calculate a total CSM mass of ˜7 × 10<SUP>-3</SUP> ${M}_{\odot }$ (M<SUB>He</SUB>/M<SUB>H</SUB> ≍6) with particle density n ≍ 10<SUP>9</SUP> cm<SUP>-3</SUP>. Radio observations indicate a significantly lower density n < 10<SUP>4</SUP> cm<SUP>-3</SUP> at larger radii r > (0.1-1) × 10<SUP>17</SUP> cm. The photometric and spectroscopic properties during the second light-curve peak are consistent with those of Ca-rich transients (rise-time of t<SUB>r</SUB> = 13.4 ± 0.210 days and a peak B-band magnitude of M<SUB>B</SUB> = -15.1 ± 0.200 mag). We find that SN 2019ehk synthesized (3.1 ± 0.11) × 10<SUP>-2</SUP> ${M}_{\odot }$ of ${}^{56}\mathrm{Ni}$ and ejected M<SUB>ej</SUB> = (0.72 ± 0.040) ${M}_{\odot }$ total with a kinetic energy E<SUB>k</SUB> = (1.8 ± 0.10) × 10<SUP>50</SUP> erg. Finally, deep HST pre-explosion imaging at the SN site constrains the parameter space of viable stellar progenitors to massive stars in the lowest mass bin (˜10 ${M}_{\odot }$ ) in binaries that lost most of their He envelope or white dwarfs (WDs). The explosion and environment properties of SN 2019ehk further restrict the potential WD progenitor systems to low-mass hybrid HeCO WD+CO WD binaries.