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 (at 3 days; Lx ∝ t−3), and a Shane/Kast spectral detection of narrow Hα and He ii emission lines (v ≈ 500) 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 < 1015 cm and the resulting cooling emission. We calculate a total CSM mass of ∼7 × 10−3 (MHe/MH ≈6) with particle density n ≈ 109 cm−3. Radio observations indicate a significantly lower density n < 104 cm−3 at larger radii r > (0.1–1) × 1017 cm. The photometric and spectroscopic properties during the second light-curve peak are consistent with those of Ca-rich transients (rise-time of tr = 13.4 ± 0.210 days and a peak B-band magnitude of MB = −15.1 ± 0.200 mag). We find that SN 2019ehk synthesized (3.1 ± 0.11) × 10−2 of and ejected Mej = (0.72 ± 0.040) total with a kinetic energy Ek = (1.8 ± 0.10) × 1050 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) 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. [ABSTRACT FROM AUTHOR]