SN 2019ehk: A Double-peaked Ca-rich Transient with Luminous X-Ray Emission and Shock-ionized Spectral Features

Abstract

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; L x  ∝  t −3 ), and a Shane/Kast spectral detection of narrow H α 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  < 10 15 cm and the resulting cooling emission. We calculate a total CSM mass of ∼7 × 10 −3 ( M He / M H ≈6) with particle density n  ≈ 10 9 cm −3 . Radio observations indicate a significantly lower density n  < 10 4 cm −3 at larger radii r  > (0.1–1) × 10 17 cm. The photometric and spectroscopic properties during the second light-curve peak are consistent with those of Ca-rich transients (rise-time of t r  = 13.4 ± 0.210 days and a peak B -band magnitude of M B  = −15.1 ± 0.200 mag). We find that SN 2019ehk synthesized (3.1 ± 0.11) × 10 −2 of and ejected M ej  = (0.72 ± 0.040) total with a kinetic energy E k  = (1.8 ± 0.10) × 10 50  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.