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 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 $\text{Swift}$-XRT discovered X-ray emission ($L_x\approx 10^{41}\mathrm{erg}{\mathrm{s}}^{-1}$ at 3 days; $L_x\propto t^{-3}$), and a Shane/Kast spectral detection of narrow H$\hat{I}\pm$ 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 $r<10^{15}$ cm and the resulting cooling emission. We calculate a total CSM mass of $\sim$ $7\times 10^{-3}$ ${\mathrm{M}}_{\odot }$ with particle density $n\approx 10^9\mathrm{c}{\mathrm{m}}^{-3}$. Radio observations indicate a significantly lower density $n<10^4\mathrm{c}{\mathrm{m}}^{-3}$ at larger radii. The photometric and spectroscopic properties during the second light curve peak are consistent with those of Ca-rich transients (rise-time of $t_r=13.4\pm 0.210$ days and a peak B-band magnitude of $M_B=-15.1\pm 0.200$ mag). We find that SN 2019ehk synthesized $(3.1\pm 0.11)\times 10^{-2}{\mathrm{M}}_{\odot }$ of ${}^{56}\text{Ni}$ and ejected $M_{\mathrm{ej}}=(0.72\pm 0.040){\mathrm{M}}_{\odot }$ total with a kinetic energy $E_{\mathrm{k}}=(1.8\pm 0.10)\times 10^{50}\mathrm{erg}$. Finally, deep $\text{HST}$ pre-explosion imaging at the SN site constrains the parameter space of viable stellar progenitors to massive stars in the lowest mass bin (~10 ${\mathrm{M}}_{\odot }$) in binaries that lost most of their He envelope or white dwarfs. The explosion and environment properties of SN 2019ehk further restrict the potential WD progenitor systems to low-mass hybrid HeCO WD + CO WD binaries.

51 pages, 27 figures. Accepted for publication in ApJ