SN 2019ehk: A Double-Peaked Ca-rich Transient with Luminous X-ray Emission and Shock-Ionized Spectral Features
Creators: name: Jacobson-Galán, Wynn V.
name: Margutti, Raffaella
name: Kilpatrick, Charles D.
name: Hiramatsu, Daichi
name: Perets, Hagai
name: Khatami, David
name: Foley, Ryan J.
name: Raymond, John
name: Yoon, Sung-Chul
name: Bobrick, Alexey
name: Zenati, Yossef
name: Galbany, Lluís
name: Andrews, Jennifer
name: Brown, Peter J.
name: Cartier, Régis
name: Coppejans, Deanne L.
name: Dimitriadis, Georgios
name: Dobson, Matthew
name: Hajela, Aprajita
name: Howell, D. Andrew
name: Kuncarayakti, Hanindyo
name: Milisavljevic, Danny
name: Rahman, Mohammed
name: Rojas-Bravo, César
name: Sand, David J.
name: Shepherd, Joel
name: Smartt, Stephen J.
name: Stacey, Holland
name: Stroh, Michael
name: Swift, Jonathan J.
name: Terreran, Giacomo
name: Vinko, Jozsef
name: Wang, Xiaofeng
name: Anderson, Joseph P.
name: Baron, Edward A.
name: Berger, Edo
name: Blanchard, Peter K.
name: Burke, Jamison
name: Coulter, David A.
name: DeMarchi, Lindsay
name: DerKacy, James M.
name: Fremling, Christoffer
name: Gomez, Sebastian
name: Gromadzki, Mariusz
name: Hosseinzadeh, Griffin
name: Kasen, Daniel
name: Kriskovics, Levente
name: McCully, Curtis
name: Müller-Bravo, Tomás E.
name: Nicholl, Matt
name: Ordasi, András
name: Pellegrino, Craig
name: Piro, Anthony L.
name: Pál, András
name: Ren, Juanjuan
name: Rest, Armin
name: Rich, R. Michael
name: Sai, Hanna
name: Sárneczky, Krisztián
name: Shen, Ken J.
name: Short, Philip
name: Siebert, Matthew
name: Stauffer, Candice
name: Szakáts, Róbert
name: Zhang, Xinhan
name: Zhang, Jujia
name: Zhang, Kaicheng
Publisher: name: arXiv
Year: 2020
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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$α$ 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 $r&lt;10^{15}$ cm and the resulting cooling emission. We calculate a total CSM mass of $\sim$ $7\times10^{-3}$ $\rm{M_{\odot}}$ with particle density $n\approx10^{9}\,\rm{cm^{-3}}$. Radio observations indicate a significantly lower density $n &lt; 10^{4}\,\rm{cm^{-3}}$ 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 $t_r =13.4\pm0.210$ days and a peak B-band magnitude of $M_B =-15.1\pm0.200$ mag). We find that SN 2019ehk synthesized $(3.1\pm0.11)\times10^{-2} ~ \rm{M_{\odot}}$ of ${}^{56}\textrm{Ni}$ and ejected $M_{\rm ej} = (0.72\pm 0.040)~\rm{M_{\odot}}$ total with a kinetic energy $E_{\rm k}=(1.8\pm0.10)\times10^{50}~\rm{erg}$. Finally, deep $\textit{HST}$ pre-explosion imaging at the SN site constrains the parameter space of viable stellar progenitors to massive stars in the lowest mass bin (~10 $\rm{M_{\odot}}$) 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.
description: 51 pages, 27 figures. Accepted for publication in ApJ