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