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

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

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 (L_x ≈ 10⁴¹ erg s⁻¹ at 3 days; L_x ∝ t⁻³), and a Shane/Kast spectral detection of narrow Hα and He II emission lines (v ≈ 500 km s⁻¹) 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 M_⊙ (M_(He)/M_H ≈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 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 M_⊙ of ⁵⁶Ni and ejected M_(ej) = (0.72 ± 0.040) M⊙ total with a kinetic energy E_k = (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 M_⊙) 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.

Additional Information

© 2020 The American Astronomical Society. Received 2020 May 4; revised 2020 June 16; accepted 2020 June 17; published 2020 August 5. We thank David Matthews, Enrico Ramirez-Ruiz, Brian Metzger, and Nathan Smith for stimulating conversations and constructive comments in regard to this work. W.J.-G. is supported by the National Science Foundation Graduate Research Fellowship Program under grant No. DGE-1842165 and the Data Science Initiative Fellowship from Northwestern University. R.M. is grateful to KITP for hospitality during the completion of this paper. This research was supported in part by the National Science Foundation under grant No. NSF PHY-1748958. R.M. acknowledges support by the National Science Foundation under Award No. AST-1909796. Raffaella Margutti is a CIFAR Azrieli Global Scholar in the Gravity & the Extreme Universe Program, 2019. The Margutti's team at Northwestern is partially funded by the Heising-Simons Foundation under grant # 2018-0911 (PI: Margutti). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. The scientific results reported in this article are based in part on observations made by the Chandra. This research has made use of software provided by the Chandra X-ray Center (CXC) in the application packages CIAO. Partial support for this work was provided by the National Aeronautics and Space Administration through Chandra Award Number DD0-21114X issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. W. M. Keck Observatory [and/or MMT Observatory] access was supported by Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). The UCSC transient team is supported in part by NSF grant AST-1518052, NASA/Swift grant 80NSSC19K1386, the Gordon & Betty Moore Foundation, the Heising-Simons Foundation, and by a fellowship from the David and Lucile Packard Foundation to R.J.F. Research at Lick Observatory is partially supported by a generous gift from Google. A.H. is partially supported by the Future Investigators in NASA Earth and Space Science and Technology (FINESST) Award No. 80NSSC19K1422. H.B.P. acknowledges support from the Kingsely distinguished-visitor program at Caltech, as well as the KITP visitor program. K.J.S. is supported by NASA through the Astrophysics Theory Program (NNX17AG28G). P.J.B. is supported by a NASA Swift Guest Investigator grant 80NSSC19K0316. TMB was funded by the CONICYT PFCHA/DOCTORADOBECAS CHILE/2017-72180113. S.C.Y. is supported by the National Research Foundation of Korea (NRF) grant (NRF-2019R1A2C2010885). M.G. is supported by the Polish NCN MAESTRO grant 2014/14/A/ST9/00121. D.A.H., J.B., D.H., and C.P. are supported by NSF AST-1911225 and NASA grant 80NSSC19K1639. J.D. and E.B. are supported in part by NASA Grant 80NSSC20K0456. S.J.S. acknowledges funding from STFC Grant ST/P000312/1. M.R.S. is supported by the National Science Foundation Graduate Research Fellowship Program under grant No. 1842400. Research by D.J.S. is supported by NSF grants AST-1821967, 1821987, 1813708, 1813466, and 1908972. This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program 15645. X. Wang is supported by the National Natural Science Foundation of China (NSFC grants 11325313, 11633002, and 11761141001), and the National Program on Key Research and Development Project (grant No. 2016YFA0400803). We acknowledge the support of the staff of the Xinglong 2.16 m telescope. This work was partially supported by the Open Project Program of the Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences. Funding for the LJT has been provided by Chinese Academy of Sciences and the People's Government of Yunnan Province. The LJT is jointly operated and administrated by Yunnan Observatories and Center for Astronomical Mega-Science, CAS. Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, Inovações e Comunicações (MCTIC) do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We wish to recognize the destructive history of colonialism endured by native Hawaiians as we strive to hear the voice of those whose sacred land we continue to utilize for scientific gain. This work includes data obtained with the Swope Telescope at Las Campanas Observatory, Chile, as part of the Swope Time Domain Key Project (PI: Piro, Co-Is: Drout, Phillips, Holoien, French, Cowperthwaite, Burns, Madore, Foley, Kilpatrick, Rojas-Bravo, Dimitriadis, Hsiao). We wish to thank Swope Telescope observers Jorge Anais Vilchez, Abdo Campillay, Nahir Munoz Elgueta, and Natalie Ulloa for collecting data presented in this paper. This research has made use of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Science Data Center (ASDC), Italy. The SN group at Konkoly Observatory is supported by the project "Transient Astrophysical Objects" GINOP 2.3.2-15-2016-00033 of the National Research, Development and Innovation Office (NKFIH), Hungary, funded by the European Union. Based on observations collected at the European Southern Observatory under ESO program 1103.D-0328. This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, and the Space Telescope Science Institute. Facilities: Hubble Space Telescope - , Spitzer Space Telescope - , Chandra X-ray Observatory - , Neil Gehrels Swift Observatory - , VLA - , Las Campanas Observatory - , Zwicky Transient Facility - , Konkoly Observatory - , ATLAS - , Thacher Observatory - , Swope:1 m - , Multi Unit Spectroscopic Explorer (MUSE) - , Shane (Kast) - , Xinglong Observatory (BFOSC) - , MMT (Binospec) - , Bok (B&C) - , Faulkes North (FLOYDS) - , NTT (EFOSC2) - , LJT (YFOSC) - , SOAR (Goodman - , Triple-Spec) - , Keck I (LRIS). - Software: emcee (Foreman-Mackey et al. 2013), SNID (Blondin & Tonry 2007), Superfit (Howell et al. 2005), IRAF, AstroDrizzle (Gonzaga 2012), photpipe (Rest et al. 2005), DoPhot (Schechter et al. 1993), HOTPANTS (Becker 2015), Sedona (Kasen et al. 2006), SYN++/SYNAPPS (Thomas et al. 2011), Castro (Almgren et al. 2010), BPASS (Eldridge et al. 2017), MESA (Choi et al. 2016), sextractor (Bertin & Arnouts 1996), HEAsoft (v6.22; HEASARC 2014).

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