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

Jacobson-Galan, Wynn V. and Margutti, Raffaella and Kilpatrick, Charles D. and Hiramatsu, Daichi and Perets, Hagai and Vinkó, József and Kriskovics, Levente and Pál, András and Sárneczky, Krisztián and Szakáts, Róbert (2020) SN 2019ehk: A Double-peaked Ca-rich Transient with Luminous X-Ray Emission and Shock-ionized Spectral Features. ASTROPHYSICAL JOURNAL, 898 (2). ISSN 1538-4357

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Abstract

We present panchromatic observations and modeling of the Calcium-rich supernova (SN) 2019ehk in the star-forming galaxy M100 (d approximate to 16.2 Mpc) starting 10 hr after explosion and continuing for similar to 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 approximate to 10(41) erg s(-1) at 3 days; L-x proportional to t(-3)), and a Shane/Kast spectral detection of narrow Ha and He II emission lines (nu approximate to 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 < 10(15) cm and the resulting cooling emission. We calculate a total CSM mass of similar to 7 x 10(-3) M-circle dot (M-He/M-H approximate to 6) with particle density n approximate to 10(9) cm(-3). Radio observations indicate a significantly lower density n < 10(4) cm(-3) at larger radii r > (0.1-1) x 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) x 10(-2) M-circle dot of Ni-56 and ejected M-ej = (0.72 +/- 0.040) M-circle dot total with a kinetic energy E-k = (1.8 +/- 0.10) x 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 (similar to 10 M-circle dot) 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.

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