Two stripped envelope supernovae with circumstellar interaction

But only one really shows it

¹ Department of Astronomy, The Oskar Klein Center, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
e-mail: jesper@astro.su.se
² Cahill Center for Astrophysics, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
³ Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
⁴ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 234 Herzl St, 76100 Rehovot, Israel
⁵ DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA
⁶ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
⁷ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁸ The eScience Institute, University of Washington, Seattle, WA 98195, USA
⁹ Astrophysics Group, Cavendish Laboratory, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
¹⁰ Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA
¹¹ IPAC, California Institute of Technology, 1200 E. California, Blvd, Pasadena, CA 91125, USA
¹² Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK

Received: 16 July 2020
Accepted: 15 September 2020

Abstract

Context. We present observations of SN 2019tsf (ZTF19ackjszs) and SN 2019oys (ZTF19abucwzt). These two stripped envelope (SE) Type Ib supernovae (SNe) suddenly showed a (re-)brightening in their late light curves. We investigate this in the context of circumstellar material (CSM) interaction with previously ejected material, a phenomenon that is unusual among SE SNe.

Aims. We use our follow-up photometry and spectroscopy for these supernovae to demonstrate the presence of CSM interaction, estimate the properties of the CSM, and discuss why the signals are so different for the two objects.

Methods. We present and analyze observational data, consisting of optical light curves and spectra. For SN 2019oys, we also have detections in radio as well as limits from UV and X-rays.

Results. Both light curves show spectacular re-brightening after about 100 days. In the case of SN 2019tsf, the re-brightening is followed by a new period of decline, and the spectra never show signs of narrow emission lines that would indicate CSM interaction. On the contrary, SN 2019oys made a spectral makeover from a Type Ib to a spectrum clearly dominated by CSM interaction at the light curve brightening phase. Deep Keck spectra reveal a plethora of narrow high-ionization lines, including coronal lines, and the radio observations show strong emission.

Conclusions. The rather similar light curve behavior – with a late linear re-brightening – of these two Type Ib SE SNe indicate CSM interaction as the powering source. For SN 2019oys the evidence for a phase where the ejecta hit H-rich material, likely ejected from the progenitor star, is conspicuous. We observe strong narrow lines of H and He, but also a plethora of high-ionization lines, including coronal lines, revealing shock interaction. Spectral simulations of SN 2019oys show two distinct density components, one with density ≳10⁹ cm⁻³, dominated by somewhat broader, low-ionization lines of H I, He I, Na I, and Ca II, and one with narrow, high-ionization lines at a density ∼10⁶ cm⁻³. The former is strongly affected by electron scattering, while the latter is unaffected. The evidence for CSM interaction in SN 2019oys is corroborated by detections in radio. On the contrary, for SN 2019tsf, we find little evidence in the spectra for any CSM interaction.