Letter to the Editor

Spectroscopic observations of progenitor activity 100 days before a Type Ibn supernova^⋆

¹ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
e-mail: sean.brennan@astro.su.se
² Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 234 Herzl St, 7610001 Rehovot, Israel
³ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave., Evanston, IL 60201, USA
⁴ The Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, 106 91 Stockholm, Sweden
⁵ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
⁶ MIT-Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave., Cambridge, MA 02139, USA
⁷ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁸ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
⁹ School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
¹⁰ IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
¹¹ Post Observatory, Lexington, MA 02421, USA
¹² Department of Physics & Astronomy, University of California Los Angeles, PAB 430 Portola Plaza, Los Angeles, CA 90095-1547, USA

Received: 26 January 2024
Accepted: 23 March 2024

Abstract

Obtaining spectroscopic observations of the progenitors of core-collapse supernovae is often unfeasible, due to an inherent lack of knowledge as to what stars experience supernovae and when they will explode. In this Letter we present photometric and spectroscopic observations of the progenitor activity of SN 2023fyq before the He-rich progenitor explodes as a Type Ibn supernova. The progenitor of SN 2023fyq shows an exponential rise in flux prior to core collapse. Complex He I emission line features are observed in the progenitor spectra, with a P Cygni-like profile, as well as an evolving broad base with velocities of the order of 10 000 km s⁻¹. The luminosity and evolution of SN 2023fyq is consistent with a Type Ibn, reaching a peak r-band magnitude of −18.8 mag, although there is some uncertainty regarding the distance to the host, NGC 4388, which is located in the Virgo cluster. We present additional evidence of asymmetric He-rich material being present both prior to and after the explosion of SN 2023fyq, which suggests that this material survived the ejecta interaction. Broad [O I], C I, and the Ca II triplet lines are observed at late phases, confirming that SN 2023fyq was a genuine supernova, rather than a non-terminal interacting transient. SN 2023fyq provides insight into the final moments of a massive star’s life, demonstrating that the progenitor is likely highly unstable before core collapse.