Dead or Alive? Long-term evolution of SN 2015bh (SNhunt275)

TL;DR

This paper studies the long-term evolution of SN 2015bh, revealing it as a faint core-collapse supernova with complex pre-explosion activity and ongoing ejecta interaction, challenging simple classifications of such transient events.

Contribution

It provides a detailed analysis of SN 2015bh's photometric and spectroscopic evolution, linking pre-explosion variability to the supernova explosion and ejecta interaction.

Findings

Pre-explosion activity observed up to 21 years before discovery.

SN 2015bh likely a faint core-collapse supernova with fallback.

Ejecta interaction with circumstellar material causes late brightening.

Abstract

Supernova (SN) 2015bh (or SNhunt275) was discovered in NGC 2770 on 2015 February with an absolute magnitude of Mr ~ -13.4 mag, and was initially classified as a SN impostor. Here we present the photometric and spectroscopic evolution of SN 2015bh from discovery to late phases (~ 1 yr after). In addition, we inspect archival images of the host galaxy up to ~ 21 yr before discovery, finding a burst ~ 1 yr before discovery, and further signatures of stellar instability until late 2014. Later on, the luminosity of the transient slowly increases, and a broad light curve peak is reached after about three months. We propose that the transient discovered in early 2015 could be a core-collapse SN explosion. The pre-SN luminosity variability history, the long-lasting rise and faintness first light curve peak suggests that the progenitor was a very massive, unstable and blue star, which exploded as a faint SN because of severe fallback of material. Later on, the object experiences a sudden brightening of 3 mag, which results from the interaction of the SN ejecta with circumstellar material formed through repeated past mass-loss events. Spectroscopic signatures of interaction are however visible at all epochs. A similar chain of events was previously proposed for the similar interacting SN 2009ip.