Dead or Alive? Long-term evolution of SN 2015bh (SNhunt275)
Brown, P. J.; Smartt, S. J.; Kotak, R.; Zaggia, S.; Ochner, P.; Turatto, M.; Cappellaro, E.; Benetti, S.; Chornock, R.; Ciabattari, F.; Fraser, M.; Berton, M.; Richardson, N. D.; Tartaglia, L.; Elias-Rosa, N.; Tomasella, L.; Dennefeld, M.; Anupama, G. C.; Terreran, G.; Bjorkman, K. S.; Milisavljevic, D.; Margutti, R.; Patnaude, D.; Sahu, D. K.; Artigau, É.; Misra, K.; Pastorello, A.; Taubenberger, S.; Itagaki, K.; Boles, T.; Cortini, G.; Briganti, F.; Dimai, A.; Martinelli, F.; Morales-Garoffolo, A.; Harmanen, J.; Polshaw, J.; Arbour, R.; Gerhartz, C. J.; Mancini, R.
Italy, Germany, United Kingdom, United States, Spain, Finland, Canada, France, India, Japan
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 an 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.