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

Abstract

Supernova (SN) 2015bh (or SNhunt275) was discovered in NGC 2770 on 2015 February with an absolute magnitude of M r ∼ −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.

Supernovae 2016bdu and 2005gl, and their link with SN 2009ip-like transients: another piece of the puzzle

“…(ii) The duration of Event A appears to vary for the five objects, lasting from a few weeks in LSQ13zm (Tartaglia et al 2016) to over 3 months, and with a bumpy rise in SN 2016bdu and SN 2015bh (Elias-Rosa et al 2016;Thöne et al 2017). The absolute magnitudes of the A events are between M R/r = −14.5 and −15.3 mag.…”

“…The duration and the luminosity of the two events are comparable in all SN 2009ip-like objects, which also show remarkable similarity in their spectral evolution (at least, at phases when spectroscopic observations exist; see Section 2.3). This observed homogeneity is even more puzzling for progenitors possessing strongly asymmetric circumstellar environments (Levesque et al 2014;Mauerhan et al 2014;Elias-Rosa et al 2016;Thöne et al 2017), whose orientation with respect to the line of sight is expected to play an important role. The striking spectroscopic and photometric similarities of the five SN 2009ip-like transients suggest that these objects very likely arise from similar stars (or stellar systems) and may have undergone a comparable fate.…”

“…(iv) When late-time observations are available, the light curves have flattened, with decline rates slower than the 56 Co decay and without any clear evidence of further light-curve fluctuations (Margutti et al 2014;Fraser et al 2015;Elias-Rosa et al 2016;Thöne et al 2017). …”

“…Fraser et al 2013b;Ofek et al 2014). Pre-SN outbursts were well observed in two Type IIn events: SN 2009ip (Fraser et al 2013a(Fraser et al , 2015Mauerhan et al 2013aMauerhan et al , 2014Ofek et al 2013b;Pastorello et al 2013a;Smith et al 2013;Graham et al 2014Graham et al , 2017Margutti et al 2014;Smith, Andrews & Mauerhan 2016b) and SN 2015bh (Elias-Rosa et al 2016;Goranskij et al 2016;Ofek et al 2016;Thöne et al 2017). Both sources had historical light curves with signatures of erratic variability over time-scales of years (SN impostor phase), followed by two luminous outbursts separated by a few weeks.…”

Optical and IR observations of SN 2013L, a Type IIn Supernova surrounded by asymmetric CSM

“…This includes SN 1998S (Leonard et al 2000), SN 1997eg (Hoffman et al 2008), SN 2009ip (Levesque et al 2014Mauerhan et al 2014;Smith, Mauerhan & Prieto 2014), SN 2010jl (Andrews et al 2011;Patat et al 2011;Smith et al 2012;Chandra et al 2015;Katsuda et al 2016), PTF11iqb and SN 2015bh (Elias-Rosa et al 2016). Recently, Mauerhan et al (2017) suggested that the IIP/L SN2013ej SN likely had an equatorial enhanced CSM close to edge on based on spectropolarimetry.…”