The supernova CSS121015:004244+132827: a clue for understanding superluminous supernovae

Abstract

We present optical photometry and spectra of the super luminous type II/IIn supernova CSS121015:004244+132827 (z=0.2868) spanning epochs from -30 days (rest frame) to more than 200 days after maximum. CSS121015 is one of the more luminous supernova ever found and one of the best observed. The photometric evolution is characterized by a relatively fast rise to maximum (∼ 40 days in the SN rest frame), and by a linear post-maximum decline. The light curve shows no sign of a break to an exponential tail. A broad Hα is first detected at ∼ +40 days (rest-frame). Narrow, barely-resolved Balmer and [O III] 5007Å lines, with decreasing strength, are visible along the entire spectral evolution. The spectra are very similar to other super luminous supernovae (SLSNe) with hydrogen in their spectrum, and also to SN 2005gj, sometimes considered a type Ia interacting with H-rich CSM. The spectra are also similar to a subsample of H-deficient SLSNe. We propose that the properties of CSS121015 are consistent with the interaction of the ejecta with a massive, extended, opaque shell, lost by the progenitor decades before the final explosion, although a magnetar powered model cannot be excluded. Based on the similarity of CSS121015 with other SLSNe (with and without H), we suggest that the shocked-shell scenario should be seriously considered as a plausible model for both types of SLSN.

Hydrogen-poor Superluminous Supernovae with Late-time Hα Emission: Three Events From the Intermediate Palomar Transient Factory

“…When such a star explodes, its early-time spectrum would have Hα emission, but not like SN IIn with both narrow and broad components indicating ejecta-CSM interaction. This type of transient may have been detected already, for example, SN 2008es, SN 2013hx, PS15br, and possibly CSS121015 (Gezari et al 2009;Miller et al 2009;Benetti et al 2014;Inserra et al 2016), which show only broad Hα emission in the photospheric phase. Another example would be an SLSN-I progenitor that has lost all of the H-envelopes, but only shortly before the supernova explosion.…”

The Most Luminous Supernovae

“…4). The sample of similar events remains small and currently includes but a handful of additional events including CSS121015 (Benetti et al 2014), SN 2013hx and PS15br ). The group is characterized by spectra with evident Balmer lines in absorption or with P-Cygni profiles, but lacking narrow IIn-like features, an important distinction as such narrow features are generally taken as evidence for strong CSM interaction.…”

“…There are only a handful of events in this class of SLSNe-II without narrow lines (Gezari et al 2009, Miller et al 2009, Benetti et al 2014). However, their spectral evolution seems to be quite similar: initially the spectra show a hot blue continuum with weak or no features.…”

Long-Duration Superluminous Supernovae at Late Times

“…Some SLSNe evolve fast on a time scale of weeks, whereas others evolve slowly on a time scale of months. There is diversity in spectral appearance, with some displaying hydrogen (Ofek et al 2007;Smith et al 2008;Gezari et al 2009;Miller et al 2009;Chatzopoulos et al 2011;Benetti et al 2014;Inserra et al 2016b), but most appearing like Type Ic SNe (Gal-Yam et al 2009;Pastorello et al I] 6300, 6364 lines appeared to be significantly stronger than the detection limits for [Fe II] 5250 and [Ca II] 7291,7323. Their expansion velocities were also of order 5,000 km s −1 .…”