Rapidly Rising Transients in the Supernova - Superluminous Supernova Gap

TL;DR

This paper reports on four rapidly rising luminous transients that challenge existing supernova models, with unique light curves and spectra, suggesting they may originate from a novel explosion mechanism or scenario.

Contribution

It introduces a new class of fast, luminous transients with properties that cannot be explained by current supernova models, proposing the need for alternative explanations.

Findings

Four rapidly rising transients with peak luminosities between SNe and superluminous SNe.

Spectral features include broad Halpha and unusual absorption lines.

Standard models like white dwarf detonations and CSM interaction do not explain these events.

Abstract

We present observations of four rapidly rising (t_{rise}~10d) transients with peak luminosities between those of supernovae (SNe) and superluminous SNe (M_{peak}~-20) - one discovered and followed by the Palomar Transient Factory (PTF) and three by the Supernova Legacy Survey (SNLS). The light curves resemble those of SN 2011kl, recently shown to be associated with an ultra-long-duration gamma ray burst (GRB), though no GRB was seen to accompany our SNe. The rapid rise to a luminous peak places these events in a unique part of SN phase space, challenging standard SN emission mechanisms. Spectra of the PTF event formally classify it as a Type II SN due to broad Halpha emission, but an unusual absorption feature, which can be interpreted as either high velocity Halpha (though deeper than in previously known cases) or Si II (as seen in Type Ia SNe), is also observed. We find that existing models of white dwarf detonations, CSM interaction, shock breakout in a wind (or steeper CSM) and magnetar spindown can not readily explain the observations. We consider the possibility that a "Type 1.5 SN" scenario could be the origin of our events. More detailed models for these kinds of transients and more constraining observations of future such events should help better determine their nature.