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.
@article{arxiv.1511.00704,
title = {Rapidly Rising Transients in the Supernova - Superluminous Supernova Gap},
author = {Iair Arcavi and William M. Wolf and D. Andrew Howell and Lars Bildsten and Giorgos Leloudas and Delphine Hardin and Szymon Prajs and Daniel A. Perley and Gilad Svirski and Avishay Gal-Yam and Boaz Katz and Curtis McCully and S. Bradley Cenko and Chris Lidman and Mark Sullivan and Stefano Valenti and Pierre Astier and Cristophe Balland and Ray G. Carlberg and Alex Conley and Dominique Fouchez and Julien Guy and Reynald Pain and Nathalie Palanque-Delabrouille and Kathy Perrett and Chris J. Pritchet and Nicolas Regnault and James Rich and Vanina Ruhlmann-Kleider},
journal= {arXiv preprint arXiv:1511.00704},
year = {2016}
}