Peculiar Supernovae

TL;DR

This paper reviews the history and classification of peculiar supernovae, illustrating how many once-peculiar types are now understood as standard variants, while highlighting the importance of multi-wavelength late-time observations for understanding their nature.

Contribution

It provides a historical perspective on supernova classification and emphasizes the role of late-time multi-wavelength observations in understanding peculiar supernovae.

Findings

Many peculiar supernovae are now recognized as standard variants.

Late-time observations are crucial for understanding supernova ejecta and progenitor systems.

Some transients like ASASSN-15lh remain scientifically contentious.

Abstract

What makes a supernova truly `peculiar?' In this chapter we attempt to address this question by tracing the history of the use of `peculiar' as a descriptor of non-standard supernovae back to the original binary spectroscopic classification of Type I vs. Type II proposed by Minkowski (1941). A handful of noteworthy examples (including SN 2012au, SN 2014C, iPTF14hls, and iPTF15eqv) are highlighted to illustrate a general theme: classes of supernovae that were once thought to be peculiar are later seen as logical branches of standard events. This is not always the case, however, and we discuss ASASSN-15lh as an example of a transient with an origin that remains contentious. We remark on how late-time observations at all wavelengths (radio-through-X-ray) that probe 1) the kinematic and chemical properties of the supernova ejecta and 2) the progenitor star system's mass loss in the terminal phases preceding the explosion, have often been critical in understanding the nature of seemingly unusual events.