# Signatures of Circumstellar Interaction in the Unusual Transient AT2018cow

## TL;DR

AT2018cow is a fast, luminous transient with properties similar to SNe Ibn, suggesting that circumstellar interaction in a hydrogen-poor environment may explain its unusual features.

## Contribution

This paper proposes that AT2018cow's properties can be explained by circumstellar interaction similar to SNe Ibn, a novel interpretation for this type of transient.

## Key findings

- AT2018cow's spectral features resemble SNe Ibn and IIn.
- Light-curve and X-ray properties are similar to interacting SNe.
- Narrow lines may originate from unresolved HII regions and slow CSM.

## Abstract

AT2018cow is a unique transient that stands out due to its relatively fast light-curve, high peak bolometric luminosity, and blue color. These properties distinguish it from typical radioactively powered core-collapse supernovae (SNe). Instead, the characteristics are more similar to a growing sample of Fast Blue Optical Transients (FBOTs). Mostly discovered at hundreds of Mpc, FBOT follow-up is usually limited to several photometry points and low signal-to-noise spectra. At only ~60 Mpc, AT2018cow offers an opportunity for detailed followup. Studies of this object published to date invoke a number of interpretations for AT2018cow, but none of these specifically consider the interacting Type Ibn SN subclass. We point out that while narrow lines do not dominate the spectrum of AT2018cow, as narrow Balmer lines typically do in SNe IIn, the narrow lines in AT2018cow may nevertheless be a mix of unresolved HII region emission and emission from slow, pre-shock CSM. We compare AT2018cow to interacting SNe Ibn and IIn and find a number of noteworthy similarities, including light-curve rise and fall times, peak magnitude, X-ray light-curves, and spectroscopic properties. In particular, the He I lines in AT2018cow closely resemble those in some examples of SNe Ibn or transitional SNe Ibn/IIn objects. We therefore explore the hypothesis that CSM interaction in a relatively H-poor system might have some merit in explaining observed properties of AT2018cow, and we go on to consider progenitor implications for AT2018cow, FBOTs, and SNe~Ibn.