AT2020caa: A Type Ia Supernova with a Prior Outburst or a Statistical Fluke?

TL;DR

AT2020caa is a supernova with a prior outburst, possibly indicating a separate supernova event or a rare pre-explosion phenomenon, challenging simple classifications of such transients.

Contribution

The paper reports the discovery of AT2020caa with a prior outburst and provides spectroscopic confirmation as a Type Ia supernova, suggesting complex progenitor scenarios.

Findings

AT2020caa exhibited two outbursts within a year.

Spectroscopy confirms AT2020caa as a Type Ia supernova.

The prior outburst is likely a separate supernova event.

Abstract

We recently discovered an extragalactic transient, AT2020caa, using the community alert broker ANTARES. This transient apparently exhibited two outbursts in a time span of a year (between 2020 and 2021). Based on a decade-long historical light curve of the candidate host galaxy, we rule out an activity from the galaxy nucleus to explain these outbursts. The measured peak magnitudes (assuming the known spectroscopic redshift of the candidate host galaxy) put AT2020caa in the realm of thermonuclear supernovae (SNe) or luminous core-collapse SNe. A handful of the latter are known to show prior outbursts (POs), thought to be linked to mass loss in massive stars. Using Gemini/GMOS, we obtained a spectrum of the current outburst that shows it to be a Type Ia supernova (SNIa). We examine the nature of AT2020caa's PO and conclude that it is likely a separate SN within the same galaxy.