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We recently discovered an extragalactic transient, AT2020caa, from the Zwicky Transient Facility alert stream using the community broker ANTARES. This transient apparently exhibited two outbursts within one year (between 2020 and 2021). Based on a decade-long historical light curve of the candidate host galaxy, we rule out 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. We examine the nature of AT2020caa’s PO seen a year earlier and conclude that it is likely a separate SN within the same galaxy.

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