Fig. 3: The multiband light curve of SN 2020eyj can be divided into a diffusion-peak phase and a long-lived interaction-powered tail phase.

The light curves of SN 2020eyj are shown with overplotted SN Ia template fits to the initial peak (see ‘Light-curve analysis’ section in Methods). The most recent mid-infrared epoch (W1 and W2) is outside the date range plotted here and is shown in Extended Data Fig. 2 . Open circles indicate synthetic photometry derived from the spectra. Phase is in rest-frame days since first detection. Apparent magnitudes on the left y axis, absolute magnitudes on the right y axis, in which μ is the distance modulus. Non-detections with 5σ upper limits are indicated by triangles. The photometry has been binned into one-night bins and has been corrected for MW reddening. The diamond markers at the top indicate the epochs of spectroscopy. The bottom panel shows the g − r colour for the nights in which both g and r photometry was obtained, overplotted with the colour evolution of a typical SN Ia. The error bars represent 1σ uncertainties.