The Star Shed Nine Suns Two Years Before It Died

The team combines optical and mid-infrared photometry with 26 spectra spanning 22 days before to 699 days after discovery. SN 2019qem reached an optical bolometric peak of 7.0 × 10^43 ergs per second and radiated 6.9 × 10^50 ergs within 575 days. In the preferred interaction model, about seven solar masses of ejecta hit roughly nine solar masses expelled two years earlier, then a dense wind, sweeping up around 32 solar masses by day 544. The inferred budget suggests a progenitor initially above 40 solar masses. Those numbers depend on the interaction model and adopted wind speed; they reconstruct one unusually luminous Type IIn event.