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Abstract

Hydrogen-rich, core-collapse supernovae are typically divided into four classes: IIP, IIL, IIn, and IIb. Recent hydrodynamic modelling shows that circumstellar material is required to produce the early light curves of most IIP/IIL supernovae. In this scenario, IIL supernovae experience large amounts of mass-loss before exploding. We test this hypothesis on ASASSN-15oz, a Type IIL supernova. With extensive follow-up in the X-ray, UV, optical, IR, and radio, we present our search for signs of interaction and the mass-loss history indicated by their detection. We find evidence of short-lived intense mass-loss just prior to explosion from light-curve modelling, amounting in 1.5 M_⊙ of material within 1800 R_⊙ of the progenitor. We also detect the supernova in the radio, indicating mass-loss rates of 10⁻⁶ to 10⁻⁷ M_⊙ yr⁻¹ prior to the extreme mass-loss period. Our failure to detect the supernova in the X-ray and the lack of narrow emission lines in the UV, optical, and NIR do not contradict this picture and place an upper limit on the mass-loss rate outside the extreme period of <10⁻⁴ M_⊙  yr⁻¹. This paper highlights the importance gathering comprehensive data on more Type II supernovae to enable detailed modelling of the progenitor and supernova which can elucidate their mass-loss histories and envelope structures and thus inform stellar evolution models.