SN 2017ati: A luminous type IIb explosion from a massive progenitor

We present optical photometric and spectroscopic observations of the Type IIb supernova (SN) 2017ati. It reached maximum light about 27 d after explosion, and the light curve shows a broad, luminous peak with an absolute r-band magnitude of M_ r = -18.48 ± 0.16 mag. At about 50 d after maximum light, SN 2017ati exhibited a decline rate close to that expected from the 56 Co ̊ightarrow$ $^ 56 Fe radioactive decay, at 0.98 mag per 100 days, as is usually observed in SNe IIb. However, it remains systematically brighter at late times by about 1--2 mag, exceeding the usual upper luminosity range of this class. As a result, modelling the light curve of SN 2017ati with a standard 56 Ni decay scenario requires a large nickel mass of up to łambdałambda6300,6364 doublet implies an oxygen mass of sim1.82-3.34,M_⊙, and the combination of a and still fails to reproduce the early-time light curve adequately. In contrast, incorporating additional energy input from a magnetar yields a significantly improved fit to the light curve of SN 2017ati, which would reduce the nickel mass to sim0.21,M_⊙, which is still close to the upper end of the range typically inferred for SNe IIb. Comparing the fitted results of SN 2017ati with the known sample of SNe IIb indicates that its luminosity evolution is best explained by a combination of neutron star spin-down energy and radioactive nickel deposition. From late-time nebular spectra of SN 2017ati, the luminosity of the / flux ratio of sim0.5 with nebular spectral model comparisons favours a progenitor zero-age main-sequence mass of geq17,M_⊙.