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

¹ School of Electronic Science and Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, P.R. China
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, P.R. China
⁴ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, P.R. China
⁵ South-Western Institute for Astronomy Research, Yunnan Key Laboratory of Survey Science, Yunnan University, Kunming, Yunnan 650500, P.R. China
⁶ Yunnan Key Laboratory of Survey Science, Yunnan University, Kunming, Yunnan 650500, P.R. China
⁷ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
⁸ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, P.R. China
⁹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, P.R. China
¹⁰ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹¹ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹² School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
¹³ Finnish Centre for Astronomy with ESO (FINCA), FI-20014 University of Turku, Vesilinnantie 5, Turku, Finland
¹⁴ Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
¹⁵ Kapteyn Astronomical Institute, University of Groningen, 9700 AV, Groningen, The Netherlands
¹⁶ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova SE-10691, Stockholm, Sweden
¹⁷ School of Sciences, European University Cyprus, Diogenes Street, Engomi 1516 Nicosia, Cyprus
¹⁸ Cosmic Dawn Center (DAWN)
¹⁹ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen N, Denmark
²⁰ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
²¹ School of Physics and Electrical Engineering, Liupanshui Normal University, Liupanshui, Guizhou 553004, P.R. China
²² Department of Mathematics and Physics, School of Biomedical Engineering, Southern Medical University, Guangzhou 510515, P.R. China

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Received: 5 February 2026
Accepted: 24 March 2026

Abstract

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 ⁵⁶Co →⁵⁶Fe 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 ⁵⁶Ni decay scenario requires a large nickel mass of up to ∼0.37 M_⊙ 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 ∼0.21 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 [O I] λλ6300, 6364 doublet implies an oxygen mass of ∼1.82 − 3.34 M_⊙, and the combination of a [Ca II]/[O I] flux ratio of ∼0.5 with nebular spectral model comparisons favours a progenitor zero-age main-sequence mass of ≥17 M_⊙.