SN 2017ckj: A linearly declining type IIb supernova with a relatively massive hydrogen envelope

We present optical observations of the type IIb supernova (SN) 2017ckj, covering approximately 180 days after the explosion. Its early-time multi-band light curves display no clear evidence of a shock-cooling tail, resembling the behaviour of SN 2008ax. The V-band light curve exhibits a short rise time of about 5 days and reaches an absolute fitted peak magnitude of MV = −18.49 ± 0.18 mag. The late-time multi-band light curves reveal a linear decline. We modelled the bolometric light curve of SN 2017ckj to constrain the progenitor and the explosion parameters. We estimated a total mass of 56Ni synthesised by SN 2017ckj of $ M_{\mathrm{Ni}} = 0.21^{+0.05}_{-0.03}\,\mathrm{M}_\odot $, with a massive H-rich envelope of $ M_{\mathrm{env}} = 0.4^{+0.1}_{-0.1}\,\mathrm{M}_\odot $. Both the 56Ni mass and the envelope mass of SN 2017ckj are higher than those of typical SNe IIb, in agreement with its peculiar light curve evolution. The early-time spectra of SN 2017ckj are dominated by a blue continuum, accompanied by narrow Hα and He <ns0:sc>II</ns0:sc> emission lines. The earliest spectrum exhibits flash ionisation features, from which we estimated a progenitor mass-loss rate of ∼3 × 10−4 M⊙ yr−1. At later epochs, the spectra develop broad P-Cygni profiles and become increasingly similar to those of SNe IIb, especially SN 2018gk. The late-time spectrum at around 139 days does not show a distinct decline in the strength of its Hα emission profile, also indicating a relatively massive envelope of its progenitor. Aside from the Hα feature, the nebular spectrum exhibits prominent emission lines of O <ns0:sc>I</ns0:sc>, Ca <ns0:sc>II</ns0:sc>, [Ca <ns0:sc>II</ns0:sc>], and Mg <ns0:sc>I</ns0:sc>], which are consistent with the prototypical SN 1993J.