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We present optical and ultraviolet photometry, as well as optical spectra,for the type II supernova (SN) 2015bf. Our observations cover the phases from$\sim 2$ to $\sim 200$ d after explosion. The first spectrum is characterisedby a blue continuum with a blackbody temperature of $\sim 24,000$K andflash-ionised emission lines. After about one week, the spectra of SN 2015bfevolve like those of a regular SN II. From the luminosity of the narrowemission component of H$\alpha$, we deduce that the mass-loss rate is largerthan $\sim 3.7\times10^{-3}\,{\rm M_\odot\,yr^{-1}}$. The disappearance of theflash features in the first week after explosion indicates that thecircumstellar material is confined within $\sim 6 \times 10^{14}$ cm. Thus, wesuggest that the progenitor of SN 2015bf experienced violent mass loss shortlybefore the supernova explosion. The multiband light curves show that SN 2015bfhas a high peak luminosity with an absolute visual magnitude $M_V = -18.11 \pm0.08$ mag and a fast post-peak decline with a $V$-band decay of $1.22 \pm 0.09$mag within $\sim 50$ d after maximum light. Moreover, the $R$-band tailluminosity of SN 2015bf is fainter than that of SNe~II with similar peak by1--2 mag, suggesting a small amount of ${\rm ^{56}Ni}$ ($\sim 0.009\,{\rmM_\odot}$) synthesised during the explosion. Such a low nickel mass indicatesthat the progenitor of SN 2015bf could be a super-asymptotic-giant-branch starthat collapsed owing to electron capture.

SN 2015bf: a fast declining type II supernova with flash-ionised signatures