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

TL;DR

SN 2017ati is a notably luminous Type IIb supernova whose light curve cannot be explained by $^{56}$Ni decay alone. A magnetar-powered component provides the best fit to the multi-band photometry, yielding $M_{\mathrm{Ni}} \approx 0.21\,M_\odot$, $B \approx 1.32\times10^{15}$ G, and $P_{spin} \approx 28$ ms, with an ejecta mass around $M_{ej} \approx 1.82\,M_\odot$. Nebular spectroscopy implies a substantial oxygen mass ($M_O \approx 1.82$–$3.34\,M_\odot$) and a progenitor ZAMS mass of $M_{ZAMS} \gtrsim 17\,M_\odot$, consistent with a relatively massive, compact progenitor that experienced partial envelope stripping. Taken together, these results support a hybrid energy-engine scenario for SN 2017ati and highlight magnetar input as a plausible driver for the high luminosity observed in some luminous IIb SNe, with important implications for the pre-SN evolution of their progenitors.

Abstract

We present optical photometric and spectroscopic observations of the Type~IIb supernova (SN)~2017ati. It reached the maximum light at about 27~d after the explosion and the light curve shows a broad, luminous peak with an absolute $r$-band magnitude of $M_{r} = -18.48 \pm 0.16$~mag. At about 50~d after maximum light, SN~2017ati exhibits a decline rate close to that expected from the $^{56}$Co $\rightarrow$ $^{56}$Fe radioactive decay, at 0.98 mag per 100 days, as 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 $\sim0.37\,M_{\odot}$ 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 $\sim0.21\,M_{\odot}$, 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 [\Oi]~$λ\lambda6300,6364$ doublet implies an oxygen mass of $\sim1.82-3.34\,M_{\odot}$, and the combination of a [\Caii]/[\Oi] flux ratio of $\sim0.5$ with nebular spectral model comparisons favours a progenitor zero-age main-sequence mass of $\geq17\,M_{\odot}$.

Figures (16)

• Figure 1: Absolute r/G-band light curve of SN 2017ati compared with other SNe IIb in r/R-band. All light curves have been corrected for reddening and shifted according to the distances listed in Table \ref{['tab:SNe_IIb']}.
• Figure 2: Colour evolution of SN 2017ati, compared to a sample of SNe IIb. Upper panel: $B~-~V$ colour evolution. Lower panel: ($R~-~I$) or ($r~-~i$) colour evolution. The colour curves are corrected for both Galactic and host galaxy extinction.
• Figure 3: Comparison of the pseudo-bolometric light curve of SN 2017ati with those of SN 1987A and other Type IIb SNe. The grey dashed line illustrates the expected slope of the light curve under the assumption that all energy from $^{56}$Co decay is fully thermalized by the ejecta.
• Figure 4: Optical spectral evolution of SN 2017ati from +27.3 days to +148.4 days since the explosion. The spectra have been corrected for reddening and redshift, and vertically shifted for better visualization. The epochs are indicated to the right of each spectrum. The positions of major telluric absorption lines are denoted by the $\bigoplus$ symbol.
• Figure 5: Line profiles of Hei , Oi , $\rm [\ion{O}{i}\,]$, $\rm [\ion{Ca}{ii}\,]$ and $\rm \ion{Ca}{ii}\,\ NIR$ within the spectra of SN 2017ati. The dashed lines indicate the velocities corresponding to the rest wavelengths of the emission lines at $\lambda\lambda5016$, 5876, 7774, 6300, 7291, and 8751. The epoch of each spectrum is given in the rest frame, relative to the estimated explosion date.