A Hydrogen-Poor Superluminous Supernova with Enhanced Iron-Group Absorption: A New Link Between SLSNe and Broad-Lined Type Ic SNe

Abstract

We present optical observations of the Type I superluminous supernova (SLSN-I) SN2017dwh at $z\approx 0.13$, which reached $M_i\approx -21$ mag at peak. Spectra taken a few days after peak show an unusual and strong absorption line centered near 3200Ã that we identify with Co II, suggesting a high fraction of synthesized ${}^{56}$Ni in the ejecta. By $\sim 1$ month after peak, SN2017dwh became much redder than other SLSNe-I, instead strongly resembling broad-lined Type Ic supernovae (Ic-BL SNe) with clear suppression of the flux redward of $\sim 5000$ Ã , providing further evidence for a large mass of Fe-group elements. Late-time upper limits indicate a ${}^{56}$Ni mass of $\lesssim 0.6$ M${}_{\odot }$, leaving open the possibility that SN2017dwh produced a ${}^{56}$Ni mass comparable to SN1998bw ($\approx 0.4$ M${}_{\odot }$). Fitting the light curve with a combined magnetar and ${}^{56}$Ni model using $\mathtt{MOSFiT}$, we find that the light curve can easily accommodate such masses without affecting the inferred magnetar parameters. We also find that SN2017dwh occurred in the least-luminous detected host galaxy to date for a SLSN-I, with $M_B=-13.5$ mag and an implied metallicity of $Z\sim 0.08$ $Z_{\odot }$. The spectral properties of SN2017dwh provide new evidence linking SLSNe-I with Type Ic-BL SNe, and in particular the high Fe-group abundance may be due to enhanced ${}^{56}$Ni production or mixing due to asphericity. Finally, we find that SN2017dwh represents the most extreme end of a correlation between continuum shape and Co II absorption strength in the near-peak spectra of SLSNe-I, indicating that Fe-group abundance likely accounts for some of the variation in their spectral shapes.

16 pages, 7 figures, Submitted to ApJ