A Hydrogen-poor Superluminous Supernova with Enhanced Iron-group Absorption: A New Link between SLSNe and Broad-lined Type Ic SNe

摘要

We present optical observations of the Type I superluminous supernova (SLSN-I) SN 2017dwh at z?≈?0.13, which reached M i ?≈??21 mag at peak. Spectra taken a few days after peak show an unusual and strong absorption line centered near 3200 ? that we identify with Co ii, suggesting a high fraction of synthesized 56Ni in the ejecta. By ~1 month after peak, SN 2017dwh 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 ~5000 ?, providing further evidence for a high mass of Fe-group elements. Late-time upper limits indicate a 56Ni mass of 0.6 M ⊙, leaving open the possibility that SN 2017dwh produced a 56Ni mass comparable to SN 1998bw (≈0.4 M ⊙). Fitting the light curve with a combined magnetar and 56Ni model using MOSFiT, we find that the light curve can easily accommodate such masses without affecting the inferred magnetar parameters. We also find that SN 2017dwh occurred in the least-luminous detected host galaxy to date for an SLSN-I, with M B ?=??13.5 mag and an implied metallicity of Z?~?0.08 Z ⊙. The spectral properties of SN 2017dwh provide new evidence linking SLSNe-I with Type Ic-BL SNe, and in particular, the high Fe-group abundance may be due to enhanced 56Ni production or mixing due to asphericity. Finally, we find that SN 2017dwh 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.