Gaia16apd – a link between fast and slowly declining type I superluminous supernovae

We present ultraviolet (UV), optical and infrared photometry and optical spectroscopy of the type Ic superluminous supernova (SLSN) Gaia16apd (=SN 2016eay), covering its evolution from 26 d before the g-band peak to 234.1 d after the peak. Gaia16apd was followed as a part of the NOT Unbiased Transient Survey (NUTS). It is one of the closest SLSNe known (z = 0.102 ± 0.001), with detailed optical and UV observations covering the peak. Gaia16apd is a spectroscopically typical type Ic SLSN, exhibiting the characteristic blue early spectra with O ii absorption, and reaches a peak Mg = −21.8 ± 0.1 mag. However, photometrically it exhibits an evolution intermediate between the fast and slowly declining type Ic SLSNe, with an early evolution closer to the fast-declining events. Together with LSQ12dlf, another SLSN with similar properties, it demonstrates a possible continuum between fast and slowly declining events. It is unusually UV-bright even for an SLSN, reaching a non-K-corrected Muvm2 ≃ −23.3 mag, the only other type Ic SLSN with similar UV brightness being SN 2010gx. Assuming that Gaia16apd was powered by magnetar spin-down, we derive a period of P = 1.9 ± 0.2 ms and a magnetic field of B = 1.9 ± 0.2 × 10$^{14}$ G for the magnetar. The estimated ejecta mass is between 8 and 16 M⊙, and the kinetic energy between 1.3 and 2.5 × 10$^{52}$ erg, depending on opacity and assuming that the entire ejecta is swept up into a thin shell. Despite the early photometric differences, the spectra at late times are similar to slowly declining type Ic SLSNe, implying that the two subclasses originate from similar progenitors.TK acknowledges financial support by the Emil Aaltonen Foundation. NB was supported by the GROWTH project funded by the National Science Foundation under Grant No. 1545949. JH acknowledges support by the Finnish Cultural Foundation. MF acknowledges the support of a Royal Society – Science Foundation Ireland University Research Fellowship. NER acknowledges financial support by the 1994 PRIN-INAF 2014 (project ‘Transient Universe: unveiling new types of stellar explosions with PESSTO’) and by MIUR PRIN 2010-2011, ‘The dark Universe and the cosmic evolution of baryons: from current surveys to Euclid’. EYH acknowledges the support provided by the National Science Foundation under Grant No. AST-1008343 and AST-1613472. MDS is funded by generous support provided by the Danish Agency for Science and Technology and Innovation realized through a Sapere Aude Level 2 grant and a grant from the Villum Foundation. This work was supported by the MINECO (Spanish Ministry of Economy) – FEDER through grants ESP2016-80079-C2-1-R and ESP2014-55996-C2-1-R and MDM-2014-0369 of ICCUB (Unidad de Excelencia ‘María de Maeztu’). NUTS is funded in part by the IDA (Instrument Centre for Danish Astronomy). ŁW was supported by Polish National Science Centre Grant No. OPUS 2015/17/B/ST9/03167