Far-ultraviolet to Near-infrared Spectroscopy of a Nearby Hydrogen-poor Superluminous Supernova Gaia16apd

We report the first maximum-light far-ultraviolet (FUV) to near-infrared (NIR) spectra (1000 - 1.62 μm, rest) of a hydrogen-poor superluminous supernova, Gaia16apd. At z = 0.1018, it is the second closest and the UV brightest SLSN-I, with 17.4 mag in Swift UVW2 band at -11 days pre-maximum. The coordinated observations with HST, Palomar, and Keck were taken at -2 to +25 days. Assuming an exponential (or t ²) form, we derived the rise time of 33 days and the peak bolometric luminosity of 3 ×10⁴⁴ erg s^-1. At the maximum, the photospheric temperature and velocity are 17,000 K and 14,000 km s^-1, respectively. The inferred radiative and kinetic energy are roughly 1 × 10⁵¹ and 2 ×10⁵² erg. Gaia16apd is extremely UV luminous, and emits 50% of its total luminosity at 1000-2500 . Compared to the UV spectra (normalized at 3100 ) of well studied SN1992A (Ia), SN2011fe (Ia), SN1999em (IIP), and SN1993J (IIb), it has orders of magnitude more FUV emission. This excess is interpreted primarily as a result of weaker metal-line blanketing due to a much lower abundance of iron group elements in the outer ejecta. Because these elements originate either from the natal metallicity of the star, or have been newly produced, our observation provides direct evidence that little of these freshly synthesized material, including ⁵⁶Ni, were mixed into the outer ejecta, and the progenitor metallicity is likely sub-solar. This disfavors Pair-instability Supernova models with helium core masses ≥ 90 M_⊙ , where substantial ⁵⁶Ni material is produced. A higher photospheric temperature definitely contributes to the FUV excess from Gaia16apd. Compared with Gaia16apd, we find PS1-11bam is also UV luminous.