2017A&A...602A...9C

We present and analyse an extensive dataset of the superluminous supernova (SLSN) LSQ14mo (z=0.256), consisting of a multi-colour light curve from -30 d to +70 d in the rest-frame (relative to maximum light) and a series of six spectra from PESSTO covering -7d to +50d. This is among the densest spectroscopic coverage, and best-constrained rising light curve, for a fast-declining hydrogen-poor SLSN. The bolometric light curve can be reproduced with a millisecond magnetar model with ∼4M_☉ ejecta mass, and the temperature and velocity evolution is also suggestive of a magnetar as the power source. Spectral modelling indicates that the SN ejected ∼6M_☉ of CO-rich material with a kinetic energy of ∼7x10⁵¹erg, and suggests a partially thermalised additional source of luminosity between -2d and +22d. This may be due to interaction with a shell of material originating from pre-explosion mass loss. We further present a detailed analysis of the host galaxy system of LSQ14mo. PESSTO and GROND imaging show three spatially resolved bright regions, and we used the VLT and FORS2 to obtain a deep (five-hour exposure) spectra of the SN position and the three star-forming regions, which are at a similar redshift. The FORS2 spectrum at +300-days shows no trace of SN emission lines and we place limits on the strength of [OI] from comparisons with otherIc supernovae. The deep spectra provides a unique chance to investigate spatial variations in the host star-formation activity and metallicity. The specific star-formation rate is similar in all three components,as is the presence of a young stellar population. However, the position of LSQ14mo exhibits a lower metallicity, with 12+log (O/H)=8.2 in both the R₂₃ and N2 scales (corresponding to ∼0.3Z_☉ ). We propose that the three bright regions in the host system are interacting, which could induce gas flows triggering star formation in low-metallicity regions.