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We present imaging polarimetry of the superluminous supernova SN 2015bn, obtained over nine epochs between $-$20 and $+$46 days with the Nordic Optical Telescope. This was a nearby, slowly-evolving Type I superluminous supernova that has been studied extensively and for which two epochs of spectropolarimetry are also available. Based on field stars, we determine the interstellar polarisation in the Galaxy to be negligible. The polarisation of SN 2015bn shows a statistically significant increase during the last epochs, confirming previous findings. Our well-sampled imaging polarimetry series allows us to determine that this increase (from $sim 0.54%$ to $gtrsim 1.10%$) coincides in time with rapid changes that took place in the optical spectrum. We conclude that the supernova underwent a `phase transition at around $+$20 days, when the photospheric emission shifted from an outer layer, dominated by natal C and O, to a more aspherical inner core, dominated by freshly nucleosynthesized material. This two-layered model might account for the characteristic appearance and properties of Type I superluminous supernovae.
The origin of the luminosity of superluminous supernovae (SLSNe) is an unresolved mystery, and a number of very different physical scenarios (including energy injection from magnetars, collision with a dense circumstellar medium and pair instability-
We present observations of SN 2015bn (= PS15ae = CSS141223-113342+004332 = MLS150211-113342+004333), a Type I superluminous supernova (SLSN) at redshift $z=0.1136$. As well as being one of the closest SLSNe I yet discovered, it is intrinsically brigh
We present the characterization of two engineered diffusers mounted on the 2.5 meter Nordic Optical Telescope, located at Roque de Los Muchachos, Spain. To assess the reliability and the efficiency of the diffusers, we carried out several test observ
This paper describes the Polarization Spectroscopic Telescope Array (PolSTAR), a mission proposed to NASAs 2014 Small Explorer (SMEX) announcement of opportunity. PolSTAR measures the linear polarization of 3-50 keV (requirement; goal: 2.5-70 keV) X-
Superluminous supernovae (SLSNe) are found predominantly in dwarf galaxies, indicating that their progenitors have a low metallicity. However, the most nearby SLSN to date, SN 2017egm, occurred in the spiral galaxy NGC 3191, which has a relatively hi