# Time-resolved polarimetry of the superluminous SN 2015bn with the Nordic Optical Telescope

## TL;DR

This study presents detailed time-resolved polarimetry of superluminous supernova SN 2015bn, revealing a phase transition in its ejecta structure around 20 days post-peak, which enhances understanding of its explosion mechanism.

## Contribution

It provides the first well-sampled imaging polarimetry series of SN 2015bn, demonstrating a significant increase in polarization linked to a structural phase transition in the supernova.

## Key findings

- Polarization increased from ~0.54% to >1.10% during late epochs.
- The phase transition occurred around +20 days, indicating a shift in the ejecta structure.
- The supernova's inner core is more aspherical and dominated by nucleosynthesized material.

## Abstract

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.