Polarimetry of the superluminous supernova LSQ14mo: no evidence for significant deviations from spherical symmetry

TL;DR

This study presents the first polarimetric observations of a superluminous supernova, LSQ14mo, revealing no significant asymmetries in its photosphere during the observed period, which challenges expectations from lower-luminosity supernovae.

Contribution

It provides the first polarimetric data for a Type I superluminous supernova, showing low polarization levels and suggesting spherical symmetry or a special viewing angle.

Findings

No significant polarization evolution detected (< 2σ)

Average polarization consistent with interstellar polarization

Photosphere likely symmetric during observations

Abstract

We present the first polarimetric observations of a Type I superluminous supernova (SLSN). LSQ14mo was observed with VLT/FORS2 at five different epochs in the V band, with the observations starting before maximum light and spanning 26 days in the rest frame (z=0.256). During this period, we do not detect any statistically significant evolution (< 2$\sigma$) in the Stokes parameters. The average values we obtain, corrected for interstellar polarisation in the Galaxy, are Q = -0.01% ($\pm$ 0.15%) and U = - 0.50% ($\pm$ 0.14%). This low polarisation can be entirely due to interstellar polarisation in the SN host galaxy. We conclude that, at least during the period of observations and at the optical depths probed, the photosphere of LSQ14mo does not present significant asymmetries, unlike most lower-luminosity hydrogen-poor SNe Ib/c. Alternatively, it is possible that we may have observed LSQ14mo from a special viewing angle. Supporting spectroscopy and photometry confirm that LSQ14mo is a typical SLSN I. Further studies of the polarisation of Type I SLSNe are required to determine whether the low levels of polarisation are a characteristic of the entire class and to also study the implications for the proposed explosion models.