Early Time Spectropolarimetry of the Aspherical Type II Supernova SN 2023ixf

Abstract

We present six epochs of optical spectropolarimetry of the Type II supernova (SN) 2023ixf ranging from ∼2 to 15 days after the explosion. Polarimetry was obtained with the Kast double spectrograph on the Shane 3 m telescope at Lick Observatory, representing the earliest such observations ever captured for an SN. We observe a high continuum polarization p cont ≈ 1% on days +1.4 and +2.5 before dropping to 0.5% on day +3.5, persisting at that level up to day +14.5. Remarkably, this change coincides temporally with the disappearance of highly ionized “flash” features. The decrease of the continuum polarization is accompanied by a ∼70° rotation of the polarization position angle (PA) as seen across the continuum. The early evolution of the polarization may indicate different geometric configurations of the electron-scattering atmosphere as seen before and after the disappearance of the emission lines associated with highly ionized species (e.g., He ii, C iv, and N iii), which are likely produced by elevated mass loss shortly prior to the SN explosion. We interpret the rapid change of polarization and PA from days +2.5 to +4.5 as the time when the SN ejecta emerge from the dense asymmetric circumstellar material (CSM). The temporal evolution of the continuum polarization and the PA is consistent with an aspherical SN explosion that exhibits a distinct geometry compared to the CSM. The rapid follow-up spectropolarimetry of SN 2023ixf during the shock ionization phase reveals an exceptionally asymmetric mass-loss process leading up to the explosion.