[Paper Review] Early-time Spectropolarimetry of the Aspherical Type II Supernova SN 2023ixf

Sergiy S. Vasylyev, Yi Yang+14 more|arXiv (Cornell University)|Jul 3, 2023

Gamma-ray bursts and supernovae4 citations

TL;DR

This study presents the earliest-ever spectropolarimetric observations of Type II supernova SN 2023ixf, revealing a rapid evolution in continuum polarization (from ~1% to 0.5%) and a 70° rotation in polarization position angle within the first five days post-explosion. The changes coincide with the disappearance of flash-ionized emission lines (e.g., He II, C IV), indicating that the aspherical SN ejecta swept through dense, asymmetric circumstellar material (CSM) within days, likely shaped by pre-explosion mass loss, possibly from a binary interaction.

ABSTRACT

We present six epochs of optical spectropolarimetry of the Type II supernova (SN) 2023ixf ranging from $\sim$ 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 $p_{ ext{cont}} \approx 1$ % 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 $\sim 70^\circ$ 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, 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.

Motivation & Objective

• To investigate the geometric asymmetry of SN 2023ixf during its early evolution using spectropolarimetry.
• To determine the origin and structure of the circumstellar material (CSM) surrounding the progenitor star.
• To understand the timing and dynamics of shock interaction with dense CSM through polarization evolution.
• To constrain the progenitor's mass-loss history based on polarization and emission line behavior.
• To test models of aspherical supernova explosions and their interaction with asymmetric CSM.

Proposed method

• Optical spectropolarimetry was conducted using the Kast double spectrograph on the Shane 3 m telescope at Lick Observatory.
• Six epochs of data were collected from ~1.4 to 14.5 days after explosion, covering the shock breakout and early interaction phases.
• Polarization parameters (polarization degree p and position angle PA) were measured across the continuum and spectral lines.
• The Stokes I, Q, and U parameters were derived to track changes in polarization across the spectrum.
• The evolution of continuum and line polarization was analyzed to infer changes in scattering geometry and CSM structure.
• Comparison of polarization evolution with spectral features (e.g., He II, C IV, Hα) helped identify the onset of CSM interaction.

Experimental results

Research questions

• RQ1 What causes the rapid change in continuum polarization and position angle in the first five days after the explosion of SN 2023ixf?
• RQ2 How does the disappearance of flash-ionized emission lines correlate with changes in polarization?
• RQ3 What is the geometry and structure of the circumstellar material (CSM) that produced the observed polarization features?
• RQ4 What physical mechanism explains the 70° rotation in polarization position angle between days +2.5 and +4.5?
• RQ5 What does the temporal evolution of polarization reveal about the aspherical nature of the SN explosion and its interaction with pre-existing CSM?

Key findings

• Continuum polarization decreased from ~1% at +1.4 days to 0.5% by +3.5 days and remained stable through +14.5 days.
• The polarization position angle rotated by approximately 70° between +2.5 and +4.5 days, indicating a change in scattering geometry.
• The polarization changes coincided with the disappearance of flash-ionized emission lines (e.g., He II, C IV, N III), signaling the end of shock interaction with dense CSM.
• The rapid evolution suggests that the SN ejecta emerged from and swept through an optically thick, aspherical CSM within the first five days.
• The polarization evolution supports a scenario in which the CSM has a disk-like or toroidal geometry, likely formed by eruptive mass loss prior to explosion.
• The final polarization state (p ≈ 0.5%, PA ≈ 225°) stabilized by day +4.6, indicating that the ejecta had fully cleared the asymmetric CSM and were now probing the aspherical ejecta directly.