Spectropolarimetric Evolution Reveals Dual-Axis Ejecta in the Atypical Magnetar-Powered SN 2012au

TL;DR

This study presents six epochs of optical spectropolarimetry for SN 2012au, revealing a highly aspherical, dual-axis ejecta geometry that evolves from an early prolate photosphere with helium-rich polar material to a distinct orthogonal inner structure in the nebular phase, consistent with magnetar-powered explosion models. By combining continuum and line polarization analyses across He I, Fe II, and O I features, the authors identify a persistent dominant axis at early times and a new axis at day 295, indicating time-dependent asymmetries layered within the ejecta. The work compares SN 2012au to other SNe Ib, highlighting its unusually high polarization and strong helium loops, and discusses implications for explosion mechanisms including magnetar activity and jet-related processes. The findings provide a link between extreme luminosity, asymmetry, and potential magnetar powering in a rare Type Ib SN, with broader relevance for understanding SESNe diversity and core-collapse physics.

Abstract

We present six epochs of optical spectropolarimetric observations of the unique and slow-evolving Type Ib supernova (SN) 2012au, between 0 and 295 days post R-band maximum. The polarization levels seen throughout our observations are on average 0.87% +/- 0.05% higher than those of any Type Ib SN~yet studied, suggesting either that it is the most asymmetric of the sample, or if all SNe Ib have similar asymmetry, that it is viewed at a more optimum angle. Significant continuum polarization indicates that the photosphere exhibited a global departure from spherical symmetry at the level of 10%-40% at the earliest times (days 0--40), which decreased to 0%-20% by days 57--90. During the early photospheric phase, the ejecta maintained a near-constant orientation of 12°-20° on the sky, as shown by the dominant axis in the Stokes q-u plane. Polarization signatures in the Fe II λλλ4924, 5018, 5169 lines shared this axis. Meanwhile, high levels of polarization associated with the He I lines traced distinct q-u loops with a dramatic rotation away from the dominant axis, indicating that the early-time ejecta were also characterized by hot, fast, helium-rich material concentrated near the poles. At day 295, during the transition to the nebular phase, a new, highly elongated structure became prominent in the ejecta, with an axis orthogonal to the dominant axis that defined the photospheric phase. This dual-axis geometry may link SN 2012au's high luminosity and asymmetric structure to a magnetar powering mechanism.

Figures (16)

• Figure 1: SN 2012au flux spectra over days 0--295 post $R$-band max, collected by the SNSPOL project. Flux spectra have been adjusted after flux calibration to stack neatly and used to identify key line features (labeled).
• Figure 2: Flux and polarization spectra for our SN 2012au observations. The relative flux spectra are in black and polarization spectra (binned to 15 Å) are in purple with error bars shaded. Polarization spectra are not ISP or continuum-subtracted; we note that from reddening considerations the maximum possible ISP is 0.66% (see § \ref{['sec: ISP']}). The rest wavelengths of prominent features in the spectra are marked by vertical lines and color-coded according to atomic species. The grey shaded regions span the ranges where we measured the continuum polarization for each epoch (§ \ref{['sec: continuum']}).
• Figure 3: ISP estimates (§ \ref{['sec: ISP']}) displayed on the $q-u$ plane for comparison. The ISP magnitude calculated from the Serkowski reddening maximum relationship is shown as a red circle to indicate no specific PA. The pink cross is the value of probe star HD 112142. The purple cross represents the value of the most distant probe star HD 112325. The small green cross represents the estimate presented in Pandey21. The yellow cross is the estimate we obtain from our days $85-90$ continuum region (§ \ref{['sec: continuum']}). The sizes of these crosses denote the uncertainty of each estimate. The solid blue and grey lines are the best fit lines to our data from days $0-7$ and 295, respectively, with the 5$\sigma$ errors shown as shaded regions around them.
• Figure 4: Continuum polarization for SN 2012au by observation day. Error-weighted means of the total polarization for the narrow continuum regions defined in § \ref{['sec: continuum']}, tabulated as $\overline{p}$ in Table \ref{['Tab:continuum_regions']}, are displayed as the red squares (7250--7350 Å) and blue diamonds (5120--5270 Å). Corresponding exponential fits are shown as black dashed lines with the corresponding exponent and residual sum of squares (RSS) displayed in black. For comparison, we also show power law fits with grey dotted lines and the power and RSS displayed in grey. Shaded regions around the data points represent the range of polarization magnitudes under different ISP assumptions (§ \ref{['sec: continuum']}; Table \ref{['Tab:continuum_regions']}). Exponential fits were not applied to these ISP-adjusted points because each point is calculated using a different ISP PA, dissolving any relation the points might have to one another over time.
• Figure 5: Full-spectrum (3890--7855 Å, binned to 15 Å) Stokes $q$ and $u$ data for all epochs of SN 2012au, color-coded by wavelength. Black diagonal lines represent the error-weighted best fit in each epoch to all the data excluding major line regions (He i$\lambda$5876, $\lambda$7065 and Fe ii$\lambda$$\lambda$$\lambda$4924, 5018, 5169; § \ref{['sec: continuum']}). The error-weighted average of points for the blue continuum region (5120--5270 Å) is displayed as a black star and for the red continuum region (7250--7350 Å) as a black X, except for day 26, where it is designated by a grey X as it was calculated by other means (§ \ref{['sec: line polarization']}). Our adopted upper-limit ISP magnitude is plotted as a red circle around the origin. The average uncertainty over all points is displayed as a cross at the top left of each diagram.