Spectropolarimetric Evolution Reveals Dual-axis Ejecta in the Atypical Magnetar-powered SN 2012au

We present six epochs of optical spectropolarimetric observations of the unique and slow-evolving Type Ib supernova (SN) 2012au, between 0 and 295 day 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.