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

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{\deg}-20{\deg} on the sky, as shown by the dominant axis in the Stokes q-u plane. Polarization signatures in the Fe II {\lambda}{\lambda}{\lambda}4924, 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.