Infrared spectropolarimetric detection of intrinsic polarization from a core-collapse supernova

Abstract

Massive stars die an explosive death as a core-collapse supernova (CCSN). The exact physical processes that cause the collapsing star to rebound into an explosion is not well-understood, and the key in resolving this issue may lie in the measurement of the shape of CCSNe ejecta. Spectropolarimetry is the only way to perform this measurement for CCSNe outside of the Milky Way and Magellanic Clouds. We present an infrared (IR) spectropolarimetric detection of a CCSN, enabled by the new highly sensitive WIRC+Pol instrument at Palomar Observatory, that can observe CCSNe (M = −17 mags) out to 20 Mpc to ∼0.1% polarimetric precision. IR spectropolarimetry is less affected than optical by dust scattering in the circumstellar and interstellar media, thereby providing a more unbiased probe of the intrinsic geometry of the SN ejecta. SN 2018hna, a SN 1987A-like explosion, shows 2.0 ± 0.3% continuum polarization in the J band oriented at ∼160°on-sky at 182 d after the explosion. Assuming prolate geometry like in SN 1987A, we infer an ejecta axis ratio of <0.48 with the axis of symmetry pointing at 70°position angle. The axis ratio is similar to that of SN 1987A suggesting that they may share intrinsic geometry and inclination angle. Our data do not rule out oblate ejecta. We also observe one other core-collapse and two thermonuclear SNe in the J band. SN 2020oi, a stripped-envelope Type Ic SN in Messier 100 has p = 0.37 ± 0.09% at peak light, indicative of either a 10% asymmetry or host interstellar polarization. The SNe Ia, 2019ein and 2020ue have p <0.33% and <1.08% near peak light, indicative of asymmetries of less than 10% and 20%, respectively.

Spectropolarimetry of the Type Ia SN 2019ein rules out significant global asphericity of the ejecta

“…5 ). We note that these values are consistent with infrared spectropolarimetry of SN 2019ein, which found a 3 σ upper limit on polarization of 1.2 per cent around the SN peak brightness (Tinyanont et al 2021 ).…”

Spectropolarimetry of the Type Ia SN 2019ein rules out significant global asphericity of the ejecta

“…5). We note that these values are consistent with infrared spectropolarimetry of SN 2019ein, which found a 3𝜎 upper limit on polarization of 1.2% around the SN peak brightness (Tinyanont et al 2021).…”

An Early-time Optical and Ultraviolet Excess in the Type-Ic SN 2020oi

“…In theory, the mass of nickel-rich material needed to explain our early-time emission is likely small (Magee & Maguire 2020). However, as we note in earlier sections, significant asymmetry in the ejecta is at odds with the negligible polarization at peak light observed by Tinyanont et al (2021). Further, we have found in Section 8 that by including C and He at significantly higher radii than the rest of the ejecta, we are able to reproduce the day 3.3 spectrum more faithfully than by considering an excess contribution of Ni and Fe.…”

“…Further, because the SN ejecta are distorted by the presence of the companion, the subsequent emission should show polarization indicative of ejecta asymmetries. Observations of SN 2020oi taken using the WIRC+Pol instrument at Palomar Observatory (Tinyanont et al 2021) near peak found a broadband polarization of p = 0.37 ± 0.09%, low enough to be explained by interstellar dust scattering and not asymmetry within the explosion itself. Because the flux excess timescale agrees more closely with the highly inclined interactions simulated in Kasen (2010), and the polarization measurements were taken long after any potential interaction, early asymmetry may be difficult to detect; further, the polarization signature of companion interaction at peak light (or lack thereof) remains unconstrained in the literature.…”

“…In this paper, we assume a redshiftindependent distance of 17.1 Mpc corresponding to the distance derived from Cepheids (Freedman et al 1994a). We note that the distance adopted in the analysis for the Ca-rich transient SN 2019ehk in the same galaxy was d ≈ 16.2 Mpc, while the distances used in the previous analyses of SN 2020oi were 14 Mpc, 16.22 Mpc, and 16 Mpc, respectively Rho et al 2021;Tinyanont et al 2021). Although these values are roughly consistent, they will be the source of some discrepancy between the SN parameters derived in this work and those from the previous studies.…”