Late-phase Spectropolarimetric Observations of Superluminous Supernova SN 2017egm to Probe the Geometry of the Inner Ejecta

Abstract

We present our spectropolarimetric observations of SN 2017egm, a Type I superluminous supernova (SLSN-I) in a nearby galaxy NGC 3191, with the Subaru telescope at +185.0 days after the g-band maximum light. This is the first spectropolarimetric observation for SLSNe at late phases. We find that the degree of the polarization in the late phase significantly changes from that measured at the earlier phase. The spectrum at the late phase shows a strong Ca emission line and therefore we reliably estimate the interstellar polarization (ISP) component assuming that the emission line is intrinsically unpolarized. By subtracting the estimated ISP, we find that the intrinsic polarization at the early phase is only ∼0.2%, which indicates an almost spherical photosphere, with an axial ratio ∼1.05. The intrinsic polarization at the late phase increases to ∼0.8%, which corresponds to the photosphere with an axial ratio ∼1.2. A nearly constant position angle of the polarization suggests the inner ejecta are almost axisymmetric. By these observations, we conclude that the inner ejecta are more aspherical than the outer ejecta. This may suggest the presence of a central energy source producing aspherical inner ejecta.

Gamma-ray Thermalization and Leakage from Millisecond Magnetar Nebulae: Towards a Self-Consistent Model for Superluminous Supernovae

“…This could result in the gamma-ray escaping earlier than predicted by our calculations, which assume a homogeneous ejecta shell. Polarization studies of hydrogen-poor SLSNe at early times following the explosion show little evidence for significant asphericity of the outer ejecta layers (Leloudas et al 2015;Saito et al 2020); however, higher polarizations have been measured during the later nebular phase revealing more aspherical inner ejecta (Inserra et al 2016;Saito et al 2020). Nevertheless, the implied deviations from spherical symmetry are not likely to alter our results at the qualitative level.…”

Three-dimensional Hydrodynamic Simulations of Supernova Ejecta with a Central Energy Source

“…The increased polarization degree from the first to the second observing epochs indicates the presence of the aspherical inner core, which is revealed as the photosphere recedes in the ejecta. The nearby SLSN-I 2017egm showed a similar increase in the polarization degree (Bose et al 2018;Maund et al 2019;Saito et al 2020). Leloudas et al (2017) point out that the increase in the polarization degree in SN 2015bn coincides with the spectral transition.…”

“…4.4.3. Inner ejecta structure probed by polarimetric observations Polarization signals from SLSNe-I are also an important probe for the multi-dimensional ejecta structure (e.g., Leloudas et al 2015Leloudas et al , 2017Inserra et al 2016;Bose et al 2018;Cikota et al 2018;Lee 2019;Maund et al 2019;Saito et al 2020). Polarimetric observations of some SLSNe-I reported no significant linear polarization around the optical maximum (e.g, LSQ14mo; Leloudas et al 2015Lee 2019), suggesting a nearly spherical photosphere.…”

Post maximum light and late time optical imaging polarimetry of type I superluminous supernova 2020znr

“…This might happen here for SN2020znr which has been observed with broad band linear polarimetry only. SN2015bn and SN2017egm were both observed with linear spectropolarimetry (Inserra et al (2016) and Saito et al (2020) respectively), which by definition is less subject to mitigation effect of the net polarization signal due to the integration of the signal over a broader band. The increase of polarization detected on SN2017egm was from the analysis of linear spectropolarimetry data only (Saito et al 2020).…”

“…SN2015bn and SN2017egm were both observed with linear spectropolarimetry (Inserra et al (2016) and Saito et al (2020) respectively), which by definition is less subject to mitigation effect of the net polarization signal due to the integration of the signal over a broader band. The increase of polarization detected on SN2017egm was from the analysis of linear spectropolarimetry data only (Saito et al 2020). On the other hand the increase of linear polarization observed on SN2015bn was detected both with linear spectropolarimetry and broad band linear polarimetry ( (Inserra et al 2016).…”