Polarimetry of hydrogen-poor superluminous supernovae

Abstract

We present linear polarimetry for seven hydrogen-poor superluminous supernovae (SLSNe-I) of which only one has previously published polarimetric data. The best-studied event is SN 2017gci, for which we present two epochs of spectropolarimetry at +3 d and +29 d post-peak in rest frame, accompanied by four epochs of imaging polarimetry up to +108 d. The spectropolarimetry at +3 d shows increasing polarisation degree P towards the redder wavelengths and exhibits signs of axial symmetry, but at +29 d, P ∼ 0 throughout the spectrum, implying that the photosphere of SN 2017gci evolved from a slightly aspherical configuration to a more spherical one in the first month post-peak. However, an increase of P to ∼0.5% at ∼ + 55 d accompanied by a different orientation of the axial symmetry compared to +3 d implies the presence of additional sources of polarisation at this phase. The increase in polarisation is possibly caused by interaction with circumstellar matter (CSM), as already suggested by a knee in the light curve and a possible detection of broad Hα emission at the same phase. We also analysed the sample of all 16 SLSNe-I with polarimetric measurements to date. The data taken during the early spectroscopic phase show consistently low polarisation, indicating at least nearly spherical photospheres. No clear relation between the polarimetry and spectral phase was seen when the spectra resemble Type Ic SNe during the photospheric and nebular phases. The light-curve decline rate, which spans a factor of eight, also shows no clear relation with the polarisation properties. While only slow-evolving SLSNe-I have shown non-zero polarisation, the fast-evolving ones have not been observed at sufficiently late times to conclude that none of them exhibit changing P. However, the four SLSNe-I with increasing polarisation degree also have irregular light-curve declines. For up to half of them, the photometric, spectroscopic, and polarimetric properties are affected by CSM interaction. As such, CSM interaction clearly plays an important role in understanding the polarimetric evolution of SLSNe-I.

No UV-bright Eruptions from SN 2023ixf in GALEX Imaging 15–20 yr Before Explosion

“…They originate from massive (≳ 8 M ⊙ ) stars, but the stellar evolution in the decades prior to explosions remains unclear. There is some observational evidence for enhanced mass-loss prior to explosion traced by narrow emission lines (e.g., Sollerman et al 2020), peculiar photometric evolution (e.g., Pursiainen et al 2023), or both (e.g., Nyholm et al 2017). However, there is also evidence that pre-explosion variability does not occur (Johnson et al 2018).…”

Type W and Type 15bn Subgroups of Hydrogen-poor Superluminous Supernovae: Premaximum Diversity, Postmaximum Homogeneity?

“…It was concluded by Könyves-Tóth (2022) that one of the main physical differences between Type W and Type 15bn SLSNe-I can be found in the photospheric temperatures: Type W SLSNe-I tend to show a hotter photosphere compared to Type 15bn objects. The two groups may differ in the presence (Type W) or absence (Type 15bn) of prepeak bumps in the LC (see Könyves-Tóth 2022 for further details and Leloudas et al 2012;Nicholl et al 2015;Smith et al 2016;Vreeswijk et al 2017;Anderson et al 2018;Lin et al 2020;Chen et al 2021;Fiore et al 2021;Pursiainen et al 2022 as original references) and in polarimetric respects as well (see Könyves-Tóth 2022 and Inserra et al 2016;Cikota et al 2018;Lee 2019;Maund et al 2021;Pursiainen et al 2023). Although the polarimetric sample of SLSNe-I is still sparse, the following hypothesis was created: Type W SLSNe-I may originate from spherically symmetric progenitors and show null polarization, while a two-component model having a more symmetric outer layer and a more asymmetric inner layer (see Inserra et al 2016) may better represent Type 15bn SLSNe-I that show increasing polarization.…”