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.

Optical polarization and spectral properties of the hydrogen-poor superluminous supernovae SN 2021bnw and SN 2021fpl

“…In addition to this point, it is interesting to note that none of the light curves of SN 2021bnw and SN 2021fpl show any strong bumps or undulations after maximum light, contrary to what can be seen, for example, from the the light curves of SN 2018bsz, SN 2017egm and 2017gci (see plot in Figure 5 of Pursiainen et al 2023). After that stage, the light curves are rather smoothly decreasing.…”

“…After that stage, the light curves are rather smoothly decreasing. It looks therefore reasonable to assume that none of the polarization measure- ments obtained on these two SLSNe were possibly 'contaminated' by polarization components that would be produced by interactions of the photospheres with the CSM, which is an important point raised by Pursiainen et al (2023). The presence of a dominant axis in the 𝑄 −𝑈 plane observed on SN 2021fpl also strengthen that hypothesis.…”

“…For SN 2015bn and SN 2021fpl, the increase of polarization, or high level of polarization, detected after optical maximum light could be favored by higher mixing and clumping than from weaker engines. On the other hand, as it is mentioned by Pursiainen et al (2023), SN 2020znr (W type) was observed with polarimetry during the nebular phase but there was no detection of an increase of polarization on that source in that phase (Poidevin et al 2022). In this case it could be that the temperature of its photosphere was still high enough to prevent the detection of the effects of a high level of mixing, or that the mass of the ejecta was so high that effects of it's clumpy structure was still not detectable in the R-filter used for linear polarimetry, or both.…”

“…Understanding polarization properties of H-poor SLSNe to retrieve information from their inner photospheres that could help to understand the nature of their progenitor is not an easy problem to deal with. Its looks reasonable to assume that before maximum light, SLSN-I photospheres are almost spherical (This work, Inserra et al 2016;Pursiainen et al 2023) with low linear polarization mainly produced by Thomson scattering of light from free electrons, and this independently of the viewing angle of the inner system. According to the SYN++ analysis the photosphere temperatures of SN 2021bnw (W type) were higher compared to those of SN 2021fpl (15bn type) consistently with Könyves-Tóth (2022) when polarimetry was obtained on each of these sources.…”

SN 2020udy: an SN Iax with strict limits on interaction consistent with a helium-star companion

“…Broad-band linear polarimetry was obtained with ALFOSC at the NOT on three epochs between g -band maximum and + 30 d. Observations were obtained in the B and V bands, except the last MNRAS 525, 1210-1228 (2023) epoch, where only V was used (see Table 3 ). All observations were obtained at four half-wave plate retarder angles (0 • , 22.5 • , 45 • , and 67.5 • ) and the data were reduced and analysed following the methods outlined in Pursiainen et al ( 2023 ). These authors presented a study of a sample of superluminous SNe, and investigated in detail NOT/ALFOSC polarimetry data including issues such as the shape of the point spread function, the optimal choice of aperture, the impact of S/N, the effect of the moon and the use of field stars to determine the (Galactic) interstellar polarization (ISP).…”