ABSTRACT Detailed spectropolarimetric studies may hold the key to probing the explosion mechanisms and the progenitor scenarios of Type Ia supernovae (SNe Ia). We present multi-epoch spectropolarimetry and imaging polarimetry of SN 2019ein, an SN Ia showing high expansion velocities at early phases. The spectropolarimetry sequence spans from ∼−11 to +10 d relative to peak brightness in the B band. We find that the level of the continuum polarization of SN 2019ein, after subtracting estimated interstellar polarization, is in the range 0.0–0.3 per cent, typical for SNe Ia. The polarization position angle remains roughly constant before and after the SN light-curve peak, implying that the inner regions share the same axisymmetry as the outer layers. We observe high polarization (∼1 per cent) across both the Si ii λ6355 and Ca ii near-infrared triplet features. These two lines also display complex polarization modulations. The spectropolarimetric properties of SN 2019ein rule out a significant departure from spherical symmetry of the ejecta for up to a month after the explosion. These observations disfavour merger-induced and double-detonation models for SN 2019ein. The imaging polarimetry shows weak evidence for a modest increase in polarization after ∼20 d since the B-band maximum. If this rise is real and is observed in other SNe Ia at similar phases, we may have seen, for the first time, an aspherical interior similar to what has been previously observed for SNe IIP. Future polarization observations of SNe Ia extending to post-peak epochs will help to examine the inner structure of the explosion.
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This content will become publicly available on June 1, 2024
Polarimetry of hydrogen-poor superluminous supernovae
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 H α 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.
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- Award ID(s):
- 1813825
- NSF-PAR ID:
- 10434270
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 674
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A81
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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