Polarimetry of hydrogen-poor superluminous supernovae

¹ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
e-mail: miipu@space.dtu.dk
² Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
³ Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, 44122 Ferrara, Italy
⁴ INFN, Sezione di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
⁵ INAF, Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini snc, 64100 Teramo, Italy
⁶ Cardiff Hub for Astrophysics Research and Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff, CF24 3AA UK
⁷ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
⁸ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Austin, TX, 78712 USA
⁹ Birmingham Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Edgbaston Park Rd, Birmingham, B15 2TT UK
¹⁰ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 234 Herzl Street, 76100 Rehovot, Israel
¹¹ Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield, S3 7RH UK
¹² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
¹³ Department of Astronomy, University of California, 501 Campbell Hall #3411, Berkeley, CA, 94720-3411 USA

Received: 19 January 2023
Accepted: 18 March 2023

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.