Pinning Down the Geometry of the Type Ic Broad-line Supernova 2026gzf

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Abstract
Type Ic broad-line supernovae (SNe Ic-BL) are often associated with energetic explosions that display a prompt outburst of high-energy emission. Since their progenitor lost the H and He envelopes before the explosion exposing the C/O core, their explosion dynamics and geometry can be seen in an unobscured and undistorted way. We present imaging polarimetry and spectropolarimetry of the Type Ic-BL SN 2026gzf obtained 4.6 and 16.5 days after the X-ray shock breakout, which was recorded by the Einstein Probe satellite as EP260321a, showing it to be one of the softest and intrinsically dimmest extragalactic fast X-ray transients. The persistent low continuum polarization indicates that the outer layer of SN 2026gzf is mostly spherical, suggesting the explosion did not significantly disrupt the progenitor envelope. At day 16.5, the calcium near-infrared triplet displays a peak polarization above 1.5%. The geometry of the associated line opacity is also compatible with an axisymmetric configuration. The spatial distribution of such oxygen-burning ashes thus indicates the presence of a symmetry axis of the excitation structure within the nearly spherical ejecta. The Ca , II triplet polarization is dominated by a primary component spanning ∼25,000–40,000 km s −1 , while a possible high-velocity polarization substructure of modest statistical significance may indicate a complex, nonaxisymmetric excitation geometry in the outer ejecta. By implementing a three-dimensional Monte Carlo calculation, we infer that a viewing angle of ∼40 ∘ from the symmetry axis of the excitation structure could plausibly reproduce the observed spectral and polarization profiles of the Ca II triplet.
Funding Information
  • National Science Foundation (2206523)
  • National Science Foundation (2311323)
  • UK Research and Innovation (1062)
  • Villum Fonden (60862)
  • DOE ∣ SC ∣ Lawrence Berkeley National Laboratory (7707915)

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