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

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 profile is dominated by a primary component spanning ~25,000--40,000 km/s, alongside a distinct secondary component extending above 28,000 km/s whose polarization implies a non-axisymmetric, complex excitation geometry toward the outer ejecta By implementing a three-dimensional Monte-Carlo calculation, we infer that a viewing angle of ~40 degree from the symmetry axis of the excitation structure could plausibly reproduce the observed spectral and polarization profiles of the Ca II triplet.

• The paper demonstrates that SN 2026gzf exhibits a quasi-spherical electron-scattering photosphere with minimal continuum polarization (<0.2%), supporting nonrelativistic shock breakout models.
• The paper uses detailed spectropolarimetric diagnostics and 3D Monte Carlo radiative transfer modeling to reveal distinct high-velocity Ca II components with differing symmetry axes.
• The paper implies that asymmetries in ionization fronts and circumstellar material interactions, rather than mere elemental abundance, are critical for interpreting explosion mechanisms.

Geometry and Excitation Structure of Type Ic-BL Supernova 2026gzf

Observational Context and Photometric Properties

SN 2026gzf, classified as a Type Ic broad-line supernova (SN Ic-BL), was contemporaneously associated with the soft extragalactic X-ray transient EP260321a. Prompt X-ray and optical follow-up revealed that SN 2026gzf underwent an energetic explosion with a shock breakout duration exceeding that expected for canonical Wolf-Rayet progenitors, implying extended circumstellar material (CSM) and prior mass loss episodes. The early spectral evolution lacked a nonthermal component, supporting a nonrelativistic shock breakout model with an upper shock velocity constraint of $U_{\text{shock}} \leq 0.08c$. Blackbody fits to the X-ray data yielded intrinsic absorption $N_H \approx 8.4 \times 10^{20}$ cm$^{-2}$ and temperatures in the range $kT \sim 112$–$124$ eV, marking SN 2026gzf as the softest and least luminous extragalactic fast X-ray transient observed by Einstein Probe (2606.18881).

Spectropolarimetric Diagnostics of Ejecta Geometry

Imaging polarimetry at days 4.6 and 16.5 post-shock breakout, coupled with VLT FORS2 spectropolarimetry, placed stringent constraints on both the global symmetry of the ejecta and the spatial distribution of key IME features. The continuum polarization remained low throughout ($p < 0.2\%$), indicating a highly spherical electron-scattering photosphere. Assuming a power-law density profile consistent with SN Ic-BL models ($n \sim 6$), the inferred axial ratio is $\sim 1.2$—compatible with a quasi-spherical configuration and minimal disruption of the progenitor envelope.

On day 16.5, the Ca II NIR triplet displayed a pronounced polarization peak exceeding $1.5\%$. Analysis of the Stokes $Q$-$N_H \approx 8.4 \times 10^{20}$0 plane confirmed an axisymmetric configuration for the Ca II line opacity, establishing the presence of a symmetry axis in the spatial distribution of oxygen-burning ashes. This axisymmetry does not extend to other IMEs; the polarization across Si II, O I, and Mg II features was negligible, signifying uniform excitation structures at the photosphere.

Modeling the Three-Dimensional Opacity Distribution

Detailed modeling using a three-dimensional Monte Carlo radiative transfer code (MCPol) incorporated electron scattering and line-resonance processes. Homologous expansion was assumed, with electrons dominating opacity at $N_H \approx 8.4 \times 10^{20}$1, and the Sobolev approximation applied for Ca II resonance lines. The best-fit model invoked two cone-shaped regions of enhanced Ca II opacity:

• Primary component: $N_H \approx 8.4 \times 10^{20}$2–$N_H \approx 8.4 \times 10^{20}$3 km/s, symmetry axis inclined by $N_H \approx 8.4 \times 10^{20}$4.
• Secondary component: $N_H \approx 8.4 \times 10^{20}$5 km/s, symmetry axis misaligned by $N_H \approx 8.4 \times 10^{20}$6.

An enhancement factor $N_H \approx 8.4 \times 10^{20}$7 was applied within $N_H \approx 8.4 \times 10^{20}$8 half-opening angles for both cones, and a viewing angle of $N_H \approx 8.4 \times 10^{20}$9 from the symmetry axis reproduced observed Ca II spectral and polarization profiles. The high-velocity component ($^{-2}$0 km/s) exhibited polarization perpendicular to the dominant axis, betraying a non-axisymmetric structure—interpreted as detached material dynamically distinct from the photosphere and incompatible with uniform clump distributions.

Physical Interpretation and Implications

The observed axisymmetric excitation structure traced by Ca II polarization is consistent with models of bipolar, jet-induced explosions, yet the lack of an on-axis relativistic jet is robustly supported by the absence of gamma-ray and radio signatures. The weak O I $^{-2}$1 feature in SN 2026gzf, combined with strong Ca II polarization, suggests an off-axis viewing geometry that is sensitive to the spatial configuration of the ionization fronts and the $^{-2}$2Ni distribution.

Polarization signatures in stripped-envelope SNe encode information not only about elemental abundance geometry but also about the complex interplay between asymmetric energy deposition and local ionization conditions. The misalignment between Ca II components implies the progenitor envelope and excitation front geometry are not coincident, potentially reflecting prior mass loss and differential shell stripping.

These results call for caution in semi-analytic modeling of SN Ic-BL shock-breakout emission using spherical symmetry and indicate that axisymmetric excitation fronts are generic but can be modulated by viewing angle and prior envelope geometry.

Numerical Results and Contradictory Claims

• Softness and low luminosity: SN 2026gzf exhibited the smallest radiated energy and softest spectrum among all known extragalactic fast X-ray transients.
• Polarization: Continuum polarization $^{-2}$3; Ca II polarization $^{-2}$4 at $^{-2}$5–$^{-2}$6 km/s.
• Ejecta velocities: TARDIS fitting yielded inner photospheric velocity $^{-2}$7 km/s, outer-edge velocity $^{-2}$8 km/s.
• Viewing angle: $^{-2}$9 from symmetry axis, matching constraints from radio nondetection ($kT \sim 112$014–45$kT \sim 112$1).

The paper challenges interpretations linking polarization directly to abundance distributions, emphasizing the dominant role of asymmetric ionization fronts and excitation structures, a position not universally corroborated in prior literature.

Practical and Theoretical Implications

The findings reinforce the utility of time-resolved spectropolarimetry in distinguishing between explosion mechanisms—especially for stripped-envelope SNe where the geometry is not masked by hydrogen/helium envelopes. They highlight the necessity of three-dimensional radiative transfer and polarization modeling to resolve multi-component excitation substructure, and advance interpretive frameworks for SN jet models, CSM influence, and progenitor evolution.

Future studies should prioritize multi-angle, multi-epoch spectropolarimetry, sophisticated hydrodynamic modeling of asymmetric shock breakout, and further investigation into the coupling between envelope stripping processes, nickel distribution, and ionization structure. Enhanced constraints on SN Ic-BL geometry will improve the calibration of these events as cosmological probes and their role as potential GRB progenitors.

Conclusion

This study of SN 2026gzf meticulously delineates the three-dimensional geometry and excitation structure within the ejecta of a Type Ic-BL supernova, supported by high-fidelity spectropolarimetric and radiative transfer analyses (2606.18881). The results provide a benchmark for interpreting asymmetric explosion physics in stripped-envelope SNe, underscore the discriminative power of polarization diagnostics, and urge refinement of models incorporating excitation-front geometry and viewing angle effects. The approach offers a template for future investigations aiming to unravel the spatial structure and energetic mechanisms underlying SN Ic-BL explosions.