An axisymmetric shock breakout indicated by prompt polarized emission from the type II supernova 2024ggi

Abstract

The death of massive stars is triggered by an infall-induced bounce shock that disrupts the star. How such a shock is launched and propagates through the star is a decade-long puzzle. Some models assume that the shock can be reenergized by absorbing neutrinos, leading to highly aspherical explosions. Other models involve jet-powered shocks that lead to bipolar explosions reflected in the geometry of the shock-breakout emission. We report measurement of the geometry of the shock breakout through unprecedentedly early spectropolarimetry of the nearby type II supernova 2024ggi starting ~1.2 days after the explosion. The measurement indicates a well-defined symmetry axis of the shock breakout, which is also shared by the hydrogen-rich envelope that emerged after the circumstellar matter was engulfed by the ejecta, revealing a persisting and prominent symmetry axis throughout the explosion. These findings suggest that the physical mechanism driving the explosion of massive stars manifests a well-defined axial symmetry and acts on large scales.

Figures (26)

• Figure 1: Temporal evolution of the polarization of SN 2024ggi after subtracting the ISP. In the top-left and the bottom-right panels, the black dashed line shows the dominant axis determined from linear fits to the small data points (the position angles and uncertainties are labeled), which cover the wavelength range 3800--7800 Å.The orientation of the dominant axis in degrees with uncertainties is indicated in the subpanels for days 1.1 and 80.8. A dashed ellipsoidal contour, whose major and minor axes respectively represent the 1$\sigma$ dispersion about the dominant and orthogonal axis, is also presented. In each panel, different symbols mark the error-weighted mean polarization calculated over the wavelength ranges identified in the color bar. A drastic change of the continuum polarization (from days 1.1 to 2.0) is followed by a gradual drift until a roughly stationary geometry is reached at day 10.9. This behavior is accompanied by a clockwise rotation of the distribution of the data points, revealing a large-scale transformation of the geometry as the CSM is swept up by the SN ejecta. Light gray lines in the upper-left and lower-right panels present the dominant axes fitted to the data through a Monte Carlo re-sampling approach using the errors in $Q$ and $U$ measured at each wavelength bin.
• Figure 2: Temporal evolution of the continuum polarization of SN 2024ggi displayed in the $Q-U$ plane.Left: The blue, green, and pink-shaded areas mark the three stages of the SN 2024ggi polarimetry. Different symbols represent the continuum polarization of SN 2024ggi at different epochs. The thin green dashed line shows the dominant axis at day 1.1 for comparison. The blue dashed line approximately follows the Stage II locus (days 2.0--6.9), when the interaction between the ejecta and CSM led to a change in overall geometry. The black arrow represents the PA of the continuum polarization of Stage III, which was estimated by the error-weighted mean of days 10.9, 19.9, and 33.0. The size of each contour is determined by the standard deviation of the polarization measured at the encircled epoch(s). Right: The upper, middle, and lower-right panels show the scaled flux-density spectra ($F_{\lambda}$) at days 1.1 (Stage I), 2.0 (Stage II), and 10.9 (Stage III), respectively, with major photoionized lines from several species labeled at velocity $v$in the rest frame. The region of the dark-gray-shaded band at day 1.1 suffers from detector saturation. Observations at day 80.8 are not presented as the polarization is affected by strong outward mixing of the inner He-rich layer and nickel clumps.
• Figure 3: Polarizations measured in the central $\pm10$ Å of various emission peaks at days 1.1 and 2.0.The emission cores as highlighted by the color-shaded spectral regions in the left subpanels are less affected by the electron-scattering emission from the wings (which are sensitive to smaller-scale structures such as lumpiness of the scattering region). Their distribution in the Stokes $Q-U$ plane as shown in the upper-right and lower-right panels for days 1.1 and 2.0 (respectively), traces the geometry of the shock-breakout ionization front with the least influence from other effects. In the upper-right panel, the green dashed line presents the dominant axes determined over the wavelength range 3800--7800 Å on day 1.1.
• Figure 4: Illustration of the expanding ejecta and the invariant CSM for different explosion schematics. In each panel, the blue dashed contour displays the location of the ionization front estimated from the isodiffusion-time surface (Methods: \ref{['sec:diffusion']}) and the solid gray circle/ellipse represents the outer boundary of the CSM, and the solid black circle/ellipse shows the outer boundary of the SN ejecta embedded in the CSM. The different schematics are ( A) spherical ejecta + spherical CSM, ( B) spherical ejecta + disk-concentrated CSM, ( C) aspherical ejecta + spherical CSM, and ( D) aspherical ejecta + disk-concentrated CSM. The axisymmetric prompt shock-breakout emission during Stage I and the time-dependent symmetry axis during the transition to Stage II suggest ( D) as the most plausible scenario.
• Figure 5: Evolution in the $Q-U$ plane of H$\alpha$ (top row) and H$\beta$ (bottom row) of SN 2024ggi from days 10.9 to 19.9. The colors encode rest-frame velocities according to the color bars. In each panel, the magenta dot-dashed line fits the polarization distribution measured at different velocities that cover the corresponding spectral feature. The green dashed lines overplot the dominant axis at day 1.1, which appears to be aligned with that of the H envelope that has progressively emerged after day 6.9.