Double shell structure in supernova 2024ggi

TL;DR

This paper tackles the problem of explaining the non-monotonic evolution of the photospheric radius $R_{ m ph}(t)$ in SN 2024ggi. It introduces a toy model with an outer fast, low-mass spherical shell (S-shell) and an inner slower, massive elongated shell (E-shell), deriving analytic expressions for each shell's photospheric radius and a method to compute the observed $R_{ m ph}$ from projected areas, including the case of equal shell temperatures which maximizes the transition radius. Fitting to SN 2024ggi data, the model reproduces the observed concave-to-convex transition in $R_{ m ph}(t)$ and supports a multi-shell ejecta structure, with polarization data further endorsing a well-defined axial symmetry compatible with the jittering-jet explosion mechanism (JJEM). The results reinforce the JJEM as a primary CCSN mechanism and suggest that double-shell ejecta may be a common outcome in jet-influenced explosions, motivating future refinements with more shells and radiation-hydrodynamics simulations.

Abstract

We built a simple toy model of a core-collapse supernova (CCSN) ejecta composed of two shells, an outer low-mass spherical shell and an inner elongated massive shell, and show that it can reproduce the evolution of the photospheric radius of SN 2024ggi, Rph(t). During the first week, the larger spherical shell, the S-shell, forms the photosphere. As the shell expands and becomes increasingly transparent, the photosphere moves inward along the mass coordinate, although it grows in size. When the photosphere reaches the long axis of the elongated inner shell, the E-shell begins to contribute to the photosphere, ultimately comprising the entire photosphere. The simple toy model explains the transition of Rph(t) from being concave (decreasing slope) to convex (increasing slope). A single-shell model predicts only concave behavior. The structure of a spherical shell with an inner elongated shell is motivated by the morphologies of several CCSN remnants whose structures have been attributed to multiple pairs of jets in the framework of the jittering jets explosion mechanism (JJEM). The deduced multiple-shell ejecta of SN 2024ggi in this study, and of SN 2023ixf in an earlier study, as well as studies of the polarization of SN 2024ggi, are better compatible with the JJEM than with the neutrino-driven mechanism. Our study supports the growing evidence that the JJEM is the primary explosion mechanism of CCSNe.

Figures (6)

• Figure 1: Two images of CCSNRs with an elongated shell inside a large-scale, more-or-less spherical one. Studies attributed their morphologies to the JJEM. In each image, we marked the two prominent shells that can form two photospheric shells: as the outer, large-scale spherical shell, S-shell (marked with the yellow-red arrows), becomes transparent, the inner, elongated one, E-Shell (marked by the double-headed pale-blue arrows), takes over. (a) A radio image of SNR G309.2–00.6 adapted from Gaensleretal1998, who noted the jet-shaped morphology. Soker2024PNSN identified the rim-nozzle symmetry. (b) An eROSITA DR1 (log scale, $0.2-2.3~\rm{keV}$) X-ray counts image of the Vela CCSNR adapted from SokerShishkin2025Vela. The lines depict the point-symmetric structure of Vela: dashed lines represent pairs of opposite structural features identified by SokerShishkin2025Vela, while the solid lines represent earlier-identified pairs. Blue dots are the centers of the lines, and the blue asterisk is the center of these dots. The inset on the bottom right ($29.2^{\prime} \times 22.7^{\prime}$) is the inner part of the Vela SNR, including the NS location (Kochanek2022; red asterisk), its projected movement direction (red arrow), and the presumed origin at explosion (Kochanek2022Dodson_etal_2003; red dot).
• Figure 2: A schematic look at the photosphere of our toy model during a time when both shells contribute to the photosphere: the blue is the S-shell with a projected (on the plane of the sky) area $A_{\rm S}$, and the brown is the E-shell with a projected area $A_{\rm E}$. The thick black line represents the photospheric limb. The line of sight is perpendicular to the long axis of the E-shell; there is an axial symmetry around this $x$-axis. The physical size of the S-shell is larger than that of the E-shell, but at this time, the outer S-shell zone is optically thin, as indicated.
• Figure 3: The photosphere limb at four times depicted by the solid lines: blue for the S-shell and red for the E-shell. Dash-blue line is the rest of the S-shell photosphere had there been no E-shell; dash-red line is the rest of the E-shell photosphere had there been no S-shell. Each shell is expanding at a constant velocity (Homologous expansion). (a) Only the S-shell contributes to the photosphere, as it is optically thick. (b) The outer S-shell becomes optically thin, and the E-shell starts contributing to the photosphere. (c) The E-shell contributes a significant fraction of the photosphere. (d) The E-shell forms the entire photosphere. The sizes of the shells and the photosphere structure presented here at the four times are of the fiducial toy model that we further describe in Section \ref{['sec:Photosphere']}.
• Figure 4: The thick black line represents the photospheric radius in our fiducial toy model (Tables \ref{['tab:FiducialModelS']} and \ref{['tab:FiducialModelE']}), according to equation (\ref{['eq:RphSE']}). The observations are from ChenTWetal2025. The dashed blue line is the photospheric radius of the S-shell according to equation (\ref{['eq:RphS']}), and the dotted red line is the photospheric radius of the E-shell according to equation (\ref{['eq:RphE']}), both in the fiducial model.
• Figure 5: The photospheric radius evolution in three models that differ in their effective temperature ratio, as indicated in the inset. When the two temperatures are unequal, the photospheric radius is according to equation (\ref{['eq:RphSET']}). All models have the same geometry as in the fiducial models (Figure \ref{['fig:SN2024ggiFigureSnapshots']}). The temperature ratio affects the photospheric radius only during the transition phase, i.e., when both shells contribute to the photosphere. The thick black line represents the fiducial model. For a given geometry, the case with equal temperatures yields the largest calculated radius in the transition phase.