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

TL;DR

This paper presents early spectropolarimetric observations of supernova 2024ggi, revealing a well-defined axial symmetry in the shock breakout, which provides insights into the explosion mechanism of massive stars.

Contribution

It provides the first measurement of the shock breakout geometry in a type II supernova using early spectropolarimetry, indicating a large-scale axial symmetry.

Findings

Shock breakout exhibits a clear symmetry axis.

The symmetry persists through the explosion.

Supports models with large-scale axial symmetry in supernova explosions.

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.