Polarization of MeV gamma-rays and 511 keV line shape as probes of SNIa asymmetry and magnetic field

TL;DR

This paper explores how gamma-ray polarization and line shape analysis can reveal asymmetries and magnetic field structures in Type Ia supernovae through detailed modeling of gamma-ray and positron transport.

Contribution

It introduces new methods to diagnose supernova asymmetries and magnetic fields using gamma-ray polarization and line shape comparisons, enhancing understanding of supernova explosion mechanisms.

Findings

Gamma-ray polarization indicates explosion asymmetries.

Line shape differences constrain positron propagation.

Magnetic field topology affects positron escape fractions.

Abstract

We discuss gamma-ray signatures associated with an asymmetric explosion and transport of positrons in SN Ia ejecta. In particular, Compton scattering of gamma-ray line photons can induce polarization in the continuum, which would be a direct probe of the asymmetries in the distribution of radioactive isotopes and/or of the scattering medium. Even more interesting would be a comparison of the shapes of $\gamma$-ray lines and that of the electron-positron annihilation line at 511 keV. The shapes of $\gamma$-ray lines associated with the decay of Co56 (e.g., lines at 847 and 1238 keV) directly reflect the velocity distribution of Co56. On the other hand, the 511 keV line arises from the annihilation of positions, which are also produced by the Co56 decay but can propagate through the ejecta before they slow down and annihilate. Therefore, the shape of the annihilation line might differ from other gamma-ray lines, providing constraints on the efficiency of positrons propagation through the ejecta and, as consequence, on the topology of magnetic fields in the ejecta and on the fraction of positrons that escape to the interstellar medium. We illustrate the above effects with two models aimed at capturing the main predicted signatures.