Galactic Positrons from Thermonuclear Supernovae

TL;DR

This work addresses the Galactic positron puzzle by quantifying positron escape from Type Ia supernovae across multiple progenitor channels (near-$M_{ m Ch}$, sub-$M_{ m Ch}$ He-triggered detonations, and WD mergers) and magnetic-field morphologies. It combines HYDRA-based hydrodynamics with Monte Carlo transport of positrons and $\\gamma$-rays, including $^{56}$Co beta$^{+}$ decay and pair production, to produce escape fractions and energy spectra, then employs population synthesis using observed CSP-I/II SN Ia class ratios to estimate the Galactic positron injection from SNe Ia. The main findings show integrated escape fractions from $<0.01\%$ to $\sim 6\%$ (dipole) or up to $\sim 20\%$ (sub-$M_{ m Ch}$ under certain morphologies), with mean positron energies around $0.35$ MeV and modest shifts due to magnetic-field strength and structure; overall, SNe Ia contribute only a small fraction of the Galactic positron budget. The results imply that the magnetic-field physics and the SN Ia rate dominate the Galactic positron contribution more than the detailed explosion mechanism, offering a framework to refine Galactic positron modeling with upcoming surveys and nebular observations. The study provides quantitative benchmarks for positron yields from SNe Ia and highlights the need for improved SN rates and 3D modeling to resolve the positron puzzle.

Abstract

Type Ia Supernovae (SNe Ia) may originate from a wide variety of explosion scenarios and progenitor channels. They exhibit a factor of about 10 difference in brightness and, thus, a differentiation in the mass of 56Ni->56Co->56 Fe. We present a study on the fate of positrons within SNe Ia in order to evaluate their escape fractions and energy spectra. Our detailed Monte Carlo transport simulations for positrons and gamma-rays include both beta + decay of 56 Co and pair production. We simulate a wide variety of explosion scenarios, including the explosion of white dwarfs (WD) close to the Chandrasekhar mass, M(Ch), He-triggered explosions of sub-M Ch WDs, and dynamical mergers of two WDs. For each model, we study the influence of the size and morphology of the progenitor magnetic field between 1 and 1E13 G. Population synthesis based on the observed brightness distribution of SNe Ia was used to estimate the overall contributions to Galactic positrons due to escape from SN Ia. We find that this is dominated by normal-bright SNe Ia, where variations in the distribution of emitted positrons are small. We estimate a total SNe Ia contribution to the Galactic positrons of < 2% and, depending on the magnetic field morphology, less than 6...20% for M(Ch) and sub-M(Ch), respectively.

Figures (6)

• Figure 1: Left: Relative strength of cross sections of positron interactions for free electron in a plasma, a single ionization, and annihilation as a function of energy. Right: Relative strength of cross sections of photon interactions for pair production and Compton scattering as a function of energy for energies greater than 1 MeV and Z=26.
• Figure 2: Abundance of $^{56}$Ni at $t \approx 0$ days after the explosion as a function of expansion velocity. Top: difference in total $^{56}$Ni mass for DDT models. Middle: difference in $^{56}$Ni mass for normal bright DDT models with different central densities. Bottom: Difference in $^{56}$Ni mass for various He triggered models and the merging scenario. The nomenclature used is described in $\S$\ref{['sect:esc_frac']}.
• Figure 3: Escape probability of positron as a function of time for all dipole models. Panel A shows difference between total $^{56}$Ni production (upper left), panel B shows difference between central density (lower left), panel C shows difference between helium detonation masses (upper right), and panel D shows the difference between a merger and a helium detonation (lower right). The nomenclature used is described in $\S$\ref{['sect:esc_frac']}.
• Figure 5: Positron flux per day from day 0 to day 1000 for all dipole models. Panel A shows difference between total $^{56}$Ni production (upper left), panel B shows difference between central density (lower left), panel C shows difference between helium detonation masses (upper right), and panel D shows the difference between a merger and a helium detonation (lower right). The nomenclature used is described in $\S$\ref{['sect:esc_frac']}.
• Figure 7: Total integrated escaped positron energy spectrum from day 0 to 2000 for each explosion dipole model. Panel A shows difference between total $^{56}$Ni production (upper left), panel B shows difference between central density (lower left), panel C shows difference between helium detonation masses (upper right), and panel D shows the difference between a merger and a helium detonation (lower right). These models are normalized to unity. The nomenclature used is described in $\S$\ref{['sect:esc_frac']}.