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511 keV Gamma Ray Echo from Particle Decays in Supernovae

Garv Chauhan, Cecilia Lunardini

Abstract

The formation of a hot and dense core in a core-collapse supernova (SN) can produce massive Beyond Standard Model (BSM) particles. These particles can decay in the stellar envelope, generating positrons either directly or through secondary processes involving neutrinos or photons. We show for the first time that such positrons regardless of their production channel, can thermalize and annihilate at rest with ambient electrons in the outer SN envelope, producing a characteristic echo of 511 keV gamma rays. For axion-like particles (ALPs), we derive bounds on the ALP-photon coupling ($G_{a γ}$) using Pioneer Venus Orbiter observations of SN 1987A. We also evaluate the sensitivity of upcoming MeV gap gamma-ray telescopes in the 511 keV range, such as COSI and AMEGO, for future Galactic SNe, which can improve existing constraints or enable ALP discovery. The echo signal is a generic prediction for any particle species that efficiently produces positrons near the stellar surface.

511 keV Gamma Ray Echo from Particle Decays in Supernovae

Abstract

The formation of a hot and dense core in a core-collapse supernova (SN) can produce massive Beyond Standard Model (BSM) particles. These particles can decay in the stellar envelope, generating positrons either directly or through secondary processes involving neutrinos or photons. We show for the first time that such positrons regardless of their production channel, can thermalize and annihilate at rest with ambient electrons in the outer SN envelope, producing a characteristic echo of 511 keV gamma rays. For axion-like particles (ALPs), we derive bounds on the ALP-photon coupling () using Pioneer Venus Orbiter observations of SN 1987A. We also evaluate the sensitivity of upcoming MeV gap gamma-ray telescopes in the 511 keV range, such as COSI and AMEGO, for future Galactic SNe, which can improve existing constraints or enable ALP discovery. The echo signal is a generic prediction for any particle species that efficiently produces positrons near the stellar surface.

Paper Structure

This paper contains 6 sections, 30 equations, 4 figures, 1 table.

Table of Contents

  1. ALP Production Rates
  2. Energy and Angular Distribution
  3. Pair Production
  4. Positron Thermalization
  5. $f_{511}$ function
  6. PVO Effective Area

Figures (4)

  • Figure 1: Constraints in the $G_{a\gamma}$-$m_a$ space from PVO observations during SN1987A (red-shaded region) and future sensitivities of COSI (blue-dashed region) and AMEGO (green-dashed region) for a SN at 10 kpc. The limits derived in our work correspond to a $3\sigma$ confidence interval. We also show existing bounds, from Refs. Jaeckel:2017tudHoof:2022xbeMuller:2023vjmDev:2023haxDiamond:2023cto (details in the text).
  • Figure 2: A schematic representation of the formation of a 511 keV signal from the ALP decay chain, Eq. (\ref{['eq:gammadecay']}). See also Eq. (\ref{['eq:diffNgamma']}).
  • Figure 3: Time-integrated ALP production rate $dN_a/dE_a$ as function of ALP energy $E_a$ through Primakoff scattering and Coalescence process for ALP masses $m_a=(50,300)$ MeV, for a reference $G_{a\gamma}=10^{-8}\text{ GeV}^{-1}$.
  • Figure 4: $f_{511}(r,E_a,m_a)$ as a function of distance from the PNS core ($r$) (defined in Eq. \ref{['eq:diffNgamma']}) for different values of ALP mass and energies.