The Search for Celestial Positronium via the Recombination Spectrum

TL;DR

This paper explores the potential for detecting celestial positronium through its recombination spectrum, which could identify sources of positrons and enhance understanding of high-energy astrophysical phenomena.

Contribution

It reviews the theory of positronium recombination lines, provides formulas for expected line strengths, and assesses observational prospects with current and future instruments.

Findings

Positronium emission lines may be detectable with recent spectroscopic advances.

Theoretical formulas enable estimation of line strengths from various sources.

Detection of Ps-alpha line could be feasible soon with improved infrared spectroscopy.

Abstract

Positronium is the short-lived atom consisting of a bound electron-positron pair. In the triplet state, when the spins of both particles are parallel, radiative recombination lines will be emitted prior to annihilation. The existence of celestial positronium is revealed through gamma-ray observations of its annihilation products. These observations however have intrinsically low angular resolution. In this paper we examine the prospects for detecting the positronium recombination spectrum. Such observations have the potential to reveal discrete sources of positrons for the first time and will allow the acuity of optical telescopes and instrumentation to be applied to observations of high energy phenomena. We review the theory of the positronium recombination spectrum and provide formulae to calculate expected line strengths from the positrons production rate and for different conditions in the interstellar medium. We estimate the positronium emission line strengths for several classes of Galactic and extragalactic sources. These are compared to current observational limits and to current and future sensitivities of optical and infrared instrumentation. We find that observations of the Ps-alpha line should soon be possible due to recent advances in near-infrared spectroscopy.