Can we see atomic positronium lines in space? What could be learned from it and what are current challenges to doing so?

Question

This answer to Have we detected spectra of exotic atoms in stars?¹ currently ends:

Estimates for observing lines corresponding to transitions between different bound states of positronium are not very promising; e.g. https://ui.adsabs.harvard.edu/abs/1996A%26AS..120C.365B/abstract

Since that paper is 1) nearly 26 years old and 2) doesn't really help me understand why it wasn't very promising then (my limitations) and there have been advances in technology, I'd like to ask:

Question: Can we see atomic positronium lines in space? What could be learned from it and what are current challenges to doing so?

According to Wikipedia's Positronium; Energy levels

$$E_n = - \frac{1}{2} \frac{m_e q_e^4}{8 h^2 \varepsilon_0^2} \frac{1}{n^2} \approx \frac{-\text{6.8 eV}}{n^2} $$

which means I'd expect the Lyman series n ≥2 → 1 transition in the UV at about 5.1, 6.0... eV (243, 205 nm) and a Balmer series n ≥3 → 2 in the near IR at 0.94, 1.28 eV (1312, 972... nm).

The UV lines would necessarily require observation from space, but perhaps some of the near IR lines might squeak through the atmosphere at high elevations and be observed from the ground.

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¹See this answer to What is a "hydrogen-like" or "hydrogenic" atom? more about "exotic" hydrogen-like atoms

Answer 1

The wavelengths of the lines you found are not problematic per se to observe, with space telescopes. If there were an isolated cloud of cold positronium nearby, we would have observed the lines in question.

The problem is to observe these lines in our real Universe. These lines should have sufficient intensity compared to the background (from other processes) so that we could discern them in the overall spectrum and plausibly ascribe to positronium transitions. These wavelengths (near UV to IR) correspond to a maximum in the blackbody spectrum at T~thousands K, vicinity of the region where this positronium resides is likely optically thick at these wavelengths (e.g. near a pulsar), the flux emitted due to positronium transitions must be sufficiently high for us to discern a corresponding bump in the spectrum on top of the continuum.

The authors of the quoted article estimate the flux in these lines produced by some known sources with positronium (namely, Galactic center, with the flux at the 511 keV line $\approx10^{-3}$ photon/cm$^2\,\!$s). They estimate the population of different excited states of Ps (Section 3: under different assumptions about the environment -- ionized or atomic or molecular gas in which positrons get decelerated and combined into positronium), from those find the fluxes of photons in the most promising lines, and see that those fluxes are too low to observe.

Whether some objects could have appeared in 26 years from which these lines could in fact be observed -- my search did not return references with successful measurements.

Space Positronium Detection by Radio Measurements [V. Burdyuzha, Ph. Durouchoux, V. Kauts (1999)] proposes that lines at radiofrequencies due to fine and hyperfine structure transitions, amplified by maser effect, might be observable with the next generation of radiotelescopes.

What could be learned [from observing the atomic lines]? -- perhaps conditions in the gas where the positronium is formed, if we manage to get good measurements of the line widths (yielding gas velocities in the source) or Zeeman splitting (magnetic fields).