In the recent paper, Postnov and colleagues proposed an idea to detect electromagnetic signal from annihilation in outer layers of antistars. Their idea is to search for antistars in the Galaxy through X-rays in the ∼ (1–10) keV energy band. The reason is, prior to annihilation, protons and antiprotons could form atomic-type excited bound states (‘protonium’, Pn), similar to 𝑒+𝑒¯-positronium (Ps) atoms, and in the process of de-excitation of protonium, an antistar could emit not only ∼ 100-MeV gamma-rays but a noticeable flux of X-rays with energies in the keV range. Their study recently appeared in Arxiv.
Antistars are objects that could have form from smaller high baryonic number (HBB). They were created in the very early universe after the QCD phase transition at 𝑇 ∼ 100 MeV & should also populate the galactic halo. Such stars are not only too old, but also they are moving very fast, and have a highly unusual chemical content. Present observations also favored the possibility of their existence.
Now, Postnov and colleagues explored the possibility that, when antistars interact with interstellar medium (ISM) gas it can give rise to excited protonium atoms. Formation of these atoms takes place most effectively during interaction of protons with neutral (or molecular) antimatter. This can happen if an antistar has a noticeable wind mass-loss.
“These (protonium) atoms rapidly cascade down to low levels prior to annihilation giving rise to a series of narrow lines which can be associated with the hadronic annihilation gamma-ray emission.”— wrote authors of the study.
They have also shown that these protonium atoms cascade to the 2p-state producing mostly L (Balmer) 3d-2p X-rays around ∼ 1.7 keV line before the 𝑝𝑝¯ hadronic annihilation.
While, antistars formed in higher HBBs should have an enhanced helium abundance. Therefore, the 4.86 keV M (4-3) and 11.13 L (3-2) narrow X-ray lines from cascade transitions in ⁴He𝑝¯ atoms can also be associated with gamma-rays from hadronic annihilations.
“These lines are interesting from the observational point of view because the protonium 3d-2p transition line energy 1.73 keV is close to the Si K-shell complex lines, which could hamper its disentangling from the background.”— wrote authors of the study.
Finally, it has been suggested, these lines can be probed in dedicated observations by forthcoming sensitive X-ray spectroscopic missions XRISM and Athena and in wide-field X-ray surveys like SRG/eROSITA all-sky survey.
Reference: Bondar et al., “X-ray signature of antistars in the Galaxy”, pp. 1-10, 2021.
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