X-ray signature of antistars in the Galaxy

The existence of macroscopic objects from antimatter (antistars) is envisaged in some models of baryogenesis. Searches for antistars have been usually carried out in gamma-rays originated from hadronic annihilation of matter. In astrophysically plausible cases of the interaction of neutral atmospheres or winds from antistars with ionized interstellar gas, the formation of excited pp¯ and Hep¯ atoms precedes the hadronic annihilation. These atoms rapidly cascade down to low levels before annihilation giving rise to a series of narrow lines which can be associated with the hadronic annihilation gamma-ray emission. The most significant are L (3p-2p) 1.73 keV line (yield more than 90%) from pp¯ atoms, and M (4-3) 4.86 keV (yield ∼60%) and L (3-2) 11.13 keV (yield about 25%) lines from 4Hep¯ atoms. These lines can be searched for in dedicated observations by the forthcoming sensitive X-ray spectroscopic missions XRISM, Athena and Lynx and in wide-field X-ray surveys like SRG/eROSITA all-sky survey.