&Bullet; physics 14, s50
The analysis of gamma-ray sources leads to an upper limit on the number of antimatter stars in the Milky Way.
Nowadays it is taken for granted that the universe does not contain significant amounts of antimatter. Most cosmological models use hypothetical physical processes to explain why matter dominates when the universe should be created with equal amounts of both. In 2018, the Alpha Magnetic Spectrometer 2 (AMS-02) experiment on the International Space Station may have detected multiple antihelium nuclei, suggesting that some primordial antimatter survived to form antistars and even antigalaxies. Now Simon Dupourqué and colleagues from the Institute for Research and Astrophysics and Planetology (IRAP), France, are identifying possible antistars based on ten years of gamma-ray observations from the orbiting Fermi gamma-ray space telescope and inferring restrictions on the number of such objects could be in our solar Neighborhood exist  .
Gamma rays are created when a particle and its antiparticle collide and annihilate. In the case of an antistar, this process is believed to take place when regular interstellar matter accumulates on the surface of the antistar. However, many other astrophysical phenomena can also emit gamma rays, with each source having a characteristic spectrum and a light curve. To identify possible antistars among the 5787 gamma-ray sources cataloged by the Fermi mission, Dupourqué and his colleagues calculated which sources have a point geometry and a spectrum compatible with baryon-antibaryon annihilation. The researchers combined their calculations with simulations of the accretion process around antistars and determined an upper limit of 2.5 antistars per million normal stars within several hundred light years of our sun – provided that the antistars have similar properties to normal stars.
While a single nearby Antistar may have produced the possible AMS-02 antihelium, the researchers suggest a more likely source outside the main galactic disk, where Antistars could more easily “hide” from gamma-ray detection.
Rachel Berkowitz is Corresponding Editor for physics based in Vancouver, Canada.