Constraints on the antistar fraction in the Solar System neighborhood from the 10-year Fermi Large Area Telescope gamma-ray source catalog

It is generally taken for granted that our Universe is free of antimatter objects and domains. This certitude has recently been challenged by the possible detection of antihelium nuclei by AMS-02. Should the observation be confirmed, the existence of nearby antistars would make a plausible hypothesis to explain the origin of the antinuclei. In this paper, we use the 10-year Fermi Large Area Telescope (LAT) gamma-ray source catalog to set constraints on the abundance of antistars around the Sun. We identify in the catalog 14 antistar candidates not associated with any objects belonging to established gamma-ray source classes and with a spectrum compatible with baryon-antibaryon annihilation. We use them along with an estimate of the LAT sensitivity to antistars to set upper limits on the local antistar fraction $f_{\overline{*}}$ with respect to normal stars. We provide parametric limits as a function of the closest antistar mass, velocity, and surrounding matter density. We also employ a novel Monte Carlo method to set limits for a few hypotheses about the antistar population. For a population with properties equivalent to those of regular stars concentrated in the Galactic disk, we obtain $f_{\overline{*}}<2.5\times {10}^{-6}$ at 95% confidence level, which is 20 times more constraining than limits previously available. For a primordial population of antistars distributed in the Galactic halo, we obtain new local upper limits which decrease as a function of antistar mass $M$ from $f_{\overline{*}}<0.2$ at 95% confidence level for $M=1M_{\odot }$ to $f_{\overline{*}}<1.6\times {10}^{-4}$ at 95% confidence level for $M=10M_{\odot }$. By combining these limits with existing microlensing constraints for lighter objects in the Magellanic Clouds, we infer that a primordial population of halo antistars must have a density lower than $\mathcal{O}({10}^{-5}{\mathrm{pc}}^{-3})$ to $\mathcal{O}({10}^{-2}{\mathrm{pc}}^{-3})$ depending on their masses. Our limits can constrain models for the origin and propagation of antinuclei in cosmic rays.

Figure 1

Positions and energy flux in the 100 MeV–100 GeV range of antistar candidates selected in 4FGL-DR2. Galactic coordinates. The background image shows the Fermi 5-year all-sky photon counts above 1 GeV (image credit: NASA/DOE/Fermi LAT Collaboration).

Figure 2

Minimum energy flux in the 100 MeV–100 GeV energy range for a pointlike source with an matter-antimatter annihilation spectrum to be detectable in the 4FGL-DR2 catalog. Galactic coordinates.

Figure 3

Left: distance $d$ of the closest antistar candidate in the 4FGL-DR2 catalog based on the luminosity relation in Eq. (4). Right: corresponding upper limit on the antistar fraction $f_{\overline{*}}$ from Eq. (5). In both panels, the quantities are shown as a function of velocity $v$ with respect to the ISM and antistar mass $M$ for an ISM density $\rho =1{\mathrm{m}}_p{\mathrm{cm}}^{-3}$.

Figure 4

Distribution of detected-antistar properties obtained from a galaxia synthetic population, by comparing the flux from Eq. (4) with the LAT sensitivity (see Fig. 2).

Figure 5

Antistar fraction (left) and mass fraction (right) upper limits as a function of antistar mass obtained from 4FGL-DR2 under the hypothesis that antistars are primordial objects evolving in the Milky Way (MW) halo (see the text for details). In the right panel, we compare gamma-ray constraints on antistars to earlier results from microlensing experiments on BDOs for the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC).