Possible counterpart signal of the Fermi bubbles at the cosmic-ray positrons

Abstract

The inner galaxy has hosted cosmic-ray burst events including those responsible for the gamma-ray Fermi bubbles and the eROSITA bubbles in X-rays. In this work, we study the AMS-02 positron fraction and find three features around 12, 21 and 48 GeV of which the lowest energy has a 1.4 to 4.9-$σ$ significance, depending on astrophysical background assumptions. Using background simulations that explain the cosmic-ray positron fraction, positron flux and electron plus positron flux, by primary, secondary cosmic rays and cosmic rays from local pulsars, we test these spectral features as originating from electron/positron burst events from the inner galaxy. We find the 12 GeV feature, to be explained by an event of age $τ\simeq 3 - 10$ Myr; in agreement with the proposed age of the Fermi bubbles. Furthermore, the energy in cosmic-ray electrons and positrons propagating along the galactic disk and not within the Fermi bubbles volume, is estimated to be $10^{51.5}-10^{57.5}$ ergs, or $O(10^{-4}) -O(1)$ the cosmic-ray energy causing the Fermi bubbles. We advocate that these positron fraction features, are the counterpart signals of the Fermi bubbles, or of substructures in them, or of the eROSITA bubbles.