Precessing Black Hole Jets and Galactic Fossils

Abstract

The Galactic Center gamma-ray excess (GCE) -- a surplus of gamma-ray radiation around SgrA$^{\star}$ -- has remained without a consensus interpretation for more than fifteen years. Dark-matter annihilation and unresolved millisecond-pulsar populations remain the leading candidates, yet neither connects the excess to the past activity of SgrA$^{\star}$ traced by the Fermi and eROSITA bubbles. We propose a common-origin scenario in which a contribution to the GCE arises as a fossil hadronic imprint of the same SgrA$^{\star}$ outburst associated with the bubbles. We develop a model of SgrA$^{\star}$ with a precessing paraboloidal Blandford-Znajek (BZ) jet launched from a tilted, magnetically arrested accretion disc during a ~ 7.5 Myr active phase ending ~2.6 Myr ago. In this picture, the jet drives the bipolar expansion of the Fermi/eROSITA bubbles -- contributing, alongside wider-angle outflows, to their observed extent -- and injects hadronic cosmic rays at the Galactic Center. We couple the analytic BZ injection to a two-zone diffusion numerical solver to compute the resulting GCE surface brightness and verify internal consistency: the proton Larmor radius remains small compared to the jet coherence scale, ensuring magnetic confinement of the cosmic-ray population, while attenuation of the produced $\gamma$ rays in the interstellar medium is negligible, leaving the medium effectively transparent. Isolating the jet contribution alone yields a spin-dependent, irreducible hadronic floor: for a SgrA$^{\star}$ spin of a$^{\star} = 0.9$, we find a robust floor at the few-percent to $\sim 10\%$ level of the observed GCE surface brightness across the inner ten degrees, highlighting a previously unexplored component relevant for comprehensive models of the GCE.