Steady-State Hadronic Gamma-Ray Emission from 100-Myr-Old Fermi Bubbles

Abstract

The Fermi Bubbles are enigmatic γ-ray features of the Galactic bulge. Both putative activity (within $\sim$ few $\times$ Myr) connected to the Galactic center super-massive black hole and, alternatively, nuclear star formation have been claimed as the energising source of the Bubbles. Likewise, both inverse-Compton emission by non-thermal electrons (`leptonic' models) and collisions between non-thermal protons and gas (`hadronic' models) have been advanced as the process supplying the Bubbles' γ-ray emission. An issue for any steady state hadronic model is that the very low density of the Bubbles' plasma seems to require that they accumulate protons over a multi-Gyr timescale, much longer than other natural timescales occurring in the problem. Here we present a hadronic model where the timescale for generating the Bubbles' hadronic γ-ray emission is $\sim$ few $\times 10^8$ years. Our model invokes collapse of the Bubbles' thermally-unstable plasma, leading to an accumulation of cosmic rays and magnetic field into localised, warm ($\sim 10^4$ K), and likely filamentary condensations of higher density gas. Under the condition that these filaments are supported by non-thermal pressure, we can predict the hadronic emission from the Bubbles to be $L_γ\simeq 2 \times 10^{37}$ erg/s $\ \dot{M}_\mathrm{in}/(0.1 \ M_{Sun}/$ year $) \ T_\mathrm{FB}^2/(3.5 \times 10^7 K) ^2 M_{fil}/M_{pls}$ ; precisely their observed luminosity (normalizing to the star-formation-driven mass flux into the Bubbles and their measured plasma temperature and adopting the further result that the mass in the filaments, $M_{fil}$ is approximately equal to that of the Bubbles' plasma, $M_{pls}$).