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 $L_{\gamma }\simeq 2\times 10^{37}$ erg/s ${\dot{M}}_{\mathrm{in}}/(0.1M_{Sun}/$ 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, $M_{pls}$).

Accepted for publication in ApJ Letters. Shortened from ver.1 with an important modification to equation 2; qualitative results the same; vers 3: title fixed

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• The Interaction of the Fermi Bubbles with the Milky Way's Hot Gas Halo
• Multi-wavelength features of Fermi Bubbles as signatures of a Galactic wind
• Modelling of the Spectral Energy Distribution of Fornax A: Leptonic and Hadronic Production of High Energy Emission from the Radio Lobes
• The Accretion Wind Model of the Fermi Bubbles (II): Radiation
• Multi-wavelength Emission from the Fermi Bubble III. Stochastic (Fermi) Re-Acceleration of Relativistic Electrons Emitted by SNRs
• Suzaku X-ray Observations of the Fermi Bubbles: Northernmost Cap and Southeast Claw Discovered with MAXI-SSC
• Multi-wavelength Emission from the Fermi Bubble II. Secondary Electrons and the Hadronic Model of the Bubble
• A multi-messenger study of the Fermi Bubbles: very high energy gamma rays and neutrinos
• Search for Blazar Flux-Correlated TeV Neutrinos in IceCube 40-String Data
• The Fermi bubbles from stochastic acceleration of electrons in a Galactic outflow
• Emission from hadronic and leptonic processes in galactic jet-driven bubbles
• Metal Enrichment in the Fermi Bubbles as a Probe of Their Origin
• The Bending Feature of the Fermi Bubbles: A Presumed Horizontal Galactic Wind and Its Implication on the Bubbles' Age
• The AGN Jet Model of the Fermi Bubbles
• Giant lobes of Centaurus A as seen in radio and gamma-ray images obtained with the Fermi-LAT and Planck satellites
• The Fermi bubbles as a Superbubble
• Multi-wavelength emission from leptonic processes in ageing galaxy bubbles