Steady-state Hadronic Gamma-Ray Emission from 100-Myr-Old Fermi Bubbles
Abstract
Fermi Bubbles are enigmatic γ-ray features of the Galactic bulge. Both putative activity (within few × Myr) connected to the Galactic center super-massive black hole and, alternatively, nuclear star formation have been claimed as the energizing 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-gigayear timescale, much longer than other natural timescales occurring in the problem. Here we present a mechanism wherein the timescale for generating the Bubbles' γ-ray emission via hadronic processes is ~few × 108 yr. Our model invokes the collapse of the Bubbles' thermally unstable plasma, leading to an accumulation of cosmic rays and magnetic field into localized, warm (~104 K), and likely filamentary condensations of higher-density gas. Under the condition that these filaments are supported by non-thermal pressure, the hadronic emission from the Bubbles is L γ ~= 2 × 1037 erg s-1 \dot{M}in/(0.1 {M_⊙ } yr-1 ) TFB^2/(3.5 × 10^7 K)2 M fil/M pls, equal to 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 the that of the Bubbles' plasma, M pls).
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2014
- DOI:
- 10.1088/2041-8205/791/2/L20
- arXiv:
- arXiv:1312.0692
- Bibcode:
- 2014ApJ...791L..20C
- Keywords:
-
- cosmic rays;
- gamma rays: diffuse background;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Cosmology and Nongalactic Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted for publication in ApJ Letters. Shortened from ver.1 with an important modification to equation 2