paper

The Spatially Uniform Spectrum of the Fermi Bubbles: the Leptonic AGN Jet Scenario

arXiv:1706.05025 · doi:10.3847/1538-4357/aa9434

Abstract

The Fermi bubbles, two giant gamma-ray bubbles above and below the Galactic center (GC), are among the most important findings of the Fermi Gamma-ray Space Telescope. Because of the proximity, spatially resolved, multi-wavelength observations offer excellent opportunities to learn about the physical origin of the bubbles. One of the unique features of the observed bubbles is that their gamma-ray spectrum, including a high-energy cutoff at ~110 GeV and the overall shape of the spectrum, is nearly spatially uniform. The high-energy spectral cutoff is suggestive of a leptonic origin as it could be a signature of synchrotron and inverse-Compton (IC) cooling of cosmic-ray (CR) electrons; however, even for a leptonic model, it is not obvious why the spectrum should be spatially uniform. In this work, we investigate the formation of the Fermi bubble in the leptonic jet scenario using a newly implemented CRSPEC module in FLASH that allows us to track the evolution of CR spectrum on-the-fly during the simulations. We show that the high-energy cutoff is caused by fast synchrotron and IC cooling near the GC when the jets were first launched. After the jets propagate away from the GC, the dynamical timescale of the jets become the shortest among all relevant timescales, and therefore the spectrum is essentially advected with only mild cooling losses. This could explain why the bubble spectrum is nearly spatially uniform: the CRs from different parts of the bubbles as seen today all share the same origin near the GC at early stage of the bubble expansion. We find that the CR spatial and spectral distribution predicted by the leptonic jet model can simultaneously match the normalization, spectral shape, and high-energy cutoff of the observed gamma-ray spectrum and their spatial uniformity, suggesting that past AGN jet activity is a likely mechanism for the formation of the Fermi bubbles.

14 pages, 7 figures, matched to the ApJ published version, added Fig. 4 of CR spectral evolution

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