The Fermi bubbles: gamma-ray, microwave and polarization signatures of leptonic AGN jets
Monthly Notices of the Royal Astronomical Society, Volume 436, Issue 3, p.2734-2746
Published in Dec 2013
The origin of the two large bubbles at the Galactic Centre observed by the Fermi Gamma-ray Space Telescope and the spatially correlated microwave haze emission are yet to be determined. To disentangle different models requires detailed comparisons between theoretical predictions and multiwavelength observations. Our previous simulations, which self-consistently include interactions between cosmic rays (CRs) and magnetic fields, have demonstrated that the primary features of the Fermi bubbles could be successfully reproduced by recent jet activity from the central active galactic nucleus (AGN). In this work, we generate gamma-ray and microwave maps and spectra based on the simulated properties of CRs and magnetic fields in order to examine whether the observed bubble and haze emission could be explained by leptons contained in the AGN jets. We also investigate the model predictions of the polarization properties of the Fermi bubbles, including the polarization fractions and the rotation measures (RMs). We find that (1) the same population of leptons can simultaneously explain the bubble and haze emission given that the magnetic fields within the bubbles are very close to the exponentially distributed ambient field, which can be explained by mixing in of the ambient field followed by turbulent field amplification. (2) The centrally peaked microwave profile suggests CR replenishment, which is consistent with the presence of a more recent second jet event. (3) The bubble interior exhibits a high degree of polarization because of ordered radial magnetic field lines stretched by elongated vortices behind the shocks; highly polarized signals could also be observed inside the draping layer. (4) Enhancement of RMs could exist within the shock-compressed layer because of increased gas density and more amplified and ordered magnetic fields, though details depend on projections and the actual field geometry. We discuss the possibility that the deficient haze emission at b 〈 -35° is due to the suppression of magnetic fields, which is consistent with the existence of lower energy CRs causing the polarized emission at 2.3 GHz. Possible AGN jet composition in the leptonic scenario is also discussed.
Galaxy: nucleus; MHD methods: numerical; cosmic rays; gamma-rays: galaxies; radio continuum: galaxies
Astrophysics - Galaxy Astrophysics; Astrophysics - High Energy Astrophysical Phenomena
2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society