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Register a new userThe Fermi bubbles from stochastic acceleration of electrons in a Galactic outflow
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Philipp Mertsch
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The discovery of the Fermi bubbles---a huge bilobular structure seen in GeV gamma-rays above and below the Galactic center---implies the presence of a large reservoir of high energy particles at $sim 10 , text{kpc}$ from the disk. The absence of evidence for a strong shock coinciding with the edge of the bubbles, and constraints from multi-wavelength observations point towards stochastic acceleration by turbulence as a likely mechanism of acceleration. We have investigated the time-dependent acceleration of electrons in a large-scale outflow from the Galactic centre. For the first time, we present a detailed numerical solution of the particle kinetic equation that includes the acceleration, transport and relevant energy loss processes. We also take into account the addition of shock acceleration of electrons at the bubbles blast wave. Fitting to the observed spectrum and surface brightness distribution of the bubbles allows determining the transport coefficients, thereby shedding light on the origin of the Fermi bubbles.
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We study stochastic acceleration models for the Fermi bubbles. Turbulence is excited just behind the shock front via Kelvin--Helmholtz, Rayleigh--Taylor, or Richtmyer--Meshkov instabilities, and plasma particles are continuously accelerated by the interaction with the turbulence. The turbulence gradually decays as it goes away from the shock fronts. Adopting a phenomenological model for the stochastic acceleration, we explicitly solve the temporal evolution of the particle energy distribution in the turbulence. Our results show that the spatial distribution of high-energy particles is different from those for a steady solution. We also show that the contribution of electrons that escaped from the acceleration regions significantly softens the photon spectrum. The photon spectrum and surface brightness profile are reproduced by our models. If the escape efficiency is very high, the radio flux from the escaped low-energy electrons can be comparable to that of the WMAP haze. We also demonstrate hadronic models with the stochastic acceleration, but they are unlikely in the viewpoint of the energy budget.
We analyse the model of stochastic re-acceleration of electrons, which are emitted by supernova remnants (SNRs) in the Galactic Disk and propagate then into the Galactic halo, in order to explain the origin on nonthermal (radio and gamma-ray) emission from the Fermi Bubbles (FB). We assume that the energy for re-acceleration in the halo is supplied by shocks generated by processes of star accretion onto the central black hole. Numerical simulations show that regions with strong turbulence (places for electron re-acceleration) are located high up in the Galactic Halo about several kpc above the disk. The energy of SNR electrons that reach these regions does not exceed several GeV because of synchrotron and inverse Compton energy losses. At appropriate parameters of re-acceleration these electrons can be re-accelerated up to the energy 10E12 eV which explains in this model the origin of the observed radio and gamma-ray emission from the FB. However although the model gamma-ray spectrum is consistent with the Fermi results, the model radio spectrum is steeper than the observed by WMAP and Planck. If adiabatic losses due to plasma outflow from the Galactic central regions are taken into account, then the re-acceleration model nicely reproduces the Planck datapoints.
The Galactic Centers giant outflows are manifest in three different, non-thermal phenomena: i) the hard-spectrum, gamma-ray `Fermi Bubbles emanating from the nucleus and extending to |b| ~ 50 degrees; ii) the hard-spectrum, total-intensity microwave (~ 20-40 GHz) `Haze extending to |b| ~ 35 degrees in the lower reaches of the Fermi Bubbles; and iii) the steep spectrum, polarized, `S-PASS radio (~ 2-20 GHz) Lobes that envelop the Bubbles and extend to |b| ~ 60 degrees. We find that the nuclear outflows inflate a genuine bubble in each Galactic hemisphere that has the classical structure, working outwards, of reverse shock, contact discontinuity, and forward shock. Expanding into the finite pressure of the halo and given appreciable cooling and gravitational losses, the contact discontinuity of each bubble is now expanding only very slowly. We find observational signatures in both hemispheres of giant, reverse shocks at heights of ~ 1 kpc above the nucleus; their presence ultimately explains all three of the non-thermal phenomena mentioned above. Synchrotron emission from shock-reaccelerated cosmic-ray electrons explains the spectrum, morphology, and vertical extent of the microwave Haze and the polarized radio Lobes. Collisions between shock-reaccelerated hadrons and denser gas in cooling condensations that form inside the contact discontinuity account for most of the Bubbles gamma-ray emissivity.
We analyse processes of particle acceleration in the Fermi Bubbles. The goal of our investigations is to obtain restrictions for acceleration mechanisms. Our analysis of the three processes: acceleration from background plasma, re-acceleration of relativistic electrons emitted by supernova remnants, and acceleration by shocks generated by processes of star tidal disruption in the Galactic Center, showed that the model of multi-shock acceleration does not have serious objections at present and therefore seems us more attractive than others.
Fermi has discovered two giant gamma-ray-emitting bubbles that extend nearly 10 kpc in diameter. We propose that periodic star capture processes by the galactic supermassive black hole, Sgr A*, with a capture rate $<10^{-5}$ yr$^{-1}$ and energy release $sim 10^{52}$ erg per one capture can produce shocks in the halo, which accelerate electrons to the energy ~ 1 TeV. These electrons generate radio emission via synchrotron radiation, and gamma-rays via inverse Compton scattering with the relic and the galactic soft photons. Estimates of the diffusion coefficient from the observed gamma-ray flux explains consistently the necessary maximum energy of electrons and sharp edges of the bubble.
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