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Register a new userParticle acceleration and the origin of gamma-ray emission from Fermi Bubbles
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Fermi LAT has discovered two extended gamma-ray bubbles above and below the galactic plane. We propose that their origin is due to the energy release in the Galactic center (GC) as a result of quasi-periodic star accretion onto the central black hole. Shocks generated by these processes propagate into the Galactic halo and accelerate particles there. We show that electrons accelerated up to ~10 TeV may be responsible for the observed gamma-ray emission of the bubbles as a result of inverse Compton (IC) scattering on the relic photons. We also suggest that the Bubble could generate the flux of CR protons at energies > 10^15 eV because the shocks in the Bubble have much larger length scales and longer lifetimes in comparison with those in SNRs. This may explain the the CR spectrum above the knee.
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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.
The Fermi Bubbles are enigmatic gamma-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 gamma -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 gamma -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}/(0.1 M_{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, $M_{pls}$).
We analyse processes of electron acceleration in the Fermi Bubbles in order to define parameters and restrictions of the models, which are suggested for the origin of these giant radio and gamma-ray structures. In the case of leptonic origin of the nonthermal radiation from the Bubbles, these electrons should be produced somehow in-situ because of relatively short lifetime of high energy electrons, which lose their energy by synchrotron and inverse Compton processes. It has been suggested that electrons in Bubbles may be accelerated by shocks produced by tidal disruption of star accreting onto the central black hole or a process of re-acceleration of electrons ejected by supernova remnants. These processes will be investigated in subsequent papers. In this paper we focus to study in-situ stochastic (Fermi) acceleration by a hydromagnetic/supersonic turbulence, in which electrons can be directly accelerated from the background plasma. We showed that the acceleration from the background plasma is able to explain the observed fluxes of radio and gamma-ray emission from the Bubbles but the range of permitted parameters of the model is strongly restricted.
We analyse new results of Chandra and Suzaku which found a flux of hard X-ray emission from the compact region around Sgr A$^ast$ (r ~ 100 pc). We suppose that this emission is generated by accretion processes onto the central supermassive blackhole when an unbounded part of captured stars obtains an additional momentum. As a result a flux of subrelativistic protons is generated near the Galactic center which heats the background plasma up to temperatures about 6-10 keV and produces by inverse bremsstrahlung a flux of non-thermal X-ray emission in the energy range above 10 keV.
We constrain the origin of Fermi Bubbles using 2D hydrodynamical simulations of both star formation driven and black hole accretion driven wind models. We compare our results with recent observations of OVIII to OVII line ratio within and near Fermi Bubbles. Our results suggest that independent of the driving mechanisms, a low luminosity ($mathcal{L} sim 0.7-1times 10^{41}$ erg s$^{-1}$) energy injection best reproduces the observed line ratio for which the shock temperature is $approx 3times 10^6$ K. Assuming the Galactic halo temperature to be $2times 10^6$K, we estimate the shock velocity to be $sim 300$ km s$^{-1}$ for a weak shock. The corresponding estimated age of the Fermi bubbles is $sim 15-25$ Myr. Such an event can be produced either by a star formation rate of $sim 0.5$ M$_odot$ yr$^{-1}$ at the Galactic centre or a very low luminosity jet/accretion wind arising from the central black hole. Our analysis rules out any activity that generates an average mechanical luminosity $gtrsim 10^{41}$ ergps as a possible origin of the Fermi Bubbles.
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