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Register a new userFermi bubbles: the collimated outburst needed to explain forward-shock edges
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The bipolar, nonthermal, high-latitude lobes known as the Fermi bubbles (FBs) are thought to originate from a massive energy release near the Galactic centre (GC). We constrain the FB engine and the circumgalactic medium (CGM) by analytically and numerically modeling the FB edges as strong forward shocks, as inferred from recent observations. A non-directed energy release produces shocks too spherical to account for observations even for a maximally massive Galactic disc, critical CGM rotation, or injection effectively offset from the GC. In contrast, collimated injection nearly perpendicular to the disc can account for observations in both ballistic (free expansion) and slowdown regimes, as we show using a simple stratified evolution model verified by hydrodynamic simulations. FBs still in their ballistic regime require injection (at $zsimeq100$ pc heights in our model) with a half-opening angle $thetasimeq4^circ$, a normalized velocity $beta_{-2}equiv v/(0.01c)gtrsim 0.4$, and an energy $Egtrsim2beta_{-2}^2times 10^{55}$ erg, launched $mathbb{T}simeq 3.3beta_{-2}^{-1}$ Myr ago, showing a distinctive low-pressure region behind the bubble head. Slowing-down (mass accumulated) FBs require a faster injection, a thinner jet, a smaller $E/(beta_{-2}theta)^{2}$, and a comparable $mathbb{T}$, and follow a ballistic stage that must reach a height $z_{s}gtrsim 5$ kpc.
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The Fermi bubbles are two giant bubbles in gamma rays lying above and below the Galactic center (GC). Despite numerous studies on the bubbles, their origin and emission mechanism remain elusive. Here we use a suite of hydrodynamic simulations to study the scenario where the cosmic rays (CRs) in the bubbles are mainly accelerated at the forward shocks driven by a pair of opposing jets from Sgr A*. We find that an active galactic nucleus (AGN) jet event happened $5-6$ Myr ago can naturally reproduce the bilobular morphology of the bubbles, and the postshock gas temperature in the bubbles is heated to $sim0.4$ keV, consistent with recent X-ray observations. The forward shocks compress the hot halo gas, and at low latitudes, the compressed gas shows an X-shaped structure, naturally explaining the biconical X-ray structure in the ROSAT 1.5 keV map in both morphology and X-ray surface brightness. CR acceleration is most efficient in the head regions of the bubbles during the first 2 Myrs. The opposing jets release a total energy of $sim 10^{55}$ erg with an Eddington ratio of $sim 10^{-3}$, which falls well in the range of the hot accretion flow mode for black holes. Our simulations further show that the forward shocks driven by spherical winds at the GC typically produce bubbles with much wider bases than observed, and could not reproduce the biconical X-ray structure at low latitudes. This suggests that starburst or AGN winds are unlikely the origin of the bubbles in the shock scenario.
We present Suzaku X-ray observations along two edge regions of the Fermi Bubbles, with eight ~20 ksec pointings across the northern part of the North Polar Spur (NPS) surrounding the north bubble and six across the southernmost edge of the south bubble. After removing compact X-ray features, diffuse X-ray emission is clearly detected and is well reproduced by a three-component spectral model consisting of unabsorbed thermal emission (temperature kT ~0.1 keV from the Local Bubble (LB), absorbed kT ~0.3 keV thermal emission related to the NPS and/or Galactic Halo (GH), and a power-law component at a level consistent with the cosmic X-ray background. The emission measure (EM) of the 0.3 keV plasma decreases by ~50% toward the inner regions of the north-east bubble, with no accompanying temperature change. However, such a jump in the EM is not clearly seen in the south bubble data. While it is unclear if the NPS originates from a nearby supernova remnant or is related to previous activity within/around the Galactic Center, our Suzaku observations provide evidence suggestive of the latter scenario. In the latter framework, the presence of a large amount of neutral matter absorbing the X-ray emission as well as the existence of the kT ~ 0.3 keV gas can be naturally interpreted as a weak shock driven by the bubbles expansion in the surrounding medium, with velocity v_exp ~300 km/s (corresponding to shock Mach number M ~1.5), compressing the GH gas to form the NPS feature. We also derived an upper limit for any non-thermal X-ray emission component associated with the bubbles and demonstrate, that in agreement with the findings above, the non-thermal pressure and energy estimated from a one-zone leptonic model of its broad-band spectrum, are in rough equilibrium with that of the surrounding thermal plasma.
Current measurements of the $gamma$-ray Fermi bubbles (FB) are based on model-dependent tracers, carry substantial systematic uncertainties, and are at some tension with each other. We show that gradient filters pick out the FB edges, which are found to smoothly connect to the bipolar X-ray structure emanating from the Galactic center, thus supporting the interpretation of the FBs as a Galactic-scale phenomenon. The sharp edges facilitate a direct, model-free measurement of the peripheral FB spectrum. The result is strikingly similar to the full FB-integrated spectrum, softened by a power law of index $etasimeq (0.2mbox{--}0.3)$. This is naturally explained, in both hadronic and leptonic models, if cosmic rays are injected at the edge, and diffuse away preferentially at higher energies $E$. The inferred, averaged diffusion function in the (more plausible) leptonic model, $D(E)simeq 10^{29.5}(E/10mbox{ GeV})^{0.48pm0.02}mbox{ cm}^2mbox{ s}^{-1}$, is consistent with estimates for Kraichnan-like turbulence. Our results, in particular the minute spatial variations in $eta$, indicate that the FB edge is a strong, Mach $gtrsim5$, forward shock.
The nature of the bipolar, $gamma$-ray Fermi bubbles (FB) is still unclear, in part because their faint, high-latitude X-ray counterpart has until now eluded a clear detection. We stack ROSAT data at varying distances from the FB edges, thus boosting the signal and identifying an expanding shell behind the southwest, southeast, and northwest edges, albeit not in the dusty northeast sector near Loop I. A Primakoff-like model for the underlying flow is invoked to show that the signals are consistent with halo gas heated by a strong, forward shock to $sim$keV temperatures. Assuming ion--electron thermal equilibrium then implies a $sim10^{56}$ erg event near the Galactic centre $sim7$ Myr ago. However, the reported high absorption-line velocities suggest a preferential shock-heating of ions, and thus more energetic ($sim 10^{57}$ erg), younger ($lesssim 3$ Myr) FBs.
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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