Gamma-ray data from Fermi-LAT reveal a bi-lobular structure extending up to 50 degrees above and below the galactic centre, which presumably originated in some form of energy release there less than a few million years ago. It has been argued that the gamma-rays arise from hadronic interactions of high energy cosmic rays which are advected out by a strong wind, or from inverse-Compton scattering of relativistic electrons accelerated at plasma shocks present in the bubbles. We explore the alternative possibility that the relativistic electrons are undergoing stochastic 2nd-order Fermi acceleration by plasma wave turbulence through the entire volume of the bubbles. The observed gamma-ray spectral shape is then explained naturally by the resulting hard electron spectrum and inverse Compton losses. Rather than a constant volume emissivity as in other models, we predict a nearly constant surface brightness, and reproduce the observed sharp edges of the bubbles.
Winds are ubiquitous in galaxies and often feature bubble structures. These wind bubbles are characterized by an external forward shock expanding in the surrounding medium and a wind termination shock separating the cool and fast wind from the hot shocked wind. While the forward shock could not be able to accelerate particles efficiently for a long time, at the wind termination shock the necessary conditions for efficient acceleration may be present. We develop a model for particle acceleration at the termination shock of such bubbles analysing the consequences of different possible engines po…
We propose the model of the global structure of the electromagnetic fields in the Fermi bubbles (FBs), which makes possible the proton regular acceleration up to ultra-high energies. The poloidal and the toroidal magnetic fields, as well as the radial electric field, turn to have a structure similar to fields that exist in jets ejected out from active galactic nuclei (AGN). A powerful source of relativistic particles observed in the centre of the Galaxy and associated with the rotating supermassive black hole (SMBH) Sgr A* can energize the FB and keeps its active for a long time. The absence o…
There are two spectacular structures in our Milky Way: the {\it Fermi} bubbles in gamma-ray observations and the North Polar Spur (NPS) structure in X-ray observations. Because of their morphological similarities, they may share the same origin, i.e., related to the past activity of Galactic center (GC). Besides, those structures show significant bending feature toward the west in Galactic coordinates. This inspires us to consider the possibility that the bending may be caused by a presumed global horizontal galactic wind (HGW) blowing from the east to the west. Under this assumption, we adopt…
Galaxy clusters are known to host many active galaxies (AGNs) with radio jets, which could expand to form radio bubbles with relativistic electrons in the intracluster medium (ICM). It has been suggested that fossil relativistic electrons contained in remnant bubbles from extinct radio galaxies can be re-accelerated to radio-emitting energies by merger-driven shocks via diffusive shock acceleration (DSA), leading to the birth of radio relics detected in clusters. In this study we assume that such bubble consist primarily of thermal gas entrained from the surrounding medium and dynamically-insi…
The Double-Episode Jet Genesis of the eROSITA and Fermi Bubbles
Henan Normal University（河南师范大学）；Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）；University of Chinese Academy of Sciences（中国科学院大学）；Tianfu Cosmic Ray Research Center（天府宇宙线研究中心）；Guizhou University（贵州大学）；Handan University（邯郸学院）；Nanjing Normal University（南京师范大学）
The newly launched X-ray satellite, eRosita, has recently revealed two gigantic bubbles extending to ~80 degrees above and below the Galactic center. The morphology of these "eRosita bubbles" bears a remarkable resemblance to the Fermi bubbles previously discovered by the Fermi Gamma-ray Space Telescope and its counterpart, the microwave haze. The physical origin of these striking structures has been intensely debated; however, because of their symmetry about the Galactic center, they likely originate from some energetic outbursts from the Galactic center in the past. Here we propose a theoret…
Simulating the Formation of the Young "Fermi Bubbles" in the Circinus Galaxy
Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）；University of Chinese Academy of Sciences（中国科学院大学）；Tianfu Cosmic Ray Research Center（天府宇宙线研究中心）；School of Physics, Henan Normal University（河南师范大学物理学院）；Center for Theoretical Physics, Henan Normal University（河南师范大学理论物理中心）；Centre for Astrophysics Research, University of Hertfordshire（赫特福德大学天体物理研究中心）；INAF - Osservatorio Astronomico di Roma（意大利国家天体物理研究所罗马天文台）；Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台）
Multi-wavelength features of Fermi Bubbles as signatures of a Galactic wind
使用流体动力学模拟，我们第一次表明，在银河系中心的恒星形成的插曲，与恒星形成率（SFR）$\sim 0.5$ M $_\odot$年$^{-1}$为$\sim 30$万年，可以产生类似于费米泡（FB）的泡沫，当从太阳的位置。FB的形态，范围和多波长观测，特别是X射线，限制了各种物理参数，如SFR，年龄和环银河系介质（CGM）密度。我们表明，CGM与银河风驱动的恒星形成在中心区域的相互作用可以解释所观察到的表面亮度和形态特征的X射线与费米泡。此外，假设宇宙射线电子通过冲击和/或湍流被加速{ \it in situ}，可以解释伽马射线发射的亮度和形态以及微波雾。在我们的模型中，冷团和暖团的运动学也与最近通过气泡观察到的吸收线相匹配。
Hydrodynamic model of nonthermal emission from the Fermi bubbles
I. E. Tamm Theoretical Physics Division of P. N. Lebedev Physical Institute（P.N.列别捷夫物理研究所I.E.塔姆理论物理部）；Institute of Astronomy, Department of Physics and Center for Complex Systems, National Central University（国立中央大学天文研究所、物理系及复杂系统中心）
The enhanced star forming activity, typical of starburst galaxies, powers strong galactic winds expanding on kiloparsec (kpc) scales and characterized by bubble structures. Here we discuss the possibility that particle acceleration may take place at the termination shock of such winds. We calculate the spectrum of accelerated particles and their maximum energy, that turns out to range up to a few hundred petaelectronvolt (PeV) for typical values of the parameters. Cosmic rays accelerated at the termination shock are advected towards the edge of the bubble excavated by the wind and eventually e…
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 heat…
Nested Fermi and eROSITA bubbles require very similar $\sim10^{56}$ erg collimated Galactic-center outbursts; their asymmetry indicates an eastern density gradient
Ben-Gurion University of the Negev（内盖夫本-古里安大学）；Indian Institute of Astrophysics（印度天体物理研究所）
The Fermi Bubbles: Giant, Multi-Billion-Year-Old Reservoirs of Galactic Center Cosmic Rays
最近，费米-LAT仪器观测到的伽马射线天空中出现了巨大特征的证据：以银河系核心为中心的双边发射“气泡”，并在银河系平面上方和下方延伸到约10千秒差距。这些结构与WMAP数据中发现的非热微波“薄雾”和ROSAT探测到的X射线辐射的扩展区域相吻合。气泡的伽马射线辐射的特征是具有坚硬且相对均匀的光谱，相对均匀的强度，总光度约为4 x 10^37 erg/s，比它们的微波光度大约大一个数量级，而比它们的X射线光度小一个数量级以上。在这里，我们表明，气泡是自然的解释，由于人口的遗迹宇宙射线质子和较重的离子注入的过程与极长的时间尺度（>~8 Gyr）和高面密度的恒星形成在银河系中心。
The supersonic winds produced by massive stars carry a large amount of kinetic power. In numerous scenarios such winds have been proven to produce shocks in which relativistic particles are accelerated emitting non-thermal radiation. Here, we report the first detection of non-thermal emission from a single stellar bubble, G2.4+1.4, associated with a WO star. We observed this source with the uGMRT in Band 4 ($550-850$ MHz) and Band 5 ($1050-1450$ MHz). We present intensity and spectral index maps for this source that are consistent with synchrotron emission (average spectral index, $α= -0.83 \p…
The Fermi bubbles are large gamma-ray-emitting structures. They are symmetric about the Galactic Centre (GC), and their creation is therefore attributed to intensive energy injection at the GC. In this study, we focus on the non-equilibrium X-ray gas structures associated with the bubbles. We show that a combination of the density, temperature, and shock age profiles of the X-ray gas can be used to distinguish the energy injection mechanisms. By comparing the results of numerical simulations with observations, we indicate that the bubbles were created by a fast wind from the GC because it gene…
We study the spatial correlation of astrophysical neutrinos detected by IceCube with the geometry of the two large globular structures located in the center of our Galaxy, known as Fermi Bubbles (FB). Using the Fermi-LAT data collected during 8 years and the upper limits derived by the High Altitude Water Cherenkov (HAWC) gamma-ray observatory, we use a hybrid (lepto-hadronic) model to investigate a possible correlation with the high-energy neutrinos in the direction of the Fermi Bubbles. We find that these events are possibly not associated with the Northern Bubble but do not dismiss a possib…
The origin of the Fermi bubbles, which constitute two gamma-ray emitting lobes above and below the Galactic plane, remains unclear. The possibility that this Fermi bubbles gamma-ray emission originates from hadronic cosmic rays advected by a subsonic Galactic outflow, or breeze, is here explored. The simulation of a breeze solution and subsequent cosmic ray transport is carried out using the hydrodynamical code, PLUTO, in combination with a cosmic ray transport code. The Galactic outflow model obtained is found to be compatible with both inferences of the decelerating outflow velocity profile …
