ORIGIN OF THE FERMI BUBBLE

“This situation is very similar to that described by Weaver et al. (1977) for a stellar wind expanding into a uniform density medium.”

“Recent analysis of Fermi/LAT data (see Su et al. 2010; Dobler et al. 2010) discovered new evidence of GC activity.”

“emission of a jet from the central black hole, wind from SN explosion close to the GC, 1st or 2nd order Fermi acceleration of CR electrons [143–147].”

“…jet from the black hole (Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012), a spherical outflow from the black hole (Zubovas et al. 2011), a wind from supernova explosions (Crocker & Aharonian 2011), or a sequence of shocks from several accretion events onto the black hole (Cheng et al. 2011).”

“Various authors have proposed mechanisms for the origin of these energetic electrons, including, for example, recent (∼ 1–3 Myr ago) AGN jet activity from the central massive black hole [48–51], a spherical outflow [52], a sequence of shocks resulting from several accretion events (involving first order Fermi accelera…”

“…recent Type II supernovae to provide the energy content of the bubbles, which is far higher than can be realistically expected from the inner ∼ 100 pc. Cheng et al. (2011) thus argued that the bubbles are inflated by episodic Sgr A∗ activity caused by tidal disruptions of stars passing too close…”

“…created the FBs, whether via AGN jets (Guo & Mathews 2012; Yang et al. 2012), feedback from nuclear star formation (Crocker & Aharonian 2011; Carretti et al. 2013; Lacki 2014), accretion flows onto SgrA∗ (Cheng et al. 2011; Mou et al. 2014), or a spherical outflow from SgrA∗ (Zubovas et al. 2011).”

“…processes associated with the inner Galaxy (Thoudam 2013) or Galactic nucleus, e.g., tidal disruption events that occur there every 104–105 yr (Cheng et al. 2011, 2015) or the concentrated and sustained star formation activity (e.g., Lacki 2014) that occurs throughout the nuclear region…”

“They can come from Fermi-1st order acceleration on shock front formed in the periodic star capture processes by Sgr A* (Cheng et al. 2011), the Fermi-2nd order acceleration through stochastic scattering by plasma instabilities (Mertsch & Sarkar 2011), directly from the jet (Guo & Mathews 2012; Guo…”

“…in the inner part of the Milky Way (Thoudam 2013) or Galactic nucleus, such as tidal disruption events regularly taking place every 104 − 105 yr (Cheng et al. 2011) or the continuous and vigorous star formation activity around the 200−300 pc diameter region around the central black hole of…”

“…star formation activity (Crocker 2012; Crocker et al. 2014, 2015; Sarkar et al. 2015; Ruszkowski et al. 2016), or in-situ as the bubbles evolve (Cheng et al. 2011, 2015; Mertsch & Sarkar 2011; Fujita et al. 2014; Lacki 2014; Sasaki et al. 2015), and compare the non-thermal emission to the…”

“Cheng et al. (2011) argue that the bubbles are inflated by episodic Sgr A∗ activity caused by tidal disruptions of stars passing too close to Sgr A∗.”

“…the bubbles (∼10 kpc from the plane), these electrons either have to be delivered very quickly – presumably on a AGN-type outflow originating at Sgr A* (Guo & Mathews 2011) – or accelerated in situ by first- (Cheng et al. 2011) or second-order (Mertsch & Sarkar 2011) Fermi acceleration processes.”

“…energy source of the FBs is still debated as either a recent outburst from the central SMBH of the MW (e.g., Zubovas et al. 2011; Wardle & Yusef-Zadeh 2014; Miller & Bregman 2016) or secular processes in the Galactic nucleus, like periodic star captures by the SMBH (e.g., Cheng et al. 2011, 2015).”

“Most theoretical work interprets these bubbles as the result of an outburst from Sgr A⋆ (Cheng et al. 2011; Zubovas, King, & Nayaksin 2011; Guo & Mathews 2012).”

“In our simulations, the magnetic energy injected with the AGN jets is constrained by the observed magnetic field strength at the GC and is only a small fraction (∼ 10−3) of the total jet power (Y12).”

“This simple two-component model of the diffuse γ-ray emission supports scenarios in which the Fermi bubbles are just outflows of the hot ISM (Yang et al. 2013; Cheng et al. 2011; Dogiel et al. 2011; Chernyshov 2011; Carretti et al. 2013).”

“Cheng et al. (2011) argued that periodic star capture processes by the Galactic supermassive black hole, Sgr A∗, release AGN winds into the Galactic halo, producing the Fermi bubbles within a few Myrs.”

“Although these models could bypass the age constraints, it has been challenging for the simplest models to produce the flat gamma-ray intensity profile (Mertsch & Sarkar 2011; Cheng et al. 2011) as well as the microwave haze emission (Fujita et al. 2014; Cheng et al. 2015).”

“A possible exception is the innermost part of the Galactic Center region where the recent discovered Fermi Bubbles may have their origin through direct bursts from Sgr A∗ (Cheng et al. 2011; Zubovas et al. 2011) or because of past starburst activities (Lacki 2014).”

“Others pointed out that the CRs originated from jets launched by the central black hole (Guo & Mathews 2012), or accelerated inside the bubbles (Mertsch & Sarkar 2011; Cheng et al. 2011).”

“They can come from: 1)Fermi first-order acceleration in shock front formed in the periodic star capture processes by Sgr A* (Cheng et al. 2011, 2014, 2015); 2) the Fermi second-order accelera- tion through stochastic scattering by plasma instabilities (Mertsch & Sarkar 2011); 3) directly carried by…”

“The shocks and turbulence could be triggered by NSF [43,49], accretion [31,50] or tidal disruption events (TDEs) [41,45] of the AGN, or an un-specified event [6,42,44,46–48].”

“Several models have been proposed to explain both the morphology and spectral properties of the FBs (Cheng et al. 2011; Mertsch & Sarkar 2011; Crocker & Aharonian 2011; Crocker 2012, 2013; Zubovas et al. 2011; Zubovas & Nayakshin 2012; Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012).”

“If all parts of a UDS possessed this velocity, a solar mass disruption would yield 1051 erg of kinetic energy (Cheng et al. 2011), but because the unbound debris has a range of velocities, with a large fraction of the mass moving significantly slower, the typical kinetic energies end up being…”

“The outflow can be generated by activity of the nucleus in our galaxy producing a jet (Guo & Mathews 2012; Guo et al. 2012), wind from long time-scale star formation (Crocker & Aharonian 2011), periodic star capture processes by the supermassive black hole in the Galactic Center (Cheng et al. 2011), or by winds produc…”

“These timing constraints are relaxed if there is distributed acceleration (Cheng et al. 2011; Mertsch & Sarkar 2011; Lacki 2013) throughout the structures.”

“The same result for the dimension of the bubble can be obtained if we use the swept-up mass model proposed by Cheng et al. (2011).”

“Moreover, the recent observation of the so-called Fermi Bubbles may be associated to recent bursts near the Galactic Centre region (see Cheng et al. 2011; Zubovas, King & Nayakshin 2011) or to past starburst activities (see Lacki 2014).”

“…from the central black hole, Sgr A*, in the past few million years (Su et al 2010; Guo & Mathews 2011; Zubovis et al 2011); the latter might be explained as due to all-pervading shocks (Cheng et al 2011) or distributed, stochastic, acceleration on plasma wave turbulence (Mertsch & Sarkar 2011).”

“It can be mentioned that neither the hadronic model presented in Crocker & Aharonian (2011) nor the leptonic model based on diffusive shock acceleration (Cheng et al. 2011) can satisfactorily explain the measured sharp edges shown in Figure 2.”

“In other models, CRs are carried by jets or outflows generated by the super-massive black hole at the GC (Zubovas et al. 2011; Guo & Mathews 2012; Yang et al. 2012) or they are accelerated in the bubbles (Mertsch & Sarkar 2011; Cheng et al. 2011).”

“Their positions are above and below the galactic plane (Cheng et al. 2011).”

“The observed spectrum of gamma-rays has a cut-off at the energy about 100 GeV (Su et al. 2010) which corresponds to the maximum of electron energy about 0.3 TeV (see e.g. Cheng et al. 2011); 2.”

“The periodic stellar capture events may explain the recent observed Fermi bubble (Cheng et al. 2011a).”

“…frequency ν = 23GHz and gamma-ray intensity = ×γ −I 4 10 9 ph cm−2 s−1 GeV−1 sr−1 at =γE 10 GeV, which is produced by scattering of optical or IR photons (see Cheng et al. 2011) whose energy density in the Halo is about ≃w 0.2 eV cm−3, for the electron spectral indexγ = 2e we obtain amagnetic…”

“Alternatively, the emission could be due to CRs which are accelerated at the Bubble edges by a shock resulting from stellar accretion onto the central black hole in the past (see Cheng et al. 2011, for the review of Bubble models, see also Yang et al. 2018).”

“…al. 2013) or leptonic (Yang et al. 2013); the underlying engine as a starburst (Carretti et al. 2013; Lacki 2014; Sarkar et al. 2015), an SMBH jet (Cheng et al. 2011; Guo & Mathews 2012; Zubovas & Nayakshin 2012; Mou et al. 2014), or steady star-formation (Crocker 2012); and the CR acceleration…”

“On longer timescales ranging from thousands to millions of years, the imprints of TDEs could be detected in the forms of enhanced electronpositron annihilation lines, high-energy gamma rays, and cosmic rays (Cheng et al. 2006, 2007, 2011).”

“The Fermi bubbles have also generated a great deal of interest on the modelling side (Crocker & Aharonian 2011; Cheng et al. 2011, 2012, 2014, 2015a,b; Zubovas et al. 2011; Mertsch & Sarkar 2011; Zubovas & Nayakshin 2012; Guo & Mathews 2012; Yang et al. 2012, 2013; Lacki 2014; Crocker et al. 2014,…”

“Recently, an alternative way of producing γ -ray emission was proposed, originally to explain the possible excess of GeV γ -rays in the GC (Cheng, Chernyshov & Dogiel 2006, 2007; Cheng et al. 2011, 2012).”

“As emission mechanisms, hadronic (Crocker & Aharonian 2011; Fujita et al. 2013; Cheng et al. 2015a) and leptonic (Cheng et al. 2011, 2014, 2015b; Mertsch & Sarkar 2011) models were proposed.”

“…2013); the underlying engine as a starburst (Carretti et al. 2013; Lacki 2014; Sarkar et al. 2015), a jet from the the central massive black hole (Cheng et al. 2011; Guo & Mathews 2012; Zubovas & Nayakshin 2012; Mou et al. 2014), or steady star-formation (Crocker 2012); and the cosmic-ray (CR)…”

“…cosmic rays (CRs) interacting with hot gas in the halo (Crocker & Aharonian 2011), to star bursts (Biermann et al. 2010), to AGN activity in our Galaxy (Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012; Cheng et al. 2011), to originating from dark matter annihilation (Dobler et al. 2011).”

“In one picture, the Fermi γ-ray bubbles are a relic of past AGN-like activity stimulated by accretion of stars or gaseous material onto the central supermassive black hole, Sgr A* (Zubovas, King and Nayakshin 2011; Cheng et al. 2011; Guo et al. 2012; Yang et al. 2012, 2013).”

“While in the leptonic models, inverse Compton scattering of relativistic electrons on optical and UV photons produces the gamma rays [2,3,13,14].”

“Other models include a starburst (Carretti et al. 2013; Lacki 2014; Sarkar et al. 2015), central massive black hole jets (Cheng et al. 2011; Guo & Mathews 2012; Guo et al. 2012; Mou et al. 2014) or outflows (Zubovas & Nayakshin 2012), and steady star-formation (Crocker et al. 2015).”

“…or hadronic processes, namely the inverseCompton scattering of interstellar radiation field and the cosmic microwave background by electrons (e.g. Cheng et al., 2011; Mertsch & Sarkar, 2011; Lacki, 2014) or the hadronuclear process of protons (and ions) colliding with ambient gas in the bubbles…”

“As Cheng et al. (2007, 2011) and Dogiel et al. (2009c) showed, this luminosity of subrelativistic and relativistic CRs in the GC can also be provided by processes of star accretion onto the central black hole, though as we mentioned these estimates cannot be considered as reliable.”

“…past accretion event on to the supermassive black hole (SMBH; Su, Slatyer & Finkbeiner 2010; Zubovas, King & Nayakshin 2011), active galactic nucleus (AGN) jets (Guo & Mathews 2012), a nuclear starburst (Su et al. 2010) and a sequence of star capture events in the last ∼10 Myr (Cheng et al. 2011).”

“HEX-P will be uniquely equipped to explore a variety of major topics in Galactic astrophysics, allowing us to 1) investigate broad-band properties of X-ray flares emitted from the supermassive black hole (BH) at Sgr A* and probe the associated particle acceleration and emission mechanisms; 2)…”

“All these point to the Fermi bubbles coming from inverse Compton scattering (ICS) of cosmic-ray electrons (e−) and positrons (e+) (Cheng et al. 2011; Crocker et al. 2011; Mertsch & Sarkar 2011; Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012; Carretti et al. 2013; Yang et al. 2013; Lacki…”

“…created by an activity in that region, such as a jet or an outflow from the supermassive black hole Sgr A*, the AGN scenario (Zubovas et al. 2011; Cheng et al. 2011; Guo and Mathews 2012; Yang et al. 2013; Sarkar et al. 2017; Yang et al. 2022; Sarkar et al. 2023), a star-burst activity near the…”

“…given the vast extension of the Bubbles, these electrons either have to be delivered very quickly – presumably on an AGN-outflow originating at Sgr A* (Guo & Mathews(2011)) – or accelerated in-situ by first (Cheng et al.(2011)) or second-order (Mertsch & Sarkar(2011)) Fermi acceleration processes.”

“An intense AGN phase at Eddington luminosity accompanied by jets or outflows a few millions years ago has been invoked (Zubovas et al. 2011) as well as the recurrent (every 104–105 years) accretion of stars captured by the black hole (Cheng et al. 2011).”

“In this chapter, we are exploring how MeV observations could be key for advancing our knowledge of the SMBH nearest surroundings, both in terms of their emission and the physics connected.”

“We discussed this model in details in Cheng et al. (2011).”

“Each capture produces a shock propagating into the Galactic halo where shocks accelerate electrons (up to E ∼ 1 TeV) and protons with energies up to E ∼ 1018 eV (see Cheng et al. 2011, 2012).”