FERMIBUBBLES INFLATED BY WINDS LAUNCHED FROM THE HOT ACCRETION FLOW IN SGR A*

Abstract

A pair of giant gamma-ray Bubbles has been revealed by Fermi-LAT. In this paper we investigate their formation mechanism. Observations have indicated that the activity of the supermassive black hole located at the Galactic center, Sgr A*, was much stronger than at the present time. Specifically, one possibility is that while Sgr A* was also in the hot accretion regime, the accretion rate should be 10 3-10 4 times higher during the past ∼10 7 yr. On the other hand, recent magnetohydrodynamic numerical simulations of hot accretion flows have unambiguously shown the existence and obtained the properties of strong winds. Based on this knowledge, by performing threedimensional hydrodynamical simulations, we show in this paper that the Fermi Bubbles could be inflated by winds launched from the "past" hot accretion flow in Sgr A*. In our model, the active phase of Sgr A* is required to last for about 10 million years and it was quenched no more than 0.2 million years ago. The central molecular zone (CMZ) is included and it collimates the wind orientation toward the Galactic poles. Viscosity suppresses the Rayleigh-Taylor and Kelvin-Helmholtz instabilities and results in the smoothness of the Bubbles edge. The main observational features of the Bubbles can be well explained. Specifically, the ROSAT X-ray features are interpreted by the shocked interstellar medium and the interaction region between the wind and CMZ gas. The thermal pressure and temperature obtained in our model are consistent with recent Suzaku observations.

A Unified Model of the Fermi Bubbles, Microwave Haze, and Polarized Radio Lobes: Reverse Shocks in the Galactic Center’s Giant Outflows

“…However, a number of recent works have claimed, on the basis of various independent analyses of X-ray, radio polarization, and other data, that it is actually a Galactic-scale feature (Kataoka et al 2013;Fang & Jiang 2014;Sun et al 2014;Sofue 2015;Tahara et al 2015) or perhaps a confusion of two structures, one local and one at Galactic scales. This finding complements the idea that (some part of) the NPS is a signature of Galactic nuclear activity (Sofue 1977(Sofue , 2000Bland-Hawthorn & Cohen 2003;Totani 2006) and perhaps even related directly to the Fermi bubbles (Guo & Mathews 2012;Kataoka et al 2013;Mou et al 2014). In addition to the NPS, there are linear γ-ray and polarized radio arcs that run concentrically between the NPS and the eastern edge of the northern bubble (Su et al 2010;Jones et al 2012); fainter radio and X-ray spurs have also been claimed as the northwestern, southeastern, and southwestern complements of the NPS (Sofue 2000(Sofue , 2015Tahara et al 2015).…”

“…Broadly speaking, the bubbles might be the signatures of a fairly recent (within a∼Myr timescale) explosive outburst (Su et al 2010;Mertsch & Sarkar 2011;Zubovas et al 2011;Guo & Mathews 2012;Yang et al 2012;Zubovas & Nayakshin 2012;Fujita et al 2013;Barkov & Bosch-Ramon 2014;Mou et al 2014;Wardle & Yusef-Zadeh 2014) from the central supermassive black hole with their γ-ray emission originating in inverse Compton (IC) collisions of a nonthermal electron population accelerated in association with this activity. Alternatively, the bubbles may result from the integrated effect of moreorless secular processes associated with the inner Galaxy (Thoudam 2013) or Galactic nucleus, e.g., tidal disruption events that occur there every 10 4 -10 5 yr (Cheng et al 2011(Cheng et al , 2015 or the concentrated and sustained star formation activity (e.g., Lacki 2014) that occurs throughout the nuclear region (inner 200-300 pc diameter region around the black hole).…”

Two-dimensional inflow-wind solution of black hole accretion with an evenly symmetric magnetic field

“…The theoretical study on the existence of strong wind has been confirmed by the detection of emission lines from the accretion flow around the supermassive black hole in the Galactic center, Sgr A* (Wang et al 2013). As an application of the accretion wind theory, more recently, the formation of the Fermi bubbles detected by the Fermi telescope in the Galaxy has been successfully explained by the interaction of wind launched from the accretion flow around Sgr A* and the interstellar medium (Mou et al 2014;2015). Compared to other theoretical models of the Fermi bubbles, this "accretion-wind" model has two advantages.…”

Multiwavelength features of Fermi bubbles as signatures of a Galactic wind

“…The high energy electrons or protons can either be accelerated in situ by internal shocks and turbulence (Mertsch & Sarkar 2011), or advected from the disc. Outflows triggered by star formation in the Galactic centre (GC) region (Crocker 2012;Lacki 2014) and by the black hole at the GC (Guo & Mathews 2012;Yang et al 2012;Mou et al 2014) have been proposed for the dynamical origin of the FBs.…”