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Register a new userMulti-wavelength features of Fermi Bubbles as signatures of a Galactic wind
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Added by
Kartick Chandra Sarkar
Publication date
2015
fields
Physics
and research's language is
English
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Using hydrodynamical simulations, we show for the first time that an episode of star formation in the center of the Milky Way, with a star-formation-rate (SFR) $sim 0.5$ M$_odot$ yr$^{-1}$ for $sim 30$ Myr, can produce bubbles that resemble the Fermi Bubbles (FBs), when viewed from the solar position. The morphology, extent and multi-wavelength observations of FBs, especially X-rays, constrain various physical parameters such as SFR, age, and the circum-galactic medium (CGM) density. We show that the interaction of the CGM with the Galactic wind driven by a star formation in the central region can explain the observed surface brightness and morphological features of X-rays associated with the Fermi Bubbles. Furthermore, assuming that cosmic ray electrons are accelerated {it in situ} by shocks and/or turbulence, the brightness and morphology of gamma-ray emission and the microwave haze can be explained. The kinematics of the cold and warm clumps in our model also matches with recent observations of absorption lines through the bubbles.
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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 a toy shock expansion model to understand two observational features: (1) the relative thickness of the NPS; (2) the bending of the {it Fermi} bubbles and NPS. In this model, the contact discontinuity (CD) marks the boundary of the {it Fermi} bubbles, and the shocked interstellar medium (ISM) marks the NPS X-ray structure. We find that the Mach number of the forward shock in the east is $sim$ 1.9-2.3, and the velocity of the HGW is ~ 0.7-0.9 $c_{s}$. Depending on the temperature of the pre-shock ISM, the velocity of the expanding NPS in Galactic coordinates is around 180-290 km/s, and the HGW is ~ 110-190 km/s. We argue that, the age of the NPS and the {it Fermi} bubbles is about 18-34 Myr. This is a novel method, independent of injection theories and radiative mechanisms, for the estimation on the age of the {it Fermi} bubble/NPS.
Fermi bubbles are giant gamma-ray structures extended north and south of the Galactic center with characteristic sizes of order of 10 kpc recently discovered by Fermi Large Area Telescope. Good correlation between radio and gamma-ray emission in the region covered by Fermi bubbles implies the presence of high-energy electrons in this region. Since it is relatively difficult for relativistic electrons of this energy to travel all the way from the Galactic sources toward Fermi bubbles one can assume that they accelerated in-situ. The corresponding acceleration mechanism should also affect the distribution of the relativistic protons in the Galaxy. Since protons have much larger lifetimes the effect may even be observed near the Earth. In our model we suggest that Fermi bubbles are created by acceleration of electrons on series of shocks born due to periodic star accretions by supermassive black hole Sgr A*. We propose that hadronic CR within the knee of the observed CR spectrum are produced by Galactic supernova remnants distributed in the Galactic disk. Reacceleration of these particles in the Fermi Bubble produces CRs beyond the knee. This model provides a natural explanation of the observed CR flux, spectral indexes, and matching of spectra at the knee.
Initial results are presented from 3D MHD modelling of stellar-wind bubbles around O stars moving supersonically through the ISM. We describe algorithm updates that enable high-resolution 3D MHD simulations at reasonable computational cost. We apply the methods to the simulation of the astrosphere of a rotating massive star moving with 30 km/s through the diffuse interstellar medium, for two different stellar magnetic field strengths, 10 G and 100 G. Features in the flow are described and compared with similar models for the Heliosphere. The shocked interstellar medium becomes asymmetric with the inclusion of a magnetic field misaligned with the stars direction of motion, with observable consequences. When the Alfvenic Mach number of the wind is $leq$10 then the stellar magnetic field begins to affect the structure of the wind bubble and features related to the magnetic axis of the star become visible at parsec scales. Prospects for predicting and measuring non-thermal radiation are discussed.
We investigate roles of magnetic activity in the Galactic bulge region in driving large-scale outflows of size $sim 10$ kpc. Magnetic buoyancy and breakups of channel flows formed by magnetorotational instability excite Poynting flux by the magnetic tension force. A three-dimensional global numerical simulation shows that the average luminosity of such Alfvenic Poynting flux is $10^{40} - 10^{41}$ erg s$^{-1}$. We examine the energy and momentum transfer from the Poynting flux to the gas by solving time-dependent hydrodynamical simulations with explicitly taking into account low-frequency Alfvenic waves of period of 0.5 Myr in a one-dimensional vertical magnetic flux tube. The Alfvenic waves propagate upward into the Galactic halo, and they are damped through the propagation along meandering magnetic field lines. If the turbulence is nearly trans-Alfv{e}nic, the wave damping is significant, which leads to the formation of an upward propagating shock wave. At the shock front, the temperature $gtrsim 5times 10^6$ K, the density $approx 6times 10^{-4}$ cm$^{-3}$, and the outflow velocity $approx 400-500$ km s$^{-1}$ at a height $approx 10$ kpc, which reasonably explain the basic physical properties of the thermal component of the Fermi bubbles.
In this article, we review some key aspects of a multi-wavelength flare which have essentially contributed to form a standard flare model based on the magnetic reconnection. The emphasis is given on the recent observations taken by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on the X-ray emission originating from different regions of the coronal loops. We also briefly summarize those observations which do not seem to accommodate within the canonical flare picture and discuss the challenges for future investigations.
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