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The diffuse X-ray emission from

the Galactic center     R. Belmont

CESR, Toulouse, France

Collaborators: M. Tagger (CEA/APC, France); M. Morris (UCLA, US); M. Muno (Caltech, US)

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Outline

• The diffuse emission issue at the Galactic center– Diffuse plasma ?– Unresolved discrete point sources ?

• Ideas to solve the diffuse plasma paradox– Confinement of the plasma Heavy helium plasma– Heating Viscous friction with dense molecular

clouds

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The Galactic Center Region

• IR view:The central molecular zone (Morris&Serabyn 1996)

Gas condensed in clouds (Bally et al. 87): N~100, R ~ 5 pc, v ~ 100 km s-1

• Central zone:

XR Bulge (25°)

XR disk (50°)

XR GC (2°)

• X-ray view:Strong emission

• Radio view:Non thermal filaments Vertical B (10G - 1mG)

100 pc

La Rosa et al. 2000

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QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Muno et al. 2004

The GC X-ray emission

Soft component:kBT ~ 0.8 keV SN remnants.

Hot component:Iron lines non thermal processes ?

unresolved point sources ? diffuse plasma (kT7 keV) ?

6.9 keV

6.7 keV

Cold component:fluorescence molecular clouds.

6.4 keV

Hard component ?:Power law ?

At the Galactic center:The diffuse emission (DE) profile is different from that of the resolved point sources (RPS) emission (Suzaku, Koyama et al. 2006).

diffuse plasma ?

-- RPS-- DE

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Problems with a diffuse plasma ?

(Kaneda et al., 1997)

• Energy problem: confinement of the plasma:– cs ~ 1500km/s ≥ vesc ~ 1100-1200 km/s the gas

escapes

– very fast escape: tesc~ 40 000 yr

– required power is huge (> 1 SN/3-300 yr in the central region)

• Heating mechanism:– If confined: radiative cooling time = 108 yr

– Heating mechanism still needed…

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Confining the plasma… (Belmont et al. 2005)

• Species of different mass have different wills:- As in planetary atmospheres confinement comparison of vth and vesc

- 1-species plasma (+e-):

Protons (=1/2) vth~ 1300 km s-1

Heavy ions (4/3-2)vth~600-750 km s-1 Escape velocity

vth~ 1200 km s-1

Selective evaporation Natural creation of a heavy He plasma (+metal), confined by gravity

• Only protons are light enough to leave the Galactic plane:

• Weakly collisional plasma: Disjoint study of the different species in the plasma

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The hot He plasma vs. Observations

• At 8 keV, H ad He are fully ionized no direct diagnostic on the major species

• Re-interpretation of spectral data: weaker number densities: n(He) ~ 0.3 n(H) Similar e- and mass densities

Smaller abundances: ([Fe]/[He])He ~ 0.3 ([Fe]/[He])H

Recent observations with Suzaku: [Fe] = 3.5 [Fe]solar He plasma with solar abundances

• Stratification: Heavy ions could sediment (sed ~ 108 yr) If the stratification is observed (He continuum, Fe line) = evidence for a plasma confined by gravity…

The origin of the continuum is uncertain (confusion from the many components). Observation at energy > 7 keV (Fe and Ni lines + continuum) with Simbol-X will clarify the spectral components in this spectral region. Spectra at several latitudes may give access to the vertical structure of the plasma for the iron line and the He continuum.

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• Effect of the magnetic field (Braginskii 1965):- Inhibited shear viscosity (by 1011 !!)- remaining bulk viscosity

Dissipation efficiency:- Strong viscous coefficient

- Subsonic motion: vc < cs < va weak compression- The precise flow structure around clouds must be studied

• Radiative cooling of the confined plasma:• Heating by the dissipation of the gravitational and kinetic energy of molecular clouds by the strong viscosity (Re ~ 10-2):

(Spitzer 1962)

A possible heating mechanism

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The inviscid Alfvén wake:

Alfvén wing (Drell et al. 1965, Neubauer 1980)

Echo-I in the earth magnetosphere Io in the Jovian magnetosphere

strong energy flux !

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Viscous dissipation :

Dissipation by : - Non linear effects- Curvature of the field lines

For most of the expected values for the magnetic field, dissipation in the Alfvén wings (Belmont&Tagger 2006):

- is very efficient

- balance the radiative cooling

- can account for the observed hot plasma

Strong outgoing Alfvén flux !

3D-MHD numerical simulations with the Zeus code are in progress to validate and extend these results…

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Conclusion

• The diffuse plasma issue is particularly interesting at the GC:– Stronger gravitational potential– High concentration of molecular gas– Vertical structured magnetic field

• Its nature is very debated. – Point sources (CVs): not enough of them ?– Diffuse plasma: should not exist since it must escape

• The escape of light protons naturally leaves a confined plasma made of He

• Its heating can be achieved by the viscous dissipation of the kinetic energy of molecular clouds.

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And Simbol-X…

• General input for the GR+GC diffuse emission (previous talks):– Thermal/Non thermal nature

• lines + high energy continuum

– Diffuse plasma/Discrete sources• High resolution mapping at high energy• Precise source identification and counting at high energy

• Specific input for the GC diffuse emission:– Good identification at high energy where the source confusion is high

– Look for vertical stratification (thanks to better constrains at high energy on the continuum origin)