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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). Outline. The diffuse emission issue at the Galactic center Diffuse plasma ?

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the diffuse x ray emission from the galactic center
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)


  • 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


the galactic center region
The Galactic Center Region

100 pc

XR GC (2°)

XR disk (50°)

XR Bulge (25°)

La Rosa et al. 2000

  • Central zone:
  • X-ray view:
  • Strong emission
  • Radio view:
  • Non thermal filaments
  •  Vertical B (10G - 1mG)
  • 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


the gc x ray emission
The GC X-ray emission

Cold component:

fluorescence molecular clouds.

Soft component:

kBT ~ 0.8 keV SN remnants.

6.4 keV

Hot component:

Iron lines non thermal processes ?

 unresolved point sources ?

 diffuse plasma (kT7 keV) ?

6.7 keV

6.9 keV

Hard component ?:

Power law ?

Muno et al. 2004

-- RPS

-- DE

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 ?


problems with a diffuse plasma kaneda et al 1997
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)
  • Heatingmechanism:
    • If confined: radiative cooling time = 108 yr
    • Heating mechanism still needed…


confining the plasma belmont et al 2005
Confining the plasma…(Belmont et al. 2005)
  • Weakly collisional plasma:
    •  Disjoint study of the different species in the plasma
  • Only protons are light enough to leave the Galactic plane:

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

  • Species of different mass have different wills:

- As in planetary atmospheres confinement  comparison of vth and vesc

- 1-species plasma (+e-):



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.


a possible heating mechanism
A possible heating mechanism
  • Effect of the magnetic field(Braginskii 1965):
    • Inhibited shear viscosity (by 1011 !!)
    • remaining bulk viscosity

(Spitzer 1962)

  • Radiative cooling of the confined plasma:
  • Heating by the dissipation of the gravitational and kinetic energy of molecular cloudsby the strong viscosity(Re ~ 10-2):
  • Dissipation efficiency:
    • - Strong viscous coefficient
    • - Subsonic motion: vc < cs < vaweak compression
    • - The precise flow structure around clouds must be studied


the inviscid alfv n wake
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 !


viscous dissipation
Viscous dissipation :
  • Dissipation by : - Non linear effects
    • - Curvature of the field lines

Strong outgoing Alfvén flux !

  • 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

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


  • 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.


and simbol x
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)