The diffuse X-ray emission from the Galactic center

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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

• Central zone

• X-ray view
• Strong emission

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

• IR view
• The central molecular zone (MorrisSerabyn 1996)
• Gas condensed in clouds (Bally et al. 87)
  N100, R 5 pc, v 100 km s-1

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The GC X-ray emission
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 ?
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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 (gt 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-)

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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
  Fesolar ? 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 gt 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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A possible heating mechanism

• 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)

• Dissipation efficiency
• - Strong viscous coefficient
• - Subsonic motion vc lt cs lt va ? weak
  compression
• - The precise flow structure around clouds must
  be studied

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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
Strong outgoing Alfvén flux !

• For most of the expected values for the magnetic
  field, dissipation in the Alfvén wings
  (BelmontTagger 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
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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 GRGC 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)