The diffuse X-ray emission from the Galactic center

1. 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) Simbol-X

2. 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 Simbol-X

3. 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 Simbol-X

4. 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 ? Simbol-X

5. 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… Simbol-X

6. 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-): Simbol-X

7. 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. Simbol-X

8. 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 Simbol-X

9. 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 ! Simbol-X

10. 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… Simbol-X

11. 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. Simbol-X

12. 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) Simbol-X