1. About the 8 keV plasma at the Galactic Center High Energy Phenomena in the Galactic Center 17th June 20005 • CEA, Saclay • Belmont R. • Tagger M. • UCLA • Muno M. • Morris M. • Cowley S.

2. The Galactic Center: R ≤ 150-180 pc (~ Central Molecular Zone) • X-ray and radio observations: • - SN remnants • discrete point sources, • gas, clouds… • Arcs, Filaments • Pervasive, vertical, magnetic field • (Morris & Serabyn 1996)

3. 6.7 keV 6.9 keV Soft phase: Ionized lines + bremstrahlung  T ~ 0.8 keV Patchy distribution = SN remnants Hot phase : 6.7 + 6.9 keV + bremstrahlung  T ~ 8 keV, Diffuse large scale: 300pc*200pc (and more) Spectral components:(Muno et al. 2004)

4. The hot phase as a diffuse plasma at 8 keV • Origin of the hard diffuse emission: (Muno et al. 2004) • Non thermal emission ? • discrete point sources ? • Chandra: Diffuse plasma. • Diffuse plasma ? (Kaneda et al., 97) • Vertical magnetic field. • Cs > 1500km/s ≥ vescape ~ 1100 km/s  not bound to the galactic plane… • Very fast escape: esc~ 40 000 yr • Heating source must be very efficient (> 30 SNe / yr in the Galaxy !!) • Also: heating mechanism ?

5. I. The confinement problem… (submitted) • Elements with different weight behave differently: • Protons alone must escape (vth > vesc) • Other ions alone would not escape (vth < vesc) • What happens for H+He ? • Can protons drag other ions ? • Faint(0.1 cm-3) + hot :~e~ 105 yr > esc • Collisionless escape => No drag. • Conclusion: plasma of helium and metals

6. A Hot Helium plasma ? • Too hot => no H- or He lines • New estimates for inferred plasma parameters: • Lower densities and abundances: • n(He) ~n(H)/3 • [Fe]/[He] for He plasma ~ 1/3*([Fe]/[He] for H plasma) Fe trapped in grains in molecular clouds ? H-like Argon line ? • Radiative cooling time ~ 108 yr = long time scale… • Reasonable energy requirement

7. B Galactic plane II. A possible heating mechanism • Gravitational energy of molecular clouds • ~100 of them • ~10 pc size • ~100 km/s relative velocity (Bally et al. 87, Oka et al. 98…) • Viscosity: (Braginskii 65) B => No shear viscosity: bulk/shear ~ 1017 !! The bulk viscosity acts on compressional motion: • Efficiency: • Subsonic motion: vc < cs < va => wea k compression • Very high viscosity: ~ T5/2 => high  • Depends on the exact flow around the clouds…

8. Alfvén wing -> wing flux: FA But incompressible ! B Slow MS wing: -> wing flux: FS And compressible V Fast MS perturbation: - 2D toy model - asymptotic expansion in vc/va -> dissipated power: QF The wake of a cloud: (in a low- plasma) Drell et al. 65, Neubauer 80, Wright & Schwartz 90, Linker 91…

9. Fast: Too weak… Slow: OK… Alfvén: 1% dissipation would be sufficient… (irregularities, curvature…) In the Central Region (h*d = 200*300 pc2): Cloud number: ~ 100 hot component luminosity: ~ 5. 1037 erg/s And more: + complex clouds structures + intermittent accretion

10. An intriguing coincidence: • The hotter, the more viscous:~ T5/2 • The hotter, the less collisional: coll~ T3/2 • forcoll >> 0 the efficiency drops most efficient for coll~0 • For the clouds: = r/v ~ 5 104 yr ~ He-He = optimal regime • Coincidence orselfregulation mechanism ? • Consequence on accretion: • emission of Alfvén waves = associated drag(cf artificial satellites) • => loss of gravitational energy andaccretion

11. Conclusions: • In the conditions deduced from observations, H must escape whereas heavier elements may remain. This solves the energetics problem. • A possible heating mechanism is the dissipation of the gravitational energy of molecular clouds by viscosity. • The associated drag on the cold clouds would help in accreting matter to the central object. • more analytical work + simulations THANK YOU !

12. Wings

13. Braginskii Viscosity • Viscosity: =~l2/~ P~ nkT ~ nvv • Perfect gas: v~cst ~ n-1T-1/2  ~ T1/2 • Ionized gas: v~v-4~ n-1T3/2  ~ T5/2 • Magnetized plasma  Braginskii viscosity (1965): • Bulk viscosity: Fi = 0didjvj • Shear viscosity: Fi = 1 dj2vi • Shear / bulk = 10-20