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

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

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

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. –C s > 1500km/s ≥ v escape ~ 1100 km/s  not bound to the galactic plane… –Very fast escape:  esc ~ yr –Heating source must be very efficient (> 30 SNe / yr in the Galaxy !!) Also: heating mechanism ?

I. The confinement problem… (submitted) Elements with different weight behave differently: –Protons alone must escape (v th > v esc ) –Other ions alone would not escape (v th < v esc ) What happens for H+He ? –Can protons drag other ions ? –Faint (0.1 cm -3 ) + hot :     e ~ 10 5 yr >  esc –Collisionless escape => No drag. Conclusion: plasma of helium and metals

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 ~ 10 8 yr = long time scale… –Reasonable energy requirement

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…) Galactic plane B Viscosity: (Braginskii 65) B => No shear viscosity: bulk/shear ~ !! The bulk viscosity acts on compressional motion: Efficiency: –Subsonic motion: v c weak compression – Very high viscosity:  ~ T 5/2 => high  – Depends on the exact flow around the clouds…

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

In the Central Region (h*d = 200*300 pc 2 ) : Cloud number: ~ 100 hot component luminosity: ~ erg/s And more: + complex clouds structures + intermittent accretion Alfvén: 1% dissipation would be sufficient… (irregularities, curvature…) Slow: OK… Fast: Too weak…

An intriguing coincidence: –The hotter, the more viscous:  ~ T 5/2 –The hotter, the less collisional :  coll  ~ T 3/2 –for  coll >>  0 the efficiency drops  most efficient for  coll  ~  0 –For the clouds :   = r/v ~ yr ~  He-He = optimal regime –Coincidence or self regulation mechanism ? Consequence on accretion: – emission of Alfvén waves = associated drag (cf artificial satellites) –=> loss of gravitational energy and accretion

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 !

Wings

Braginskii Viscosity Viscosity:  =  ~  l 2 /  ~ P  ~ nkT   ~ n  v v –Perfect gas:  v ~cst  ~ n -1 T -1/2  ~ T 1/2 –Ionized gas:  v ~v -4  ~ n -1 T 3/2  ~ T 5/2 Magnetized plasma  Braginskii viscosity (1965) : –Bulk viscosity: F i =  0 d i d j v j –Shear viscosity:F i =  1 d j 2 v i –Shear / bulk =