Hard X-Ray Emission of Quasi-Thermal Electrons from the Galactic Ridge

1. Hard X-Ray Emission of Quasi-Thermal Electrons from the Galactic Ridge V. A. Dogiel1,2, Hajime Inoue1, Kuniaki Masai3, V. Schoenfelder4, and A. W. Strong4 1 Institute of Space and Astronautical Science, Sagamihara, Japan 2 P.N.Lebedev Physical Institute, Moscow, Russia 3 Tokyo Metropolitan University, Tokyo, Japan 4 Max-Planck Institut fuer extraterrestrische Physik, Garching, FRG

2. Galactic Ridge X-Ray Emission • 30 years since its discovery (Bleach et al., 1970), but the origin has not been resolved yet. • The total energy flux in the range 2-10 keV is Qx=1038erg/s • Distribution |l|<50o, |b|<10o.

3. Origin of the Ridge X-Ray Flux • Discrete sources. Galactic point-like sources with required properties are not found from the ASCA and CHANDRA observations. Excluded. • Inverse Compton Scattering. Inconsistent with the observed Galactic radio emission. Excluded. • Thermal bremsstrahlung origin.X-ray emission from hot plasma with the temperature 5-10 keV. Too high rate of SN explosions. Excluded. • The ridge emission is truly diffuse and nonthermal. • Nonthermal bremsstrahlung radiation of subrelativistic electrons or protons. Q~1042-43 erg/s> QSN. A new class “unseen” of CR sources? (or exluded).

4. Thermal vs Nonthermal • Multi-temperature interpretation • Regions with temperatures 0.75, 1.8 and 10 keV are needed to reproduce the Ridge spectrum (Tanaka 2001) • The position of the Fe-line, 6.61 keV corresponds to a highly ionized hot medium (Kaneda et al.1997) with the temperature 5-10 keV 10 kev plasma is unstable!

6. The ridge spectrum is reproduced by a two-temperature plasma (0.6 and 2.8 keV) + a hard flux of nonthermal subrelativistic electrons (Valinia et al. 2000) Thermal vs Nonthermal A flux of 6.4 keV Fe-line has to be generated by nonthermal electrons. The energy output of the electrons as high as 1043 erg/s is needed, i.e. more than can be supplied by SN stars!

8. Particle Acceleration from Background Plasma • The large scale association of the hard X-ray emission with the thermal X-rays implies that these two components are linked • This leads to the idea that thermal particle in the hot plasma are accelerated. • The X-ray flux is produced in the regions where particles are freshly accelerated (Yamasaki et al. 1997). • There is an extended transition region of quasi-thermal particles between the energy ranges of thermal and non-thermal particles (Gurevich, 1960 – Fermi acceleration, Bulanov and Dogiel, 1979 – shock wave acceleration)!

10. Bremsstrahlung of Quasi-Thermal Particles • Equation for accelerated particles • E<kT/(a/n)0.4- thermal particles • E>kT/(a/n)0.66- nonthermal particles • kT/(a/n)0.66>E>kT/(a/n)0.4- quasi-thermal particles • Bresstrahlung emission of quasi-thermal particles – the ridge X-ray emission?

12. List of Problems has to be Resolved • Energetical problem • Problem of plasma hydrostatic stability • Problem of multi-temperature medium • Problem of highly ionized medium • Single X-ray spectrum from different regions of the Galactic Ridge

13. Multi-Temperature X-Ray Spectra • Two processes form the particle spectrum: Coulomb collisions which form the background spectrum; Stochastic acceleration which forms a power-law “tail” of non-thermal particles. • The acceleration violates the equilibrium state of the background plasma that produces a particle “run-away” flux into acceleration region. • Coulomb collisions form an extended transition region of quasi-thermal particles that mimics the effect of many temperature distribution.

14. TH QTH NTH

15. Nonthermal particles x x N 0, t=ti, ti/tbr~10-5 ti x Fx~N/tbr~10-5N/ti x Q=10 5 Qx=10 43 erg/s x x x x Thermal particles N N , t=tbr ti x x Fx=10-5 N/ti Q= Qx =N/tbr~1038 erg/s x x Energy Output

16. Bremsstrahlung of quasi-thermal electrons Q=Qx tbr/te=Qxtbr/ti ti/te= =105Qxti/te Qx=1038erg/s Q<1042erg/s Quasi-thermal particles N N’< N, ti< t < tbr ti ti x Fx=10-5 N Fx=10-5 N/ti, Q=N/t x 10 38erg/s<Q<10 43erg/s x

17. Electrons or protons? • 10 keV photons are emitted either by a ~10 keV electron or by a 20 MeV proton. • For a 0.3 keV plasma the range of quasi-thermal electrons 5<E<50 keV; >50 keV the range of nonthermal particles. 20 MeV protons are nonthermal. • Qp~1043 erg/s Qe<1042 erg/s !!!

18. electrons protons

19. Pressure of quasi-thermal particles Region of X-ray emission of thermal and quasi-thermal particles Region of X-ray emission of nonthermal particles Acceleration region Surrounding medium Particle lifetime in acceleration region: tth= tbr; tbr< tqth< ti; tnth= tacc<ti Particle pressure in acceleration region: Pth=1, Pqth<0.3, Pnth~0 . Plasma hydrostatically stable!!!

20. Quasi-Thermal Origin of the Line Emission • Three components of the electron spectrum: thermal (T~0.6 keV), quasi-thermal, and nonthermal • Thermal component – ionization state of iron nuclei +16 • Nonthermal component - 6.4 keV line • Quasi-thermal component – additional ionization of Fe nuclei. Result – 6.61 keV line emission in relatively cold plasma!

22. T=0.3 keV

23. T=0.6 keV

25. List of Resolved Problems • Energetical problem - <1042 erg/s • Problem of plasma hydrostatic stability - plasma temperature T<Tgr • Problem of multi-temperature medium – Artifact. Emission of quasi-thermal electrons • Problem of highly ionized medium- Ionization by quasi-thermal electrons • Single spectrum – single process of the electron spectrum formation

26. Conclusion • Emitting particles - electrons • Emitting space – regions of particle acceleration • Parameters of the space – T~ 0.6 - 1 keV • Energy range of emitting particles – quasi-thermal electron (with E~5-50 keV) • Acceleration time necessary to produce the ridge X-ray flux – te=6 1012 s • The energy output of the emitting electrons – (1-3) 1041 erg/s