Hard X-Ray Emission of Quasi- Thermal Electrons from the Galactic Ridge V. A. Dogiel 1,2, Hajime Inoue 1, Kuniaki Masai 3, V. Schoenfelder 4, and A. W. Strong 4 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

Galactic Ridge X-Ray Emission 30 years since its discovery (Bleach et al., 1970), but the origin has not been resolved yet. 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 Q=10 The total energy flux in the range 2-10 keV is Q x =10 38 erg/s Distribution |l|< 50 o, |b|< 10 o. Distribution |l|< 50 o, |b|< 10 o.

Thermal bremsstrahlung origin. 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. Discrete sources. Discrete sources. Galactic point-like sources with required properties are not found from the ASCA and CHANDRA observations. Excluded. Inverse Compton Scattering. Inverse Compton Scattering. Inconsistent with the observed Galactic radio emission. Excluded. Nonthermal bremsstrahlung radiation Nonthermal bremsstrahlung radiation of subrelativistic electrons or protons. Q~ erg/s> Q SN. A new class “unseen” of CR sources? (or exluded). Origin of the Ridge X-Ray Flux

Therma l 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!

A flux of 6.4 keV Fe-line has to be generated by nonthermal electrons. The energy output of the electrons as high as erg/s is needed, i.e. more than can be supplied by SN stars! Thermal vs Nonthermal 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)

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)!

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

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

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.

THQTHNTH

Energy Output Thermal particles x titi x x x F x =10 -5 N/t i N N, t=t br Q= Q x = N/t br ~10 38 erg/s x Q=10 5 Q x =10 43 erg/s Nonthermal particles titi xx x x x x x N 0, t=t i, t i /t br ~10 -5 F x ~N/ t br ~10 -5 N/ t i

Bremsstrahlung of quasi-thermal electrons erg/s<Q<10 43 erg/s Quasi-thermal particles x titi F x =10 -5 N titi F x =10 -5 N/ t i, Q=N/ t x x N N’< N, t i < t < t br Q=Q x t br /t e =Q x t br /t i t i /t e = Q x =10 38 erg/s Q<10 42 erg/s =10 5 Q x t i / t e

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 50 keV the range of nonthermal particles. 20 MeV protons are nonthermal. Q p ~10 43 erg/s Q e <10 42 erg/s !!!

electrons protons

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: t th = t br ; t br < t qth < t i ; t nth = t acc < t i Particle pressure in acceleration region: P th =1, P qth <0.3, P nth ~0. Plasma hydrostatically stable!!!

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 keV line Quasi-thermal component – additional ionization of Fe nuclei. Result – 6.61 keV line emission in relatively cold plasma!

T=0.3 keV

T=0.6 keV

List of Resolved Problems Energetical problem - <10 42 erg/s Energetical problem - <10 42 erg/s Problem of plasma hydrostatic stability - plasma temperature T<T gr Problem of plasma hydrostatic stability - plasma temperature T<T gr Problem of multi-temperature medium – Artifact. Emission of quasi-thermal electrons Problem of multi-temperature medium – Artifact. Emission of quasi-thermal electrons Problem of highly ionized medium- Ionization by quasi-thermal electrons Problem of highly ionized medium- Ionization by quasi-thermal electrons Single spectrum – single process of the electron spectrum formation Single spectrum – single process of the electron spectrum formation

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