Hard X-Ray Emission of Hard X-Ray Emission of Quasi-Thermal Electrons from Quasi-Thermal Electrons from

the Galactic Ridgethe Galactic Ridge

V. A. DogielV. A. Dogiel1,21,2, Hajime Inoue, Hajime Inoue11, , Kuniaki MasaiKuniaki Masai33, V. , V.

SchoenfelderSchoenfelder44, and A. W. , and A. W. StrongStrong44

1 Institute of Space and Astronautical Science, Sagamihara, Japan

2 P.N.Lebedev Physical Institute, Moscow, Russia3 Tokyo Metropolitan University, Tokyo, Japan 4 Max-Planck Institut fuer extraterrestrische Physik, Garching, FRG

Galactic Ridge X-Ray Galactic Ridge X-Ray EmissionEmission

30 years since its 30 years since its discovery (Bleach et discovery (Bleach et al., 1970), but the al., 1970), but the origin has not been origin has not been resolved yet.resolved yet.

The total energy flux The total energy flux in the range 2-10 in the range 2-10 keV is keV is QQx=10=1038erg/s

Distribution Distribution |l|<|l|<5050oo, , |b|<|b|<1010oo..

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 radiationNonthermal bremsstrahlung radiation of subrelativistic electrons or protons. Q~1042-43 erg/s> QSN. A new class “unseen” of CR sources? (or exluded).

Origin of the Ridge X-Ray Origin of the Ridge X-Ray FluxFlux

ThermaThermal vs Nonthermall 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 1043 erg/s is needed, i.e. more than can be supplied by SN stars!

Thermal vs NonthermalThermal 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 Particle Acceleration from Background PlasmaBackground 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 Bremsstrahlung of Quasi-Thermal ParticlesParticles

Equation for accelerated particles

E<kT/(0.4 - thermal particles E>kT/(0.66 - nonthermal particles kT/()0.66>E>kT/(0.4 - quasi-thermal

particles Bresstrahlung emission of quasi-thermal

particles – the ridge X-ray emission?

0ˆˆˆPr

fLLLt AccCoul

List of Problems has to be List of Problems has to be ResolvedResolved Energetical problemEnergetical problem

Problem of plasma hydrostatic Problem of plasma hydrostatic stabilitystability

Problem of multi-temperature Problem of multi-temperature mediummedium

Problem of highly ionized Problem of highly ionized mediummedium

Single X-ray spectrum from Single X-ray spectrum from different regions of the Galactic different regions of the Galactic RidgeRidge

Multi-Temperature X-Ray Multi-Temperature X-Ray Spectra 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.

TH QTH NTH

Energy OutputEnergy Output

Thermal particlesx

x

xx i

N N , br

Q= Qx =br~1038 erg/s x

Q=10 5 Qx=10 43 erg/s

Nonthermal particles

xx

x

xx

xx

N 0, i, ibr~10-5

Fx~N/br~10-5N/i

Bremsstrahlung of quasi-Bremsstrahlung of quasi-thermal electronsthermal electrons

10 38erg/s<Q<10 43erg/s

Quasi-thermal particles

x

Fx=10-5 N Fx=10-5 N/i, Q=N/xx

N N’< N, ibr

Q=Qx

bre=Qxbriie=

Qx=1038erg/sQ<1042erg/s

=105Qxie

Electrons or protons?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 !!!

electrons

protons

Pressure of quasi-thermal Pressure of quasi-thermal particlesparticles

Region of X-ray emission of thermal and quasi-thermal particles

Region of X-ray emission of nonthermal particles

Acceleration region Surrounding mediumParticle lifetime in acceleration region:th=br;br<qth<i;nth=acc<i

Particle pressure in acceleration region: Pth=1, Pqth<0.3, Pnth~0 . Plasma hydrostatically stable!!!

Quasi-Thermal Origin of the Line Quasi-Thermal Origin of the Line EmissionEmission

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!

T=0.3 keV

T=0.6 keV

List of Resolved ProblemsList of Resolved Problems Energetical problem - Energetical problem - <10<104242 erg/s erg/s Problem of plasma hydrostatic Problem of plasma hydrostatic

stability - stability - plasma temperature T<Tplasma temperature T<Tgrgr

Problem of multi-temperature Problem of multi-temperature medium – medium – Artifact. Emission of Artifact. Emission of quasi-thermal electronsquasi-thermal electrons

Problem of highly ionized medium- Problem of highly ionized medium- Ionization by quasi-thermal Ionization by quasi-thermal electronselectrons

Single spectrum – Single spectrum – single process of single process of the electron spectrum formation the electron spectrum formation

ConclusionConclusion Emitting particles - Emitting particles - electronselectrons Emitting space – Emitting space – regions of particle regions of particle

accelerationacceleration Parameters of the space – Parameters of the space – T~ 0.6 - 1 keVT~ 0.6 - 1 keV Energy range of emitting particles – Energy range of emitting particles –

quasi-thermal electron (with E~5-50 keV)quasi-thermal electron (with E~5-50 keV) Acceleration time necessary to produce Acceleration time necessary to produce

the ridge X-ray flux – the ridge X-ray flux – ee=6 10=6 101212 s s The energy output of the emitting The energy output of the emitting

electrons – electrons – (1-3) 10(1-3) 104141 erg/s erg/s