Thermal and Non-Thermal X-Rays from the Galactic Center V. Dogiel, - - PowerPoint PPT Presentation

• V. Dogiel, P.N.Lebedev Institue of Physics

In collaboration with: A. Bamba, K.-S. Cheng, D. Chernyshov, A. Ichimura,

(alphabetical order)

• H. Inoue, W.-H. Ip, K. Koyama,C.-M. Ko, M. Kokubun,
• Y. Maeda, K. Mitsuda, K. Nakazawa, M. Nobukawa,
• D. Prokhorov, V. Tatischeff, N. Y. Yamasaki, T. Yuasa

Centre de Spectrometrie Nucleaire et de Spectrometrie de Masse, Orsay, France Institute of Astronomy, National Central University, Taiwan Institute of Space and Astronautical Science, Japan Kyoto University, Japan Moscow Institute of Physics and Technology, Russia University of Hong Kong, China University of Tokyo, Japan

Many thanks to: A. Bykov, K. Ebisawa, M. Ishida, M. Revnivtsev, and S. Yamauchi

for discussions and to K. Koyama and R. Terrier for several pictures in the talk

Thermal and Non-Thermal X-Rays from the Galactic Center

Vulcano Workshop 2010

Galactic Center as a Harbour of High Energy Activity

−1 −0.5 0.5 1 10

10 longitude (degree) Counts s−1

TeV emission >100 MeV emission 511 keV line emission 6.4 keV line emission 2-10 keV thermal emission 14-40 keV nonthermal emission 1037erg/s 1037erg/s 2 1036erg/s 4 1036erg/s 1035erg/s

GC Thermal Emission

(Koyama, 1996-2009, Muno, 2004) – T ~ 108 K – L2-10 ~ 2x1036ergs/s – Size ~ 50pc x 30pc – nave ~ 0.1cm-3 – npeak ~ 0.4cm-3 – Egas ~ 3x1052ergs

• The energy input needed to heat the gas up to T~10 keV is about

1041 erg/s. This energy supply cannot be produced by SN explosions. Other more powerful sources of energy are required to support the energy balance there (Sunyaev et al. 1993; Koyama et al. 1996; Muno et al. 2004). The source of energy with an output ~1041erg/s is required!!!

Non-Thermal X-Ray (Belanger et al. 2006,Yuasa et al. 2008)

• Region
• Spectrum

ΔE=12-40 keV, Γ=1.4, Ec =19-50 keV

• Luminosity W~4 1036 erg/s

l<2o, b<2o

• Origin of X-rays from the GC
• Point sources (Revnivtsev et al. 2009)
• Diffuse of unknown origin (Koyama et al.

2009)

Point sources

• Revnivtsev et al. from the Chandra data: 88% of the disk

emission is produced by dim and numerous point sources. Therefore, at least in the ridge emission, accreting white dwarfs and active coronal binaries are considered to be main emitters.

Image

Point Sources

2 scale-heights of 6.7 keV line

~1/7

Point sources Revnivtsev

However!

Relativistic protons X-rays Flux of subrelativistic protons

Star capture (Diener et al. 1997, Ayal et al. 2000, Alexander 2005 etc.)

A half of the star matter (i.e. ~ 1057 protons when a one solar mass star is captured) escapes with a subrelativistic velocity.

• Passing the pericenter, a star is tidally disrupted into a very long

and dilute gas stream.

• Tidal disruption processes were perhaps already observed in

cosmological galaxy surveys (see, e.g. Donley et al., 2002).

Energy Release from Tidal Disruption

(see e.g. Alexander, 2005)

2 1 1 / 3 2 52 6

/ 4 10 10 0.1 periastron tidal

• ut

p t p t

M R m M b W erg M R r b r r radius of r radius of disruption

    

                           

 



MeV/n ) 1 . / ( 68

2 

  b Eesc

• A total tidal disruption of a star occurs when the penetration parameter b<1.

The tidal disruption rate n can be approximated to within an order of magnitude from an analysis of star dynamics near a black hole via the Fokker-Planck

• equation. For the parameters of the GC it gives the rate n ~10-4 years-1 (see the

review of Alexander, 2005).

Subrelativistic Protons in GC

• Equation

ion cap

  

ion ~3 106 years; cap <105 years Quasi-stationary energy release in the GC in the form

• f subrelativistic protons supplies to the background

plasma ~1041-1042erg/s (Coulomb energy losses).

• Energy distribution
• Spatial distribution

0.01 0.02 0.03 0.04 0.05 0.06 10

−4

10

−2

10 10

2

E (GeV) f(E, r=0) GeV−1 cm−3

0.1 0.5 1 5 10 50 100 rpc

• 5. 106

0.00001 0.00002 0.00005 NpMeV1cm3

• The energy necessary for plasma heating ~1041-1042

erg/s can easily be provided by subrelativistic protons through their Coulomb losses.

protons

Coulomb collisions Gas heating

protons Bremsstrahlug losses Hard X-ray emission

For Ep =100 MeV Ex ~70 keV

Inverse Bremsstrahlung?

• IB cross-section
• IB intensity in the direction l

Energy Spectrum of IB emission in GC

WIB =3.4 1036erg/s ! Emax ~100 MeV

6.4 keV line from molecular clouds

Origin: (a) Sunyaev et al. 1993, Koyama et al. 1996 – 2008: Reflection of X-ray flux produced by past activities of a SN or Sgr A*? (b) Yusef-Zadeh et al. 2007 – subrelativistic electrons? (c) Dogiel et al. 2009 – subrelativistic protons?

Time Variable

6.4 keV line from molecular clouds

Origin: (a) Sunyaev et al. 1993, Koyama et al. 1996 – 2008: Reflection of X-ray flux produced by past activities of a SN or Sgr A*? (b) Yusef-Zadeh et al. 2007 – subrelativistic electrons? (c) Dogiel et al. 2009 – subrelativistic protons?

?????

Fe Kα variations from MC at the GC

Fe Kα

• and bremsstahlung cross-sections

Equivalent Width:

   

6.4 6.4

line Fe cont

F keV eW F keV

 Fe abundance hFe

' ' e e p p

E E for electrons m E E for protons m  

Equivalent width of the Fe Kα line

For the Compton scattering scenario (Koyama et al.)

1.6

Fe

 

Charge particle scenario

protons Bremsstrahlung continuum+ 6.4 Kα

• line

Molecular cloud

Sgr B2: lx ~L, lp <<L

Sgr B2 HESS J1745-303

Continuum and 6.4 keV line emission from the clouds

Observations Model

Other sources of the emission?

Ionization Rate produced by Subrelativistic Protons in Molecular Clouds at the GC

0.02 0.05 0.1 0.2 0.5 1 2 xpc

• 1. 1010
• 1. 108
• 1. 106

NpMeV1cm3

0.01 0.02 0.05 0.1 0.2 0.5 1 2 xpc

• 1. 1016
• 5. 1016
• 1. 1015
• 5. 1015
• 1. 1014
• 5. 1014
• 1. 1013

Ζs1

Proton distribution as a function of cloud depth Ionization rate z

De-Excitation Gamma-Ray Line Emission from the GC

protons

Nuclear collisions

De-excitation gamma-ray lines Fast protons with energies about 10-100 MeV excite nuclei of particles from background plasma. The excitation can lead to triggering of nuclear reaction or to formation of de-excitation line.

Spectrum of De-Excitation Lines

• The gamma-ray lines will be emitted from the region of maximum 5

angular radius. Such an emission would appear as a small-scale diffuse emission for gamma- ray instruments like SPI.

• The total gamma-ray line flux below 8 MeV to be ~10-4 photons cm-2 s-1
• The most promising lines for detection are those at 4.44 and 6.2 MeV,

with a predicted flux in each line of 10-5 photons cm-2 s-1.

• GRIPS mission proposed for ESA's ''Cosmic eristics Vision'' program

could achieve after 5 years in orbit a sensitivity, which would allow a clear detection of the predicted gamma-ray line emission at 4.44 and 6.2 MeV from the GC region. The Advanced Compton Telescope project proposed as a future NASA mission aims at even better sensitivity, near 10-6 photons cm-2 s-1 for 3% broad lines. A future detection of the predicted gamma-ray lines would provide unique information on the high-energy processes induced by the central black hole.

Can we see these de-excitation lines from the GC?

Conclusion

• Accretion processes in GC release in average 1041-1042erg/s in the

form of subrelativistic protons with energies E~100 MeV;

• Protons lose almost all their energy by ionization and, thus, heat the

plasma up to the temperature ~ 10 keV;

• Inverse bremsstrahlung of these protons generates hard X-ray flux in

the energy range above 10 keV with the total flux 3 1036erg/s;

• Protons penetrating into the dense molecular clouds produce

Kα

• vacancies. The flux of 6.4 keV line at Earth from Sgr B2 is

expected at the level of 10-4 ph cm-2s-1 , and the hard X-ray continuum flux due to proton bremsstrahlung is about 1035erg/s, just as observed;

• These protons heat the molecular gas in the GC up to the

temperature about 100 K.

• They produce intensive de-excitation gamma-ray lines which, in

principle, can be observed from the central 1ox1o region.

• Out of the scope: the origin of anniohilation emission from the GC

and predictions of the de ecitatrion line emission from there

• Astronomy and Astrophysics, vol.473, p.351, 2007
• Publications of the Astronomical Society of Japan, Vol.61, p.901, 2009
• Publications of the Astronomical Society of Japan, Vol.61, p.1093, 2009
• Publications of the Astronomical Society of Japan Vol 61 p 1099 2009