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

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1 Thermal and Non-Thermal X-Rays from the Galactic Center 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 Vulcano Workshop 2010

2 Galactic Center as a Harbour of High Energy Activity TeV emission >100 MeV emission 511 kev line emission erg/s erg/s erg/s 2-10 kev thermal emission kev nonthermal emission kev line emission Counts s longitude (degree) erg/s erg/s

3 GC Thermal Emission (Koyama, , Muno, 2004) T ~ 10 8 K L 2-10 ~ 2x10 36 ergs/s Size ~ 50pc x 30pc n ave ~ 0.1cm -3 n peak ~ 0.4cm -3 E gas ~ 3x10 52 ergs The energy input needed to heat the gas up to T~10 kev is about 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 ~10 41 erg/s is required!!!

4 Non-Thermal X-Ray (Belanger et al. 2006,Yuasa et al. 2008) Region l<2 o, b<2 o Spectrum ΔE=12-40 kev, Γ=1.4, E c =19-50 kev Luminosity W~ erg/s

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

6 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.

7 However! Image 2 scale-heights of 6.7 kev line Point sources ~1/7 Revnivtsev Point Sources

8 Relativistic protons X-rays Flux of subrelativistic protons

9 Star capture (Diener et al. 1997, Ayal et al. 2000, Alexander 2005 etc.) 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). A half of the star matter (i.e. ~ protons when a one solar mass star is captured) escapes with a subrelativistic velocity.

10 Energy Release from Tidal Disruption (see e.g. Alexander, 2005) W p out b r p t 2 1 1/ M R m / M b erg 6 M R radius of periastron r radius of tidal disruption t r r E esc 68 ( b / 0.1) 2 MeV/n 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).

11 Subrelativistic Protons in GC Equation ion ~ years; cap <10 5 years ion cap Quasi-stationary energy release in the GC in the form of subrelativistic protons supplies to the background plasma ~ erg/s (Coulomb energy losses).

12 Energy distribution Spatial distribution f(e, r=0) GeV 1 cm Np MeV 1 cm E (GeV) r pc

13 protons Coulomb collisions Gas heating The energy necessary for plasma heating ~ erg/s can easily be provided by subrelativistic protons through their Coulomb losses.

14 protons Bremsstrahlug losses Hard X-ray emission For E p =100 MeV E x ~70 kev

15 IB cross-section Inverse Bremsstrahlung? IB intensity in the direction l

16 Energy Spectrum of IB emission in GC W IB = erg/s! E max ~100 MeV

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18 Origin: 6.4 kev line from molecular clouds (a) Sunyaev et al. 1993, Koyama et al : Reflection of X-ray flux produced by past activities of a SN or Sgr A *? (b) Yusef-Zadeh et al subrelativistic electrons? (c) Dogiel et al subrelativistic protons?

19 6.4 kev line from molecular clouds Origin: (a) Sunyaev et al. 1993, Koyama et al : Reflection of X-ray flux produced by past activities of a SN or Sgr A *? (b) Yusef-Zadeh et al subrelativistic electrons? (c) Dogiel et al subrelativistic protons? Time Variable

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35 Fe K α variations from MC at the GC

36 Fe K α - and bremsstahlung cross-sections ' E Ee for electrons m ' e E E p for protons mp Equivalent Width: ew Fe F F line cont kev kev Fe abundance h Fe

37 Equivalent width of the Fe K α line For the Compton scattering scenario Fe 1.6 Charge particle scenario (Koyama et al.)

38 protons Molecular cloud Bremsstrahlung continuum+ 6.4 K α -line Sgr B2: l x ~L, l p <<L

39 Continuum and 6.4 kev line emission from the clouds Observations Sgr B2 HESS J Model Other sources of the emission?

40 Ionization Rate produced by Subrelativistic Protons in Molecular Clouds at the GC Np MeV 1 cm Ζ s x pc Proton distribution as a function of cloud depth x pc Ionization rate z

41 De-Excitation Gamma-Ray Line Emission from the GC protons Nuclear collisions De-excitation gamma-ray lines Fast protons with energies about MeV excite nuclei of particles from background plasma. The excitation can lead to triggering of nuclear reaction or to formation of de-excitation line.

42 Spectrum of De-Excitation Lines

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

44 Conclusion Accretion processes in GC release in average erg/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 erg/s; Protons penetrating into the dense molecular clouds produce K α vacancies. The flux of 6.4 kev line at Earth from Sgr B2 is expected at the level of 10-4 ph cm -2 s -1, and the hard X-ray continuum flux due to proton bremsstrahlung is about erg/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 1 o x1 o 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