1. High Energy emission from the Galactic Center
Jason Ybarra

2. The Galactic Center Contains a supermassive black hole
M = 3.6 × 106 M
3 main radio sources Sgr B, SgR C, SgR A
Very dense molecular clouds
Supernova remnants

3. Observations
INTEGRAL IBIS/ISGRI
( keV)
HESS
( TeV)

4. INTEGRAL observations
IBIS/ISGRI imager
4.6 Ms total exposure time for observations between
Range keV
(Bélanger et al 2006)

5. INTEGRAL IBIS/ISGRI mosaic of GC in in the 20–40 keV range.
(Bélanger et al 2006)

6. INTEGRAL
20-30 keV
30-40 keV
56-85 keV
40-56 keV
(Bélanger et al 2006)

7. Spectrum of IGR J17456−2901
Red is the ISGRI data. Green is a power law fit with index Γ = 3.04 ± 0.08
(Bélanger et al 2006)

8. Spectrum of IGR J17456−2901
1-10 keV from XMM-Newton keV from ISGRI
Power law Γ2=3.22
Spectrum
High-temp plasma (6.6 keV)
Low temp plasma (1 keV)
6.4 keV Fe line
Power law Γ=1.51
(Bélanger et al 2006)

9. Spectrum of IGR J17456−2901
The two-temperature plasma component does a decent job of modeling the 1-10 keV spectrum, but cannot account for the emission flux > 20 keV
(Bélanger et al 2006)

10. Possible Sources? X-Ray Transients Sgr A* flares
Charged-Particle Acceleration

11. X-Ray Transients Large number of X-Ray transients near Sgr A*
INTEGRAL
Large number of X-Ray transients near Sgr A*
4 within 30″ of Sgr A*
Light curves were constructed from Chandra and XMM-Newton data
CXOGC J −290124
CXOGC J −290005
CXOGC J −290031
CXOGC J −290022
(Bélanger et al 2006)

12. X-Ray Transients Contemporaneous XMM-Newton data for J174540.0−290031
IGR J
Contemporaneous XMM-Newton data for J −290031
Estimated flux ~ 5 x 1034 erg s-1 is still an order of magnitude too low.
Spectrum of transient unlikely to be a pure power law > 100 keV
CXOGC J −290124
CXOGC J −290005
CXOGC J −290031
CXOGC J −290022
(Bélanger et al 2006)

13. Possible Sources? X-Ray Transients Sgr A* flares
Charged-Particle Acceleration
Sgr A East

14. X-Ray Flares Flares occur on average once per day
Average L ~ 1035 ergs s-1, but last for a few thousand seconds.
The constant luminosity of IGR J cannot result from successive flares
(Bélanger et al 2006)

15. Possible Sources? X-Ray Transients Sgr A* flares
Charged-Particle Acceleration

16. Charged Particle Acceleration
Perhaps same origin as the HESS TeV source
The TeV emission is thought to come from the acceleration of particles (protons) to very high energies
(Bélanger et al 2006)

17. Proton-Proton Collisions
Accelerated protons are thought to collide with ambient protons p p

18. Proton-Proton Collisions
Accelerated protons are thought to collide with ambient protons
This interaction produces neutral pions p p p π0 p

19. Proton-Proton Collisions
Neutral pions decay very quickly into two gamma rays p p p γ π0 γ p

20. Proton-Proton Collisionsπ+ p

21. Proton-Proton Collisionsμ+ π+ νμ p

22. Proton-Proton Collisions
Secondary electrons and positrons can produce gamma-rays through bremsstrahlung or inverse Compton scattering p p νe e+ n μ+ νμ π+ νμ p

23. Diffuse Emission
Belanger et al. (2006) argue that the emission is diffuse
Absence of variability
Not detected by JEM-X (3′ resolution)

24. HESS High Energy Stereoscopic System (HESS)
This is an array of 4 atmospheric Cherenkov Telescopes

25. HESS High Energy Stereoscopic System (HESS)
This is an array of 4 atmospheric Cherenkov Telescopes

26. SNR/Pulsar Wind Nebula
Galactic center
HESS detected a point-like source of very-high energy gamma rays a the galactic center (HESS J ).
(Aharonian et al 2006)

27. The white contours indicate molecular gas traced by CS emission
SNR/Pulsar Wind Nebula
Galactic center
The white contours indicate molecular gas traced by CS emission
The correlation between molecular material and the faint γ-ray emission indicates cosmic ray origin
(Aharonian et al 2006)

28. (Aharonian et al 2006)

29. Energy distribution
The diffuse material exhibits the same power law index as HESS J
This suggests that J is the source of cosmic rays that slowly diffuse out
(Aharonian et al 2006)

30. What can accelerate the particles?
Two possibilities:
Supernova Remnant Sgr A East
SMBH Sgr A*

31. Inverse Compton scattering
If a high-energy photon and a low-energy electron interact, the electron receives energy
If a low-energy photon and a high-energy electron interact, the photon will increase it energy.

32. Inverse Compton scattering
Average energy lost by the photon
ΔEγ/Eγ = - Eγ / mec2
Average energy gained by the photon
ΔEγ/Eγ = 4/3 β2 γ2
ΔEγ/Eγ = 4/3 β2 γ2 - Eγ / mec2

33. Inverse Compton Scattering
(Hinton & Aharonian 2007)

34. Inverse Compton Scattering
The magnetic field strength is fixed at 105 μG
(Hinton & Aharonian 2007)

35. Inverse Compton
Solid line – very young source with B = 50μG, electron spectrum α =0.3
Dashed line – old source B = 110 μG, α =1.5
(Hinton & Aharonian 2007)

36. Dark Matter Annihilation
Green - Minimal Supersymmetric Standard Model annihilation of 14 TeV neutralinos

37. Dark Matter Annihilation
Blue – mixed final state, DM masses 6-30 TeV

38. Summary Inverse Compton scattering Two leading theories
Gamma rays from accelerated particle interactions (p-p → p + p + π0, π0 → 2γ )
Inverse Compton scattering

39. References Aharonian et al (HESS Collaboration) 2006, PRL, 97, 221102
Belanger et al 2006, ApJ, 636, 275
Hinton, J. A. & Aharonian F. A. 2007, ApJ, 657, 302

40. Diffuse TeV Emission from the Galactic Center
Jason Ybarra
High Energy Astrophysics Seminar
April 30, 2008

41. Previously …
SNR/Pulsar Wind Nebula
Galactic center
HESS detected a point-like source of very-high energy gamma rays at the galactic center (HESS J ).
(Aharonian et al 2006)

42. The white contours indicate molecular gas traced by CS emission
SNR/Pulsar Wind Nebula
Galactic center
The white contours indicate molecular gas traced by CS emission
The correlation between molecular material and the faint γ-ray emission indicates cosmic ray origin
(Aharonian et al 2006)

43. Energy distribution
The diffuse material exhibits the same power law index as HESS J
This suggests that J is the source of cosmic rays that slowly diffuse out
(Aharonian et al 2006)

44. Can protons accelerated by Sagittarius A
Can protons accelerated by Sagittarius A* account for the diffuse emission seen by HESS?

45. Simulations Distribution of molecular clouds
Magnetic field modeled with Kolmogrov turbulence
Wommer et al (arXiv: v1 [astro-ph] 18 Apr 2008)

46. Energy loss rates p-p scattering p-γ scattering Synchrotron cooling
Compton scattering
(Wommer et al. 2008)

47. Energy loss rates p-p scattering Compton scattering
Cooling rates within the clouds
Synchrotron cooling
(Wommer et al. 2008)

48. Energy loss rates p-p scattering Compton scattering
Cooling rates between the clouds
Synchrotron cooling
(Wommer et al. 2008)

49. Proton propagation F = qv × B Diffusion equation
1,000 protons were followed with the Lorentz force equation in order to determine diffusion coefficients
(Wommer et al. 2008)

50. Proton Distribution
(Wommer et al. 2008)

51. Simulated Gamma-ray intensity map
Intensity assuming Sagittarius A* as source of relativistic protons, B ~ 10μG
(Wommer et al. 2008)

52. Simulated Gamma-ray intensity map
Intensity assuming Sagittarius A* as source of relativistic protons matched to intensity range of HESS
(Wommer et al. 2008)

53. Simulated Gamma-ray energy map
Diffuse emission from Sagittarius A* in simulation extends only a fraction of a degree
Diffusion from galactic center is too slow to account for emission beyond a fraction of a degree
Morphology is inconsistent with HESS data
(Wommer et al. 2008)

54. Simulated Gamma-ray intensity map – multiple injection sites
Intensity assuming 5 distinct sources of protons (from HESS observations), B = 10μG, intensity range of HESS
(Wommer et al. 2008)

55. Simulated Gamma-ray intensity map – multiple injection sites
Intensity assuming 5 distinct sources of protons (from HESS observations), B = 100μG, intensity range of HESS
(Wommer et al. 2008)

56. Simulated Gamma-ray emission map – multiple injection sites
Emission is concentrated at the injection sites
Morphology is centrally peaked and inconsistent with HESS data
(Wommer et al. 2008)

57. Simulated Gamma-ray intensity map
Protons injected throughout the inter-cloud medium accelerated through second-order Fermi acceleration, B ~ 10μG, intensity range of HESS
(Wommer et al. 2008)

58. Simulated Gamma-ray intensity map – protons accelerated throughout inter-cloud medium
Protons accelerated throughout the inter-cloud medium by second-order Fermi acceleration can produce a diffuse emission consistent with the HESS data
(Wommer et al. 2008)