The 511 keV Annihilation Emission From The Galactic Center

1. The 511 keV Annihilation Emission From The Galactic Center Department of Physics National Tsing Hua University G.T. Chen 2007/1/2

2. References: • K.S. Cheng et al. 2006 ApJ “Annihilation emission form the galactic black hole” • W. Wang et al. 2006 A&A “Could electron-positron annihilation lines in the galactic center result from pulsar winds ? ” • W. Wang 2005 astro-ph/0510461 “Millisecond pulsar population in the galactic center and high energy contributions” • N. Guessoum et al. 2005 A&A “The live and deaths of positrons in the interstellar medium” • Jean et al. 2006 A&A “Spectral analysis of the galactic e+e- annihilation emission” • Knodlseder et al. 2006 A&A “The all-sky distribution of 511 keV electron-positron annihilation emission”

3. Outline • Introduction • Models — (I) (II) • Discussion and Future Work

4. Introduction • Radiation of e+-e- annihilation from the galactic center was first reported in 1972 • Recently, we have the observation data by SPI of the INTEGRAL

5. Board line : Narrow line : (Jean et al. 2006 A&A) Introduction • The bulge emission is spherically symmetric and is centered on the galactic center with an extension of ∼ 50-80 (FWHM) • Total flux=

6. Board line : Narrow line : Introduction • Line width ~ (Churazov et al. 2005 MNRAS) (Jean et al. 2006 A&A) • Positronium fraction ~ (Jean et al. 2006 A&A) • production positron rate ~ (J. Knodlseder et al. 2005 A&A)

7. Introduction Weidenspointner et al. 2006 A&A

8. Introduction Jean et al. 2006 A&A

9. Introduction • Problems: • The production of positrons • The galactic map of the annihilation line • The propagation/evolution of the positrons between their production sites and annihilation places

10. Introduction • Positrons can be produced by: • β+ decay • Pion decay • Pair production (e＋- e- ) through photon-photon interactions • Pair production by the interaction of an e- with a strong magnetic field

11. Introduction • Positronium (PS): • It is the bound state of e＋and e- Para-PS state Ortho-PS state

12. Introduction • In order to determine the overall spectrum of the line produced by galactic positrons, we must consider the model of ISM  distributions, temperatures and ionization of the gases and dusts. • Taking all the physical information (processes, cross sections, line widths, gas distributions, etc.) into account , we can reproduce the observational data

13. Introduction • The ISMs in the galactic center • Cold neutral component (n=10 cm-3, T=80K) • Warm neutral component (n=0.3 cm-3, T=8000K) • Warm ionized component (n=0.17 cm-3, T=8000K) • Hot component (n=3*10-3 cm-3, T=4.5*105K)

14. Models — (I) (II)

15. Models • ………… • Star accretion by central supermassive black hole (Model I) (K.S. Cheng et al. 2006 ApJ ) • Millisecond pulsars in the galactic bulge region (Model II) (W. Wang et al. 2006 A&A)

16. Model (I) • The assumptions in this model: • The rate of production of positrons~ • If positrons are produced via p-p collisions, the total energy of protons must not exceed • The spatial distribution of positrons should extend over a region with angular radius 50-80 around GC when they cool down to thermal energies

17. Model (I) • The capture of stars by Galactic black hole provides relativistic protons • Collisions of these protons with surrounding gas result in relativistic positrons with energy E>30 MeV

18. Model (I) Electron/Positron Production Spectrum electrons positrons K.S. Cheng et al. 2006

19. Model (I) • Consider the interactions of positrons and gases ---ionization loss+ synchrotron +inverse Compton loss +bremsstrahlung loss , we can describe the evolution of the positrons spectrum from relativistic to thermal energies

20. Model (I) ne=1 cm-3 T=2.5 eV K.S. Cheng et al. 2006

21. Model (I) • Later Coulomb collision continue to form the equilibrium distribution and accumulate positrons in the thermal energy range, then these thermal positrons decrease because of annihilation

22. Model (I) Guessoum et al. 2005 A&A

23. Model (I) • In this case, they assume the annihilation spectrum is caused by in-flight annihilation with e- and charge exchange with H

24. Model (I) ne=1 cm-3 T=2.5 eV K.S. Cheng et al. 2006

25. Model (I) ne=3*10-3 cm-3 T=100 eV K.S. Cheng et al. 2006

26. Model (II) • There possibly exists a population of millisecond pulsars (MSPs) in the Galactic center region. • MSPs could emit GeV gamma-rays through synchrotron-curvature radiation as predicted by outer gap models • MSP winds provide good candidates for the e＋- e- sources in the Galactic center

27. Model (II) • They consider the e＋- e- pair production occurs in the pulsar outer-magnetosphere region • MSP winds are positron sources which result from pair cascade near neutron star surface • MSPs are active near Hubble time, so they are continuous positron injecting sources

28. Model (II) • Typical parameters: • How many MSPs in this region ? P=3 ms B=3*108 G  Injection rate

29. Discussion & Future Work • Study much details in the model(I) • Apply some tools learning from model(I) to MSPs model

30. >>Thank You<<

31. Guessoum et al. 2005 A&A

32. Guessoum et al. 2005 A&A

33. Guessoum et al. 2005 A&A

37. Guessoum et al. 2005 A&A

38. Annihilation rel. electrons Ionization loss of positrons Ionization loss of electrons Syn.+Inverse Compton rel. electrons K.S. Cheng et al. 2006

39. Energy threshold of reactions induced by positrons Guessoum et al. 2005 A&A

40. Guessoum et al. 2005 A&A