The 511 keV Annihilation Emission From The Galactic Center

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The 511 keV Annihilation Emission From The
Galactic Center

• Department of Physics
• National Tsing Hua University
• G.T. Chen
• 2007/1/2

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References

• K.S. Cheng et al. 2006 ApJ
• Annihilation emission form the galactic black
  hole
• W. Wang et al. 2006 AA
• 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 AA
• The live and deaths of positrons in the
  interstellar medium
• Jean et al. 2006 AA
• Spectral analysis of the galactic ee-
  annihilation emission
• Knodlseder et al. 2006 AA
• The all-sky distribution of 511 keV
  electron-positron annihilation emission

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Outline

• Introduction
• Models (I) (II)
• Discussion and Future Work

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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

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Introduction

• The bulge emission is spherically symmetric and
  is centered on the galactic center with an
  extension of 50-80 (FWHM)
• Total flux

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Introduction

• Line width

(Churazov et al. 2005 MNRAS)
(Jean et al. 2006 AA)

• Positronium fraction

(Jean et al. 2006 AA)

• production positron rate

(J. Knodlseder et al. 2005 AA)
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Introduction
Weidenspointner et al. 2006 AA
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Introduction
Jean et al. 2006 AA
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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

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Introduction

• Positrons can be produced by
• ß decay
• Pion decay
• Pair production (e- e- ) through photon-photon
  interactions
• Pair production by the interaction of an e- with
  a strong magnetic field

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Introduction

• Positronium (PS)
• It is the bound state of e and e-

Para-PS state
Ortho-PS state
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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

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Introduction

• The ISMs in the galactic center
• Cold neutral component (n10 cm-3, T80K)
• Warm neutral component (n0.3 cm-3, T8000K)
• Warm ionized component (n0.17 cm-3, T8000K)
• Hot component (n310-3 cm-3, T4.5105K)

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Models (I) (II)
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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 AA)

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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
  

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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 Egt30
  MeV

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

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Model (I)
ne1 cm-3 T2.5 eV
K.S. Cheng et al. 2006
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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

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Model (I)
Guessoum et al. 2005 AA
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Model (I)

• In this case, they assume the annihilation
  spectrum is caused by in-flight annihilation with
  e- and charge exchange with H

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Model (I)
ne1 cm-3 T2.5 eV
K.S. Cheng et al. 2006
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Model (I)
ne310-3 cm-3 T100 eV
K.S. Cheng et al. 2006
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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

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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

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Model (II)

• Typical parameters
• How many MSPs in this region ?

P3 ms B3108 G ? Injection rate
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Discussion Future Work

• Study much details in the model(I)
• Apply some tools learning from model(I) to MSPs
  model

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gtgtThank Youltlt
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Guessoum et al. 2005 AA
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Guessoum et al. 2005 AA
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Guessoum et al. 2005 AA
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Guessoum et al. 2005 AA
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Annihilation rel. electrons
Ionization loss of positrons
Ionization loss of electrons
Syn.Inverse Compton rel. electrons
K.S. Cheng et al. 2006
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Energy threshold of reactions induced by positrons
Guessoum et al. 2005 AA
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Guessoum et al. 2005 AA