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THE RIDDLE OF P0SITRONS IN THE GALACTIC
BULGE M.Cassé Service dAstrophysique,CEA,
Institut dAstrophysique de Paris
OUTLINE SPI /INTEGRAL OBSERVATIONS POSITRON
INJECTION RATE MORPHOLOGY POSITRON
SOURCE(S) SNIa fall short Hypernovae, GRB, LMXB,
microquasars NEUTRALINOS even less
convincing Pave the way to LIGHT DARK M
ATTER (P. Fayet)
Treated by J. Knödlseder
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OSSE/GRO map
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Knödlseder et al 2005
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SPI Observation of 511 keV line from the Galactic
bulge
INTEGRAL/SPI

• Centroid
• 511.060.17/-0.10 keV
• Line width
• 2.950.45/-0.51 keV (FWHM)
• Flux
• (1.050.06)10-3 ph cm-2 s-1

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ANNIHILATION MEDIUM
P.Jean et al 2005
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Positronium Fraction

• Annihilation of e in the gas after
  thermalisation
• direct annihilation e e- 2g (511keV)
• formation of ??Positronium (para)
• 1/4 of the Ps (s0)
• annihilation via 2g (511keV line)
• formation of ??Ps (s1)
• 3/4 of the Ps (triplet)
• annihilation via 3g (0 - 511 keV)
• fraction de positronium
• fPs 2 / (1.5 2.25 F511 / F3g)
• CGRO/OSSE
• Kinzer et al, ApJ 559 (2001)
• thin line ? 3 keV
• fPs 0.93 0.04

GRO/OSSE data
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26Al map (1809 keV) CGRO/COMPTEL
Knödlseder, 1999, Proceedings 'Astronomy with
Radioactivities
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FLUX MORPHOLOGY (summary)
Jean et al, 2003, AA, 407 Knödlseder et al 2003,
AA, 407 Knödlseder et al 2005,
astro-ph/0506026 Bulge Flux (1.09 0.04) ph
cm-2 s-1 fp 930.04 Le (2-1.5fp).L511 Annihi
lation rate (1.50.1) 1043 e s-1 No evidence
for a point like source ?l (FWHM) 8.1 0.9 ?b
(FWHM) 7.2 0.9 No other sources (Cygnus,
Vela, LMC) WHAT IS THE SOURCE OF POSITRONS?
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GAS DENSITY(rough estimate)

• Gaz density (through dE/dx) determines behaviour
  of positrons in the GB
• together with Diffusion coefficient D, both
  poorly known
• No clear observation of HI, H2(foreground disk),
  except in the CMZ
• Bulge elliptical galaxy in a spiral (see
  models by Wyse Silk )
• Age of stars 10 Gyr, Metallicity solar
• IMF dN/dm k Ma (Gould 2000)
• (a -2.0 for Mgt0.7Mo, a -1.3 for Mlt0.7Mo)
• 1MoltMlt8Mo stars? WD 22
• Each WD ejects (PN phase) 0.5 Mo ? Mg 109 Mo
• Bulge Radius 2 kpc
• Gas Density 1 cm-3 (upper limit)
• Accretion by stars? Sinking in CMZ ? new stars
• Necessity dedicated calculations

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PROPAGATION OF LOW E POSITONS

• Diffusion Ionisation losses
• D 3 1027 cm2s-1
• (Parizot et al 2005) TENTATIVE (Bykov)
• Ionisation
• dE/dt 2 10-8 (Ln gamma 6.6)nH eVs-1
• tD R2/D ? 108 ans
• tstop E/(dE/dt)
• 1 0.1 0.01 cm-3
• 1 2.2 105 2.2 106 2.2 107
• 10 1.7 106 1.7 107 1.7 108
• 100 1.4 107 1.4 108 1.4 109
• MeV Escape

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Potential sources of e

• ACTIVE OBJECTS WITH A BLACK HOLE OR A NEUTRON
  STAR INSIDE
• Microquasars to few, not powerful enough
• LMXRB (Prantzos 2004) No 511 keV emission
  detected
• HN/GRB (Schanne et al 2204,Parizot et al 2005)
• Central Source
• Pb of diffusion des positons
• D3 1027cm2s-1
• Rate of HN/GRB? Disk emission
• NO GOOD CANDIDATE
• ? RADIOACTIVITY by order of importance
• 22Na Novae ? very small amounts
• 26Al SN et WR ? (1809 keV emission) not
  sufficient
• 44Ti SN smaller contribution than 26Al
• 56Ni SN II MNi0.1 M? Thick envelope (10Mo)
• positrons released 0
• SN Ia MNi 0.6 M? Thinner envelope (1 Mo)
• positrons released ?

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Production of positrons by SNIa (Milne et al,
Schanne et al )

• TYPICAL PARAMETERS OF SNIa
• M1.34 Mo ejected mass
• E1.171051 erg Kinetic energy (E1/2 M v2)
• MNi 0.6 Mo mass of 56Ni synthesized
• ENERGY SOURCE
• 56Ni ? 56Co (t1/26.1 j) ? 56Fe (t1/277j) e
• Simple Model of explosion (spherical symmetry )
• - free homologous expansion R v t
• radioactive nuclei (56Ni) deeply buried
• - production of positrons 56Co ? 56Fe e
  (19)
• - absorption r R l mean free path l 0.6 g
  cm-2
• -Escape time M E-1/2
• t390 d? consistent with inflexion of L.C..
• 3.3 of 56Co remaining

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RATE OF SNIa IN THE BULGE
Calibration with M (1010Mo) and B-K (4.8)
Schanne et al 2004, 2005)  0.030.02/century,
in agreement with theoretical models (Matteucci,
Nomoto)
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HYPERNOVA SN2003dh

• SNIA
• PUREMELY RADIOACTIVE OBJECTS
• L.C. fed by gammas positrons
• 56Ni ? 56Co (t1/26.1 j) ? 56Fe (t1/277j) e
• Eg847 keV,
• ltEegt 640 keV (19), Emax1.4 MeV
• Envelope dense opaque to g e ,
• then progressive leak out
• Light curve of SN2003dh declines faster than
  typical SNIa
• ? faster ejection (earlier transparency)
• ? faster escape of g and e
• ? larger number of released positrons
• 25 times more than in SNIa

Time (days since max)
Magnitude difference (V-band)
56Ni ? 56Co (t1/26.1 j) ? 56Fe (t1/277j) e
(Br19)
Schanne et al 2004
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hypothesis   ANNIHILATION OF DARK MATTER
NATURE ? NEUTRALINOS (favorite candidate of
particle physicists) 551keV EMISSION DOUBLE
ANNIHILATION   ?? ? ee- ? ?? or ??? ? a ?
b Z c Ho,1 d Ho,2 photino
higgsino    MAJORANA PARTICULE (particle
antiparticle)    Positrons produced by
different channels ?? ? WW,ZZ,tt,??-   (qq-)
(bb-)? ? ?? 2? ?? ? ? e ?-? ?- ? e-  final
product ?, (e,e-), (?, ?-), (p,p-)




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NEUTRALINO m? 500GeV 10TeV e injected ar
high Energy
Hooper et al., 2004
Energy 1043 Ee ergs-1 if Ee 1
GeV   Lbol gt 1040 erg s-1 gtgt Lbol(non
thermal)   ? Radio-synchrotron Bremmstrahlung
Not observed   Branching (e e-)/? 1 ?huge
flux of HE ? Not observed
GB Positrons do not emanate from the annihilation
of neutralinos Egt30 MeV excluded by
bremmstrahlung (EGRET 30-100 MeV)
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BREMMSTRAHLUNG (EGRET)
Hunter et al 1997
No hint of a bulge
Lb(bulge) lt 2 1036 erg/s Thick target Lb tb/ti
Li Li 5 1039 erg/s Lb (100-0 MeV) 1039
erg/s Lb (100-30 MeV) 2 1038 erg/s Mx lt 100 MeV
Ionisation
Bremsstrahlung
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CONCLUSION OTHER SOURCE(S) OF POSITRONS
NEEDED LIGHT DARK MATTER (TO AVOID PRODUCTION OF
p and HIGH ENERGY GAMMA RAYS) ? Pierre Fayet