Post on 31-Jan-2016
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M.Cassé@iap.fr 1
THE RIDDLE OF P0SITRONS IN THE GALACTIC BULGE
M.Cassé
Service d’Astrophysique,CEA, Institut d’Astrophysique de Paris
OUTLINE
SPI /INTEGRAL OBSERVATIONSPOSITRON INJECTION RATE
& MORPHOLOGY
POSITRON SOURCE(S)SNIa fall short
Hypernovae, GRB, LMXB, microquasars & NEUTRALINOSeven less convincing
Pave the way to LIGHT DARK M ATTER
(P. Fayet)
Treated byJ. Knödlseder
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OSSE/GRO map
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Knödlseder et al 2005
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Centroid: 511.06+0.17/-0.10 keV
Line width: 2.95+0.45/-0.51 keV (FWHM)
Flux:(1.05±0.06)×10-3 ph cm-2 s-1
SPI Observation of 511 keV line from the Galactic bulge
INTEGRAL/SPI
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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- : 2(511keV)• formation of Positronium (para)
• 1/4 of the Ps (s=0)• annihilation via 2(511keV line)
• formation of Ps (s=1)• 3/4 of the Ps (triplet)• annihilation via 3(0 - 511 keV)
fraction de positronium :fPs= 2 / (1.5 + 2.25 511 / 3)
CGRO/OSSE Kinzer et al, ApJ 559 (2001)• thin line 3 keV
• fPs= 0.93 ± 0.04
Annihilation in a warm medium (~5×104 K).
L511 =(2-1.5 fp) L e+
Positron injection rate: (1.5 ± 0.1)1043 s-1
GRO/OSSE data
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Knödlseder, 1999, Proceedings 'Astronomy with Radioactivities’
26Al map (1809 keV) CGRO/COMPTEL
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Jean et al, 2003, A&A, 407Knödlseder et al 2003, A&A, 407
Knödlseder et al 2005, astro-ph/0506026
BulgeFlux = (1.09 ± 0.04) ph cm-2 s-1
fp = 93±0.04Le+ = (2-1.5fp).L511
Annihilation rate= (1.5±0.1) 1043 e+ s-1
No evidence for a point like source
Δl (FWHM) = 8.1°± 0.9°Δb (FWHM) =7.2 ± 0.9°
No other sources (Cygnus, Vela, LMC…)
WHAT IS THE SOURCE OF POSITRONS?
FLUX & MORPHOLOGY (summary)
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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 ~ solarIMF dN/dm =k Ma (Gould 2000)
(a =-2.0 for M>0.7Mo, a= -1.3 for M<0.7Mo)
1Mo<M<8Mo stars WD: 22%
Each WD ejects (PN phase) ~ 0.5 Mo Mg ~109 MoBulge 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 lossesD ~ 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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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 SourcePb of diffusion des positons: D~3 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 MNi=0.1 M Thick envelope (10Mo)
positrons released~ 0%
SN Ia MNi= 0.6 M Thinner envelope (1 Mo) positrons released ?
Potential sources of e+
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TYPICAL PARAMETERS OF SNIa: M=1.34 Mo ejected mass E=1.17×1051 erg Kinetic energy (E=1/2 M v2) MNi = 0.6 Mo mass of 56Ni synthesized
ENERGY SOURCE :
56Ni 56Co (t1/2=6.1 j) 56Fe (t1/2=77j) + 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 = = mean free path ~ 0.6 g cm-2
-Escape time: M E-1/2 • t~390 d consistent with inflexion of L.C..• 3.3% of 56Co remaining
Number of positrons released Ne+ = 8×1052
Production of positrons by SNIa (Milne et al, Schanne et al )
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RATE OF SNIa IN THE BULGE
Calibration with M (10Calibration with M (101010Mo) and B-K (+4.8) ;Mo) and B-K (+4.8) ;Schanne et al 2004, 2005Schanne et al 2004, 2005)) : : 0.030.03±±0.02/century0.02/century,, in agreement with theoretical models in agreement with theoretical models (Matteucci, Nomoto…)(Matteucci, Nomoto…)
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HYPERNOVA SN2003dh
SNIA: PUREMELY RADIOACTIVE OBJECTS
L.C. fed by gammas & positrons
56Ni 56Co (t1/2=6.1 j) 56Fe (t1/2=77j) + e+
E=847 keV, <Ee+>= 640 keV (19%), Emax=1.4 MeV
Envelope dense & opaque to & e+ ,then progressive leak out
Light curve of SN2003dh declines faster than typical SNIa faster ejection (earlier transparency) faster escape of and e+
larger number of released positrons
25 times more than in SNIa
~ 0.02/century required
BUT: Rate? Diffusion of positrons? Disk/Bulge ratio?
Ma
gn
itud
e d
iffe
ren
ce (
V-b
an
d)
Time (days since max)
56Ni 56Co (t1/2=6.1 j) 56Fe (t1/2=77j) + e+ (Br=19%)
-3,50
-3,00
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
-20,00 -10,00 0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00
SN03dh-modelSN03dh-pointsSN92alSN96x
Schanne et al 2004
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hypothesis : ANNIHILATION OF DARK MATTER
NATURE ? NEUTRALINOS (favorite candidate of particle physicists)
551keV EMISSION DOUBLE ANNIHILATION
e+e- 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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Energy ~ 1043 Ee+ ergs-1 if Ee+ ~ 1 GeV
Lbol > 1040 erg s-1 >> Lbol(non thermal)
Radio-synchrotron + Bremmstrahlung Not observed
Branching (e+ e-)/ ~ 1 huge flux of HE Not observed
NEUTRALINOm = 500GeV – 10TeV
e+ injected ar high Energy
GB Positrons do not emanate from the annihilation of neutralinosE>30 MeV excluded by bremmstrahlung (EGRET 30-100 MeV)
Hooper et al., 2004
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BREMMSTRAHLUNG (EGRET)
Ionisation
Bremsstrahlung
No hint of a bulge
Lb(bulge) < 2 1036 erg/sThick target
Lb =τb/τi Li
Li = 5 1039 erg/sLb (100-0 MeV) ~1039 erg/sLb (100-30 MeV) ~2 1038 erg/s
Mx < 100 MeV
Hunter et al 1997
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CONCLUSION
OTHER SOURCE(S) OF POSITRONS NEEDED
LIGHT DARK MATTER(TO AVOID PRODUCTION OF π°
and HIGH ENERGY GAMMA RAYS)
Pierre Fayet